RETed

8<

Bad example...
What do you think "air" is?
It's just another fluid...displacement of fluid...


-Ted

gio64

iTrader: (1)

Sorry, I was skiing (and still is, but I'm finally done getting my connection ready...).
OK.
I went to look at some other information, because the data just didn't add up.

So, I managed to find out how the dyno works and that explains why we're talking about stuff and not getting each other.
The dyno does its calculations based on the ability of the wheels of the car to accelerate the roller/drum.
Knowing what it weighs, what the friction is, it calculates power and torque.
What this means, is that all calculations are done while the car is accelerating. This means that all the work needed to spin the tranny (and the alternator and everything esle) simply increases due to the fact that the car is accelerating faster than before.
So, your examples were right, but nobody bothered telling me that the dyno works under acceleration and not under constant load.
There was no way I would have understood your examples, for that simple reason.
Now, while the tranny power consumption on the dyno is affected by increase of power, I believe that the calculations are somewhat misleading.
If the dyno worked under constant load -for example, supply increasing resistance to the wheels until it finds a balance that keeps the engine revved at top rpms WOT, without accelerating/decelerating- then the results of the test would be different. In fact, under those conditions, your increased crank to wheel power loss would be very minimal (compared to what it was before power increase).
I also believe that the increase in tranny power loss is not even considerable as such, for the many reasons I mentioned before.
The dyno test practically verifies what power is available at the wheels to accelerate the rest of the car, when you are accelerating.

Now this might be my misperception, but I believe that the actual power should be measured under constant load, rather than under acceleration.
measuring power this way is almost like bringing the car to the strip and factoring out the ability of the driver.

What I need to find out now, is wheter the power at the crank is calculated under constant load or under acceleration.

I thank you all, and I apologize for "not getting it", but there was a fundamental piece of information missing (the way the dyno does its measurements).
Under those conditions, I understand what happens (and I did before too, it's just that I assumed the dyno would work under constant load).

gio64

iTrader: (1)

BTW, assuming that you're mentioning the fact that the air you need to move with the car also requires work, then, yes you'd have to displace more air/second, therefore some of your power would be lost through the work the air requires to be moved.
That means, in my "paradox" that it is like saying that the air has become thicker.
In any case, it is just another parameter that requires work and therefore more work when/if accelerating faster, just like the weight of the rest of the car requires more work to be accelerated faster than before, which is what I said. But the car does require more work to be accelerated, regardless of the air resistance (if you were able to do the experiment in a vacuum, the amount of work requried to accelerate the car faster would still be higher, so my example is not bad simply incomplete, if you will.

Like I said in the previous post, this entire discussion could have been avoided if somebody would have said:
the dyno measures the capacity of the engine of accelerating the "roller" and derives the values from that, so the gearbox (as well as any other component rotating) requires more power to be accelerated than it did when the car was making less power, because now it is accelerated faster.
But nobody could come up with that.

My next question is (and I hope I'll get straight answers):
How is the power at the crank measured? is it under acceleration or is it at constant velocity?
Because if it is constant velocity, then the relation between power at the crank and power at the wheels (measured on a dyno) means absolutely nothing.

Anyone knows?
I'll be checking it out...

Mahjik

iTrader: (1)

Read this site and maybe you'll see why it's a function of acceleration:

http://www.fjr1300.info/misc/torque-power.html

gio64

iTrader: (1)

OK, maybe you missed my last 2 posts, where I said, now that I know how the dyno works, I understand what you guys were talking about (and why, which is more important).

However, my objections and questions are not wrong at all and, if I may say, I was right in the first place, in saying that the matter was not the way you were putting it.

I just read the article attached to your post.

Here's an excerpt from it:

With an inertial dynamometer, the engine is allowed to spin up as quickly as it can, accelerating the drum as quickly as possible. Consequently, the measured results reflect the engine's ability to increase its work output rapidly, which ability is more greatly influenced by the engine's own internal inertia than is a measurement of steady-state work output. This factor is treated less rigorously than it could be: there is generally no attempt to quantify the effect that the engine's reluctance to rapidly increase its output has on the measured results, and the measured results are not carefully distinguished from a measurement of steady-state output such as would be obtained on a brake dynamometer. Be that as it may, an inertial dynamometer is a more realistic test of an engine's ability to accelerate a vehicle, whereas a brake dynamometer is at least as realistic when the question at hand is an engine's ability to climb a steep hill at steady speed. Turbine engines have a very high thrust to weight ratio, but due to the combined moments of inertia of all of the turbine blades, they are slow to speed up and slow to slow down. Jay Leno's turbine-powered motorcycle would fare much better on a brake dynamometer than it would on an inertial dynamometer.

A few points:
1) My questioning wasn't inappropriate, and here's why: there is a difference between measuring the engine's rate of acceleration and its ability to perform constant work. This means that unless the system used to measure the power is clarified (constant or acceleration) there is no right or wrong.

2) Those people that were saying that a gearbox will use more power to be moved when you make more power were wrong. The gearbox uses proportionally increasing power if you are increasing acceleration, while it would not be like that if you were to calculate that at constant speed (and please, we are all aware of fluids, friction and the likes, so to be precise, we would have to factor in several things, I agree, we are talking in general and since viscosity of oil decreases with increasing temperatures while moving it faster requires more work who the heck knows how to factor those 2 in and find out whether you are actually using more or less power than you were before).

3) My latest question, how is the power of the engine measured, blah blah blah,
is very appropriate and here's why:
The engine power is bhp=brake horse power. Referring to the excerpt from the article you linked, the engine crank output is brake horse power, which means that it is measured at constant rate. After the last 2 days findings, the dyno measures power under acceleration and not at a costant rate. So, when someone talks about crank to wheel power loss, is talking about stuff that he has no appropriate data to measure.
To do a comparison and try to understand what is happening you would have to have the same type of data to measure: either both at constant rate, or both under acceleration. Now, I might be stretching my legs here, but I believe that if you measure the power output of the engine at the crank under acceleration, the number you would find would be lower than the one at constant speed, because while accelerating, the engine would use more power to accelerate crank, connecting rods, piston pins, pistons, rings, valves, camshaft (or rotors, seals and what not in a rotary).

4) Yes, you were right in saying that the powertrain losses will increase when you increase the power. But that's a gross generalization. Your drivetrain losses will increase if you incrase the power, when you are accelerating, for all the good reasons discussed previously. Whether or not you'd need more power to move your gearbox at constant speed depends on many factors and, if you do need more, it is certainly not a significant increase compared to baseline, for all the reasons I discussed before.
In all actuality, what should be said is that under maximum acceleration, all the rotating elements attached to the engine will require more power than they did when the engine was making less power.

5) The chassis dyno is a tool that measures ability of accelerating, so, my comparison (it's like going to the strip and factoring out the driver) is very appropriate and here's why:
the only factual thing that the dyno tells (if all has been done properly) is whether or not a car will accelerate faster after something has been changed on it. It will do so, because it will give a measure of what energy is usable to move the car, which means that if there is more energy now, the car will move faster. The strip has air, yes, but in the end, if a driver is capable of doing 2 identical runs (if all else being the same), you'll be able to tell if the car accelerates faster than before something was changed on it.

My point was -I hope I remember this right- that the power loss from crank to wheels doesn't increase proportionally when the power of the engine is increased.
Now, I should have added to my point a few (very important things). I meant to say that the measuraments were done at a constant rate, both at the crank and -obviously- at the wheel. In any case, most of the power losses registered by the dyno are due to the fact that the dyno measures power under acceleration and not at a constant rate.
Now, I will be the first one to say: to hell with all those considerations, if you are making more power at the wheels, you're going to go faster.
But I started this discussion because I was in disbelief in relation to the fact that you could have 370 hp at the wheels and 445 at the crank. Fact is, that's way overinflated and the power calculation should not even be done, for the reasons mentioned above (comparing apples with Rubik's cube).

gio64

PS it's very crazy in here I believe I repeated myself sometimes, I apologize.

SPICcnmGT

Measuring hp at a constant load as in running 50mph and setting the cruise and then measuring the hp is pointless to find the max output of the engine. because if you have a 200hp or a 1000hp engine, the work required to stay at a constant equal. Because of this if the 200hp car is using 50hp to stay at 50mph the 1000hp car with equal weight and resistance forces will also require 50hp to stay at 50mph. So by doing that all you would then is figure out how much hp each car requires at a constant speed.

You are right, of course, that just because you increase the hp at the engine the loss to the crank under a constant load will not increase. But with more power being transfered through the drivetrain, which will cause more work/force/acceleration you are going to lose more hp.

Also if you are trying to keep say a constant speed with the car pushing on say a large truck to keep it at a constant speed with you giving it more and more gas, or a higer hp car wot. vs. a lower hp car at wot. The drivetrain again will lose more hp with a higher hp car because of the extra forces, mostly being increased friction.

Anyway you look at it the higher hp car will lose more through the same drivetrain as a lower hp car.

gio64

iTrader: (1)

Well, that's not quite what a measurement at constant load/speed is.
As a matter of fact, as I stated before, every engine is rated by the factory under constant load, on a brake dyno.
I believe that this value is not insignificant, otherwise nobody would think that just because the engine on an FXX Ferrari made 800 hp at the crank under constant load, there is a good possibility that it will accelerate that car faster than the DOHC in a Neon with 150 hp under constant load.
The example of the cruising does not even apply.
What a brake dyno does to measure the power output of an engine is to apply increasing and measured resistance to the rotation of the engine until it balances the power made by the engine at rev limiter with corresponding resistance (engine power rating is given as XXbhp@XXrpms, constant velocity). That is not like cruising -which, I agree with you- would take the same at whatever identical speed, given that the car is the same, the weight is the same and so forth.
To transfer that on a chassis dyno, you'd have to use a similar system, where while you accelerate at WOT, the dyno drum supplies increasing resitance, until it balances out the power output of the wheels at rev limiter, WOT.

If the measurement of the power output of an engine is done by everybody that way it must be for a good reason. Once again, what do we do when we look around and we see that the specs of the 3rd gen RX? We think: "255 hp with 2750 lbs, must be fast". And that value there, is not taken under acceleration, but under constant load.

Now, I think nobody is arguing that under acceleration a car will have a "powertrain loss" that is higher if the car has higher output, for all the reasons stated before (and if you go back in the thread, you'll see that I never said this was wrong, I only assumed the dyno would be a "brake dyno" therefore the powerloss due to acceleration would not be a factor in the measurement, therefore, the powerloss could not increase linearly).

My "beef" -if you will- was with the fact that somebody would say that you have a 20% powerloss from engine to wheels.
The reason is the following: engine power rating from factory is not accurate. While we assume that an FD makes 255 at the crank, it might make less (or more). Most importantly, however, it does make 255 under constant load and, it is turbo charged (which I'll go back to later).
Now, you buy the car, you put it on a dyno and you turn 218.
So you figure, 255-218=37 hp lost from crank to wheels. 37/255=14.5% approx.
You mod, you test again, you put out 300 hp at the wheels, so you say that your engine makes 343 hp or so.
Meanwhile, this is what can happen: your car had 245 hp instead. That's a rating under constant load, not under acceleration. The engine power rating is done with several things attached, which would rob more power if you were to do the measurement under acceleration; the engine internals themselves will take more power under acceleration. So, just for the sake of the argument (no precise numbers here), let's say that -had you rated your 245 hp engine under acceleration- you actually would have turned 235 hp instead.
What does this mean? Well, 235-218=17. 17/255=6.6%. After modding, 6.6% power loss through the drivetrain equals 20 hp, so your engine is now making 320 hp and not 343 (13 hp is 4% less than what you're making).

I wrote in my previous post that I agree, if you put the car on the dyno and turn more power, you must be going faster. And certainly, measuring the power on an
"inertial" dyno is probably the most useful test you could do, because your mods are aimed at accelerating faster
But that's different from using those values to say you have astronomical power at the crank, with unbelievable power losses through your drivetrain.

That "turbo thing" I was talking before, is this: when the engine power rating is done (at constant load), the turbos are spooled and at full capacity.
When you do the test under acceleration, the turbos are not working as efficiently, which in the case of the RX7 contributes to lower the power value at the crank.
I did not make this up, it is in the article referenced by another member previously.

Lastly, I am quite sure that the majority of performance nuts have done some something to reduce the power loss, like smaller pulleys, lighter flywheel, lighter crankshaft and even brake rotors and wheels.
This means that their original power loss through the drivetrain -even accepting that the original value was 15%- it is now lower, so the original percentage should be reduced.
I am going to look for an article that I stumbled into yesterday night, where the writer was pointing out how high percentages of drivetrain losses are simply absurd, he was making an example with a 200 hp car explaining how the amount of heat generated by the transmission would be impossible to be dissipated through the casing itself, I am going to look for it again.

SPICcnmGT

I agree that under a constant load(brakes appled) that the engine is working harder. If that was not true then boost creep would not be a problem. But because of the harder work the engine puts out more exhaust gases, which intern cause the turbos to spin faster and the cycle goes on. Thats why boost creep is more of a problem in the higher gears.

I will also say that just because at 200hp you are losing 15% that at 500hp you may not be, but it will be very close for a steetable car. Like most everything else it is more of a curve than a straight line on a graph, and come to a point where the drivetrain can't take it and it would just twist into because of the power from the engine, causing the curve to shoot to infinity.

BTW I also agree with everything you just wrote, and now I think we are at the point where you seem to understand whatever you were looking for and I'm probably just saying stuff trying to explain to you and you explain to me, and it kinda seems to be running in circles. Not to mention I'm not the best at presenting stuff, especially on the internet.

gio64

iTrader: (1)

I think you're right.
I was watching football and then NE scored their fourth TD, so I left the game.
I did manage to find the article BTW, so I am going to put the link and an excerpt up (not to try to argue anything with anyone, just for info/reference for whoever finds it useful -and maybe some people already know about this-).

Link: http://www.sdsefi.com/techdyno.htm

Excerpt:
Flywheel vs. Wheel HP

As most people know, there are power losses through the drivetrain so wheel hp is always lower than flywheel hp. Front wheel drive cars with transverse engines tend to be more efficient than most rear drive configurations due to the layout of components. However most publications overestimate these losses considerably.

Most rear drive cars have a 1 to 1, 4th gear which means that the power path goes directly through the mainshaft of the transmission. The only losses here are bearing drag which is less than 0.5% and the viscous drag of the gears running through the oil which is about 1% with hot oil. Indeed, published data indicates a transmission efficiency of 98 to 98.5% for conventional transmissions in 4th gear.

Losses within the driveshaft account for about 0.5% if they are properly aligned, balanced and with fresh U-joints.

Differential losses in the commonly used Hypoid type gearset is in the order of 6 to 10%.

The worst scenario case for a rear drive setup is on the order of 12.5% in 4th gear, not the 20 -25% often published. If 25% was being lost in the drivetrain, the oil would boil in the differential housing in short order and aluminum transmission cases would fatigue and break from the temperatures generated. On a 200 hp engine, something on the order of 37,000 watts would have to be dissipated out of the transmission and differential housings. Obviously, this is not the case.

Transverse, front drive transaxles usually have no direct lockup gears and no 1 to 1 ratio, however, since the torque path is never turned 90 degrees as in the rear drive setup and efficient helical gears are usually employed for the final drive set, losses are more on the order of 6 to 9 percent in the upper ratios.

Tire pressure and wheel alignment can have very significant effects on losses at the rollers. Tire pressures should be set the same between each test. Tire rolling resistance varies inversely with speed, another factor not taken into account by most chassis dynos when applying phantom flywheel hp formulas.


What you probably don't know, is the reason why I started this thread. I was reading one of the "for sale" articles and somebody put a car up for sale, said the car had 440 hp, then it surfaced that it wasn't rear wheel hp, but estimated crank, given that he was saying he maxed out on the stock turbos, which -according to him- give up at 370 rwhp. That's pretty much 20% loss through the drivetrain, as 20% loss would be the equivalent of having 444 hp. So I thought it was a bunch of bull and I started the thread. At this point I have learned a few things (how a dyno works is -I guess- the most important, and that the power loss through drivetrain cannot be as high as stated in several places, including that car thread.
And I think it was a lot of work, but I feel receiving input from many was a very helpful thing, so I once again thank everyone, even those I disagreed with. I agree with you, we're trying to kind of say the same stuff for the most part...

gio64

iTrader: (1)

BTW, using the 12.5% suggested as "worst case scenario for a rear wheel drive car, the FD would put 223.5 hp at the wheels. If I am not mistaking, 218 is common in stock version. If you consider the fact that engine output rating might not be correct and that even if it were its value under acceleration would certainly be less than 255, then it looks like that number works pretty well...

Mahjik

iTrader: (1)

Most engine dyno's run the car with all accessories on them (i.e. belts to turn the waterpump and oil pumps flowing to keep the engine lubricated and cool during the tests). With an engine dyno, you are getting what the engine is "capable" of without the drivetrain (which we have already established will zap some of the power to turn the wheels).

As far as the company "mis-quoting" engine power, sure. Many cars come out and dyno higher or lower than "what is printed on the add". However, that is typically do to mass manufactoring. i.e. while they do try, not every car/engine is built 'exactly' the same. But that has nothing to do with measuring BHP to RWHP. They both work together to demonstrate what a car can do. Having one without the other (in true performance building/tuning) is not good. I would think that if you engine dyno'd at 400 BHP, and you dynamometered at 150 RWHP, you might want to know something isn't as good as it can be between the two.

People throw around percentages for drivetrain loss as it's their 'best guess' from what the manufacturer has published the car can do. Are you going to buy a car, pull the engine, dyno it, then reinstall the engine, slap the car on a dynamometer and then find the true drivetrain loss for your car? Heck no. There are special people on this planet that will, but for most sane people, that's not going to happen. So, people dynometer 'new' cars and then use the manufacturers quote for the BHP. If the BHP is wrong, big frick'n deal. Any time you rely on data from another source, you'll never know if that source is correct unless there are other impartial tests confirming the data. That's just life. Even the 'coasting' tests are just a calculation. It's not a real BHP dyno test like this:

http://kccompacts.com/kcrx7/modules....iewtopic&t=959

So those 'coasting' tests are a 'guestiment' as well.

Don't even start that topic (as lighten flywheels don't increase rwhp). The conversation of sprung and unsprung weight into this converstation would be a nightmare. The major losses through the drivetrain aren't going to be effected enought to really show a benefit with simple bolt-on mods that us 'normal folk' have access to.

Yes, that's why we have transmission oils as thick as we do. If all that heat could be radiated out, we wouldn't be running 75w90 or higher weight oils for the tranny. We'd be running something much less.

Mahjik

iTrader: (1)

There have been stock FD's that dyno'd in the lower 220's. However, just like everything else, dyno calibrations, the difference in dyno manufacturers all together (as well as the people running the dyno) can all give different results on the same car.

Mahjik

iTrader: (1)

That's why people always ask "what are you making to the wheels", as BHP is an estimate unless you really did engine dyno it (and that is most likely not going to happen unless it's a race car).

gio64

iTrader: (1)

"Most engine dyno's run the car with all accessories on them (i.e. belts to turn the waterpump and oil pumps flowing to keep the engine lubricated and cool during the tests). With an engine dyno, you are getting what the engine is "capable" of without the drivetrain (which we have already established will zap some of the power to turn the wheels)."

I am not going to find the link, but there is a clear explanation in several places out there about what engine bhp is and what's attached to the engine while testing. Before early '70s it was net, before you'd attach anything to it; after that time most accessories are factored in. It is the standard SAE way of measuring, not what "most engine dynos" do and the reason why it is now done this way is because practically every engine has that stuff attached to it regardless, so giving net hp is quite a volatile value, considering that the engine will never throw that kind of power to the tranny.
What does this ad to the discussion?


"As far as the company "mis-quoting" engine power, sure. Many cars come out and dyno higher or lower than "what is printed on the add". However, that is typically do to mass manufactoring. i.e. while they do try, not every car/engine is built 'exactly' the same."

Power output is not only influenced by production, that being a relatively small factor, but also by advertising and "final production changes". If you access SportCompactCar magazine, I believe they have quite a few articles available online and they might have the one that deals with the "whys" of "advertised power" against actual power (marketing and miscommunication being the 2 most important factors).
Again, doesn't this actually confirm what I wrote (power ratings aren't reliable)?


"But that has nothing to do with measuring BHP to RWHP."

??? What does this mean? Like you say later, the only way to know for sure is by putting the engine on a brake dyno, right? Like you say, most people don't, right? So, the way they estimate driveline loss is by using what is written on the papers that the car comes with, right? So, if the paper is wrong, their entire estimation is out of wack, right?
Plus, BHP=steady output, RWHP=accelerating output, that's like comparing two different things.
One of the points I was making is that your power at the crank under acceleration is lower than your power at the crank under constant velocity. So, not factoring that in when trying to calculate drivetrain loss leads to overestimating the drivetrain loss, which is what I was arguing in the first place.


"They both work together to demonstrate what a car can do. Having one without the other (in true performance building/tuning) is not good. I would think that if you engine dyno'd at 400 BHP, and you dynamometered at 150 RWHP, you might want to know something isn't as good as it can be between the two."

They don't work together to demonstrate what a car can do.
When you get the literature regarding a car, they usually tell you the power (BHP@crank) and they tell you 0-60, 400 m-1/4 mile, 0-1000m, 0-200 Kmh, blah blah blah. That's what tells you what the car can do.
You're right about the tuning, however, like I said a few times, relying on some data could be misleading at times. In fact, if you're going to do tuning seriously, you should be pretty precise and meticulous regarding to what numbers you start with and what you end up with and since usually the discrepancies between advertised power and rear wheel power are not as big as in your example, a misquote from the factory can be very influential in your ability of understanding exactly what's happening, especially considering that dynos are very sensitive to a bunch of little-not so little factors related to all those parts that go from the power plant to the dyno roller.


"People throw around percentages for drivetrain loss as it's their 'best guess' from what the manufacturer has published the car can do. Are you going to buy a car, pull the engine, dyno it, then reinstall the engine, slap the car on a dynamometer and then find the true drivetrain loss for your car? Heck no. There are special people on this planet that will, but for most sane people, that's not going to happen. So, people dynometer 'new' cars and then use the manufacturers quote for the BHP. If the BHP is wrong, big frick'n deal. Any time you rely on data from another source, you'll never know if that source is correct unless there are other impartial tests confirming the data. That's just life. "

Well, then why are you arguing? If it is their best guess, then this is my best guess, so we're all right and that's it.
After looking around hard for a few days, it seems that there are people that can tell you with much better precision than your guess, what those losses are. I posted a link and an excerpt from a page, is there anything you'd like to debunk about what is written in that page?
It looks like the worst case scenario for RWD is 12.5%. There's plenty of explanation there, it's not just oil viscosity (and I'm sure that dissipating 37000 watts from a tranny would boil any oil, even if it is as thick as butter; try to put your hand on a 100 watt bulb after it's been on for 20 seconds, then imagine if there were 370 of those under your hand instead.
Once again, the discussion is: don't come around and throw a 20% drivetrain loss, so you can say that your car has 440 hp when you either know you don't have that much or you don't know what you're talking about. Don't make claims based on thin air.
BTW, if the rating from the factory is wrong, IT IS A BIG FRIKIN' DEAL!
That throws all your analysis of your power losses out of the window

"Even the 'coasting' tests are just a calculation. "

And that's fine, they are all calculations, that take precise parameters in consideration. That's why, I said in the first place, using engine BHP to compare the dyno to, is nonsense. You are the one that gave me a link of a page where it explains how dynos do a certain type of calculation instead of another and so on...


"Don't even start that topic (as lighten flywheels don't increase rwhp). The conversation of sprung and unsprung weight into this converstation would be a nightmare. The major losses through the drivetrain aren't going to be effected enought to really show a benefit with simple bolt-on mods that us 'normal folk' have access to."

I am not sure I wrote that flywheels increase horse power, but if I did, please show me where I did. I also don't understand the unsprung weight, so I have to plea ignorance on that (if we're talking about power making).
This is what I know about lighter flywheel and other lighter rotating components: the work necessary to spin them up is less than the work necessary to spin the stock counterpart. They don't let the engine make more power, but they allow the engine to deliver a higher percentage of the power the engine makes. We've been on angular acceleration and the related work necessary for days, now you would like to tell me that under acceleration a lighter flywheel would not show its benefits? And if it does, guess where you're going to see it: when you put the car on the dyno, since, as I have finally learned, the dyno measures the energy available to move the car forward.
When you install lighter rotating components, you see an increase of the power at the wheels, because less work goes into spinning them under acceleration. The engine was making 200hp? Still is, but what was lost through moving some pulleys, is now going down the driveline, so you see the difference.
The flywheel per se is a different matter. As power at the crank is measured under constant speed, the difference between a light and a heavy flywheel is not particularly high. But if you were to put the engine on a dyno "chassis style" then you would see a difference in the output under acceleration, as less work is necessary to spin up a lighter flywheel.
For this, once again, check SportCompactCar, as they had an incredibly good article in what difference a lighter flywheel makes and how this difference is much higher in low gears as opposed to high gears, since the angular acceleration of the engine tends to diminish in higher gears as opposed to low gears (the engine will reach redline in, say, 2 seconds in first, while it will take it 8 seconds in 5th).


"Yes, that's why we have transmission oils as thick as we do. If all that heat could be radiated out, we wouldn't be running 75w90 or higher weight oils for the tranny. We'd be running something much less."

For this, I invite you to go back to the page I linked, please do, it is a very interesting article (and I didn't write it). Keep in mind, that energy doesn't disappear. If your oil can take it, it'll deliver it to something else (housing). If there is an impossible amount of energy to dissipate, something will simply break. The point of that excerpt is not that you need a thick oil, but that you simply cannot dissipate all that energy even when the engine is "only" making 200 hp, so that rate of power loss (20/25%) would simply be mindblowing for anything that has more power than that.


"There have been stock FD's that dyno'd in the lower 220's. However, just like everything else, dyno calibrations, the difference in dyno manufacturers all together (as well as the people running the dyno) can all give different results on the same car."

OK, I believe I wrote down: 218 is common. That means (once again, if it's right) that it is the average output, which means it is a pretty reliable value. I agree with the variables with the dyno, but 3 things need to be noted:
1) An average value tends to factor the variables you mention out.
2) This confirms that drivetrain percentage loss under acceleration is low, closer to 10 than 20%, so, once again, I was right in saying that those crazy 20/25% are simply stupid
3) If the dyno is unreliable, both 1) and 2) cancel out together with any kind of estimate on driveline loss under acceleration, power increase measured on the dyno and whatever else you want to put in here.


"That's why people always ask "what are you making to the wheels", as BHP is an estimate unless you really did engine dyno it (and that is most likely not going to happen unless it's a race car)."

...That's great. And what's the point of this statement?
I am aware of this, I simply made an observation in regards to crank to wheel power losses.
You don't have to be interested or find it important. However, whether it is important or just an evening silly pass time, I made a statement for the sake of learning something and confirming/debunking some things I thought/knew/thought I knew.


This is what transpired:

Engine power output is measured in BHP, steady rate.
Power is calculated at the crank, with most components attached.
Chassis dynos measure power at the wheels under acceleration.
The power loss through drivetrain is somewhat proportional if calculated under acceleration, while it is somewhat constant if calculated under constant load.
20/25% power loss through drivetrain is "impossible"
Realistic worst case scenario for RWD power loss through drivetrain under acceleration is 12.5%

I started saying that I couldn't understand why the crank to wheel power loss was proportional.
I did not know at that time that the dyno measures the power under acceleration.
Nobody ever posted that, I managed to find out through my own research.
At that point, I posted it and I said that it was clear to me.
I also reiterated that under constant velocity the losses wouldn't be proportional.
I was then sent by you to a site that pretty much reiterates what I just wrote on the line above. This site also explains how measuring on dyno or on engine dyno give different aspects of an engine's performance.
I kept looking around and found out the page I linked earlier, where there is an explanation of why drivetrain power losses cannot be as high as most people like to think they are.

I find a long list of responses that really don't change any of the matters that were peculiar to this discussion.
I replied as I saw fit, but I still wonder why those replies came about.

Mahjik

iTrader: (1)

Yep.

If the load is constant, so is the friction. It doesn't go away once you stop accelerating or it wouldn't take any work to keep it at that constant load. That's common physics. If you increase the load (i.e. accelerating) then the friction increases which maintains the proportion.

False, check AWD cars. For RWD, 15% is commonly used as it's a "worst case guestiment".

Pick a number. That number is not any more right or wrong than most any other numbers commonly used unless you yourself have done the tests on the engine/car.

I posted many links of formulas that make up the calculations. Acceleration was always a factor in the calculations.

Well, like I said, that's basic physics; same load, same friction. Friction is directly proportion to the force applied. That is backed by physics. If you don't change the force, then the friction doesn't change which is why it would take the same power for a 200hp car and a 1000HP car to keep it at 3000 RPM in the same gear.

Dyno's are really measuring the rest of the car. Basically a dyno is showing how efficient or non-efficient everything is "after" the engine. If you engineered a brand new car and just did a dyno, that wouldn't give you the complete picture. A standard dyno combined with an engine dyno would.




Your whole point was that someone used a mythical number to calculate BHP and you don't believe that was the correct number to use. Honestly, unless you plan to pull his motor and engine dyno it, you have no leg to stand on as to what his numbers really are as your numbers would be just a guess as his are.

gio64

iTrader: (1)

"If the load is constant, so is the friction. It doesn't go away once you stop accelerating or it wouldn't take any work to keep it at that constant load. That's common physics. If you increase the load (i.e. accelerating) then the friction increases which maintains the proportion."

Go back to link I posted, follow the attached links and you'll see how this relationship is not proportional as you make it sound.
In any case, I say "A", you quote me and say "A". What is the point?

What my thesis was in the beginning, is this:
When you put an engine on a brake dyno, you force the engine to produce its highest output at constant speed, which is what the "power spec" from factory is.
What this measurement does, is to quantify the engine output eliminating the losses due to acceleration. The engine is still UNDER FULL LOAD, but it is not accelerating.
What I said was, if you are steady at 6000 rpm, WOT and you are on a brake dyno, the amount of work to keep the tranny at that speed does not require increased power, because the work required to keep (not accelerate) the tranny at a certain speed doesn't change when the power of the engine increases. This is, naturally factoring out some increase in friction, change in oil viscosity and what not. Which doesn't mean that there is ABSOLUTELY NO DIFFERENCE. It means that although it does increase as an absolute value, it does not increase linearly as a percentage, that's all.
The example of the 2 engines with different power keeping a tranny at 3000 rpm doesn't relate to this and the reason is that if you keep any engine at a certain rpm level with the throttle, you're putting out of that engine only the power you need to do that specific work, so the overall output of the engine is factored out. If you are using a brake dyno, you are forcing the engine to put out all the power it can put out (at 3000 or any other speed), therefore there are some increased loads on the tranny, just not steady go percentage, that's all.


"False, check AWD cars. For RWD, 15% is commonly used as it's a "worst case guestiment"."

Now, this is borderline stupid. Obviously a 4WD car has higher driveline loss, it has twice as many halfshafts and three times as many differentials (and some other spinning components)!! What kind of an objection is this? It's like saying that if you strap the FD engine to the tranny of a transport truck, there will be more heat generated by the tranny then...
Now, if you have 3 differentials, you have three times as much oil and casing surface to dissipate the heat! So, I quote a number for a RWD, you say it is false, because of the number of an AWD is higher... Sure, that makes sense.
As far as the 15% goes, note it is only 2.5% higher than 12.5, which on a 400 hp car would make up for 10 hp difference, which is not all that relevant.
In any case, your 15% is "commonly used" (?) while the page I linked gives quite a good explanation of the why and how. I don't see you disputing the facts written on it, you are simply saying that "commonly used" is 15%. Could you show me where this magical 15% is calculated with some precision and perhaps show me where the page I linked is wrong? Wouldn't that account for 415 instead of 440 if you are making 370 at the wheels (which means the power claim is still bull, like I said)?


"Pick a number. That number is not any more right or wrong than most any other numbers commonly used unless you yourself have done the tests on the engine/car."

????
First of all, I believe we all agree on the fact that an actual calculation is so complex that in practicality it is almost impossible.
Having said that, one thing is to say that xx% is more or less what the actual value is, another is to say "pick a number"?
I found some pretty clear and factual information. You obviously don't like it, so you continue to argue about stuff you cannot argue about. If you can demonstrate that the number figures in this page are wrong, either with your own scientific ability or with someone else's, that's fine, I'll look it up and I'll discuss the two arguments with you, my pleasure. If you cannot, why are you arguing? 1000 years ago, most people (if not all) thought Earth was flat, like a coin. I guess they were wrong, because they actually did not know exactly what they were talking about, they were just going with what their experience told them.



"I posted many links of formulas that make up the calculations. Acceleration was always a factor in the calculations."

True, no argument about that. But if I say that powertrain loss cannot be increasing proportionally because the work needed to make the components move at a given speed doesn't change and you tell me that it does under acceleration, how am I to understand that you are saying that because you know/assume that the dyno does its calculation under acceleration?
I don't think you are so patronizing towards me to assume that I don't know my physics (at least to a certain extent, that is) and therefore you are assuming that I am not aware of the fact that accelerating faster requires more work, right? And still, this doesn't give me any info on why you're using acceleration formulas (especially considering that I was already under the impression that the engine dyno uses steady rate output, bhp).
Plus, many times earlier, I continued to object that what you all were saying was true, but under acceleration, not under constant load. WHY DIDN'T ANYONE SAY: "look, you might be right under constant load, but the dyno measures the power under acceleration, so it is what we say that applies, and not what you say".
No, everyone rumbled around and around, trying to dispute what I was saying pulling little details here and there when what should have been disputed was the big picture.
What I think is that you all did not have a clear understanding of the overall picture and continued to repeat what you heard from somebody else, without having fully understood it and without having ever wondered if there could have been something else around the entire story.
Honestly, if somebody would tell me the same question today, I would say: "look engine power is measured at constant rate, dyno goes under acceleration." And so on so forth. But nobody said that to me. Was I that unclear in explaining my point?


"Well, like I said, that's basic physics; same load, same friction. Friction is directly proportion to the force applied. That is backed by physics. If you don't change the force, then the friction doesn't change which is why it would take the same power for a 200hp car and a 1000HP car to keep it at 3000 RPM in the same gear. "

This is completely out of wack.
First, the power losses through the driveline are not only due to friction, but also due to rotational inertia.
Friction is the result of two counteracting forces. Rubbing hands, like we said. If you push harder, more friction, more heat. But when you push harder, you are changing both the forces that contribute to friction: right hand push and left hand push. When you accelerate the engine faster, the weight of the car and its components, doesn't change, so while the left hand pushes harder (the engine) the right hand doesn't resist back with more force (the car). What does change considerably however, is the amount of work needed to spin all rotating components faster, the rotational inertia.
On the other hand, what changes the friction considerably is the measurement under constant load. In fact, this measurement is done supplying the engine (or the rear wheels) with increasing resistance, until a balance is reached between engine/wheel output and dyno resitance, so that engine/wheels are not accelerating/decelerating while at WOT @ max rpm. This would be like saying that the car has become heavier, like in my previous example. So, in this type of test, the power loss through friction would increase while the engine power output increases, for the reason I just explained.
The last part of this statement shows that you don't understand what a measurement at constant load is.
If it were done like you say, then every engine output would be equal to the amount of work needed to keep all of its components spinning at max rpm, while the power at the wheels would be equal to the amount of power needed to spin all the car components at max rpm.
What that would mean is that a manufacturer would say to you that the engine output was 0 hp, the engine would not be able to move anything but itself.
Keep in mind, before I read about senseless "point-outs" that my explanation above doesn't consider several things, it is a generalization, blah, blah blah. If you want to dispute it, then you show me that there is an incredible increase in friction between gears when increasing power and measuring under acceleration, or that there is a transcurable increase in friction when increasing power and measuring under constant load. Everything else you might think about pointing out, I will say I agree with, so I don't have to specify things. Please look at the big picture.
Once again, please go to the page I linked, read; go to the included links, read; if you know (and not feel) that what is written there is wrong, then please explain it to me, or send me to a page that clearly demonstrates that those calcualtions in the page I linked are wrong.
Please note how among those pages you'll see that the authors also mention how increases are actually not proportional even under acceleration, I'll leave it up to you to find it, but it is in one of those pages.


"Dyno's are really measuring the rest of the car. Basically a dyno is showing how efficient or non-efficient everything is "after" the engine. If you engineered a brand new car and just did a dyno, that wouldn't give you the complete picture. A standard dyno combined with an engine dyno would. "

Agreed. Again, somewhat.
I don't argue with the fact that if I can control all variables in an acceptable manner, I baseline a car and continue to check it on the same dyno while modding, I'll have a very useful tool to understand whether or not I am going faster. And this is done "scientifically", since if you were to go to a track, uncontrollable factors would affect your results and would not tell you the truth, unless you are making gobs of power more.
What the dyno cannot allow you to say, is that you've now got xx hp at the crank.
Measurements are done under different circumstances, so it is aleatory to say I am making this and that.



"Your whole point was that someone used a mythical number to calculate BHP and you don't believe that was the correct number to use. Honestly, unless you plan to pull his motor and engine dyno it, you have no leg to stand on as to what his numbers really are as your numbers would be just a guess as his are."

This is not true. Once again, just because it is a complicated operation, doesn't mean it is impossible, or that there is no way to tell approximately.
First, that car has a stock tranny. so the way the tranny will take power out is pretty standard; whether I want to agree with you or you want to agree with me, fact is, a value exists to approximate that engine output.
I do have not one leg to stand on, but many, as per links attached.
Second, like I said before, the power consumption through the driveline changes when you install lighter components. This means that -even if we go with a commonly agreed percentage number- there is no way you lose 20% because an FD stock doesn't and we all agreed that even if we accepted power loss as a percentage, it is a fixed percentage; a 255 FD with 20% power loss, means a stock FD with 201 rwhp. If you can show me a large majority of stock FD runs around that figure, then you have something to argue about; if not, the claim is simply outrageous and it would be so even if I used your brain instead of mine to do the calculation, you know it; I have no ground? What does that mean? Is it that you can claim whatever power and everyone has to agree with it? You said, drivetrain loss is a percentage. You said it is common to use 15%. You said stock FDs score low 220s, which is less than 13%, and that is accepting that Mazda was right on with the claimed power and that for those cars that they screwed up with, the actual power was higher than claimed.
This guy says 20% and you say, well, there is no leg you can stand on.
Does that mean you don't believe what you say?

http://www.sdsefi.com/techdyno.htm
http://www.pumaracing.co.uk/power3.htm
http://www.pumaracing.co.uk/coastdwn.htm
Read on, debunk with fact/calculation, averages (not single strange examples),
then I'll agree with you. Note the part that says how even under acceleration, the percentage loss doesn't stay the same, but actually drops.

Mahjik

iTrader: (1)

Your link says exactly what I said:


And here:

Physics wise, it is proprotional (assuming no large changes in the drivetrain while the test is happening). What you are confusing is "how much friction". i.e. as they mentioned things like tire inflation have given different results. That is between different runs, not the same run. That's different. Your variables have changed on the different runs which gives a different proportion for the frictional component.

That has nothing to do with the frictional aspect. That just shows the test conditions were not the same which is what I mentioned earlier about different dynos yielding different results.

My objection was to your comment:

The statement as typed, is not correct. AWD cars typically see much more than 20%. So, your statement was false.


Yes, that can happens as the car squats which changes it's contact on the rollers, which can/will affect the total amount friction from the engine to the ground. However, there are many things which you can't measure which are going on. This is what I referred to above as "changing variables". The equations are set, they are real. It's the variables that change. However, the equations still have friction directly proportional to force.

http://hyperphysics.phy-astr.gsu.edu/hbase/frict.html

gio64

iTrader: (1)

Before I go any further, let me say this:
I read your post and I don't see anything that debunks the statements regarding the "worst case scenario 12.5% RWD power loss".
This means that you either think it's right, or you don't know how to demonstrate that what you believe (for whatever reason) is right. All you posted is arguing or trying to make senseless points.
Let's continue with the nonsense then, since at this point there is no other word to define this part of the discussion

"Your link says exactly what I said:

Quote:
So is there any way of really measuring the true transmission loss of a car? Yes - only one - by measuring the flywheel power on an accurate engine dyno, the wheel power on an accurate chassis dyno and taking one away from the other. There is no way on God's green earth of finding out the true transmission loss just by measuring the power at the wheels."

Note as the it says "really" and then "true"?
So, unless you are simply talking for the sake of talking (or you don't understand what you read), you see that this statement says it is an exact number that cannot be calculated, don't you?
So, (and I have asked you several times already) what are you trying to say with this quote?
Did I say the 100% accurate calculation is possible? No, so you posted something to rebuttal what I said, which is however, the same thing you said, so there was nothing to argue about approximation. If you find a place where I stated (or one of the pages I linked states) that precise calculations are possible, then you're right in posting this quote with rebuttal; if not, I hope you'll apologize for the futility of your statement, which cannot even be called an argument, since I don't disagree.
Look at this again:
Does this excerpt you are using say that 12.5% worst case scenario for a RWD is wrong? No
Does saying 12.5% for RWD worst case scenario means that the mentioned percentage is the exact number? No.
Does this excerpt mean that 12.5% worst case scenario for the power loss is too little? No.
Does this excerpt mean that since exact calculation cannot be done, any number will do? Because you all "attacked" me when I said that losses weren't a percentage, remember? Does this mean that I can say that since the exact value can't be established?
Did I ever say that the percentage loss can be easily and exactly calculated?
NO!!!!!!
SO, ONCE AGAIN, WHAT IS THE POINT OF POSTING SOMETHING WE BOTH BELIEVE TO BE RIGHT (in the attempt of antagonize me)?
Do you feel that this proves any of your other points? Because I'll repeat it here if you didn't understand it before, we are talking about approximate values. But the fact that they are approximate values, doesn't mean that you can use any value. Pi is approximated to 3.14. Does that mean that when calculating a circumference I can use 400 for a Pi value instead? No. Or, hold on! I must calculate the percentage difference between 12.5 and 20, then factor it into Pi, and use that or you will argue that my example is wrong based on the number difference? Or should I assume that you are intelligent enough to understand my paradox beyond the numbers involved, so I can leave it like that? Let's risk it and see what happens.
I disagree only with the approximate loss percentage number, so why are you putting this up?
Here's what you did:
I wrote "go read what it says on the links" in a post that was sustaining that the power loss is 12.5% worst case scenario for RWD. The section of the post you quoted says that the exact value cannot be calculated simply interpolating the data from engine dyno and chassis dyno. This doesn't mean that 12.5 worst case scenario for RWD is wrong; the only thing that you could say based on your quote is that it the value you will find will not be a precise value; but since the statement itself doesn't imply precision, (it is a limit, which means it could be 10, 11, 12, but not 15), so there is no point in putting this quote up. The person that wrote it has done calculations based on the realm of possibility and has found out that the percentage loss cannot exceed 12.5. Stating that precise calculation cannot be made doesn't mean that any number could be appropriate, but this requires some brain to understand, it is not a matter of arguing, so let's see if you have it.


"And here:

Quote:
Originally Posted by gio64
Go back to link I posted, follow the attached links and you'll see how this relationship is not proportional as you make it sound.
In any case, I say "A", you quote me and say "A". What is the point?


Physics wise, it is proprotional (assuming no large changes in the drivetrain while the test is happening). What you are confusing is "how much friction". i.e. as they mentioned things like tire inflation have given different results. That is between different runs, not the same run. That's different. Your variables have changed on the different runs which gives a different proportion for the frictional component.

That has nothing to do with the frictional aspect. That just shows the test conditions were not the same which is what I mentioned earlier about different dynos yielding different results."

First off, I said I had agreed with the fact that under acceleration there is a proportional loss; I added that based on what is written in one of the pages I linked, the loss is not exactly proportional, so the substance is, I agree, but it is not exactly a percentage value; now, if you read the question, I am trying to get an explanation from you regarding the reason why you post things to rebuttal me on something I don't disagree with you on. But as you and anyone else can see, you quote a question and instead of posting the answer to that question, you talk to me about other stuff.
As far as your friction losses, I am not sure you read the links I posted, or you wouldn't simply be talking about tire pressures and grip (and relative friction).
Also, I did not mention differences between runs (in fact, the only time I referred to something in this respect, I was talking about what dynos are useful for and I opened saying "all variable being kept the same", which is actually the opposite of talking about differences of controllable variables).
As far as friction is concerned, I didn't even think about the tires, so I am not confusing anything. I also was not referring to the variability of conditions on the pages I linked, but I was referring to the part that deals with increase friction within the tranny and how although some values increase with power increase, thus making a percentage value acceptable as power loss, there are others (seals in particular, if I remember correctly) that do not have a proportionally increasing amount of power consumption, so they don't justify the use of a percentage power loss.
-Actually, I went back and found it, here it is:

Now it is true that not every component in a transmission system absorbs a fixed % of the input power. Some components like oil seals and non driven meshed gears (as in a normal car multi speed gearbox) have frictional losses which are not affected by the input torque. These losses do increase with speed of course but at a given rpm can be taken to remain constant even if the engine is tuned to give more power. We'll look at real world transmission loss percentages later. Finally, the biggest source of loss in the entire transmission system of a car is in the tyres - they account for half or more of the total losses between the flywheel and the rollers. Each set of driven gears, i.e. the final drive gear or the particular gearbox ratio that you happen to be testing the car in, only absorbs about 1% to 2% of the engine's power.

The frictional losses I was mentioning were those inside of the tranny and all the bearings, which I was saying I believe will increase under constant load.
I was saying that if you provide increased resistance to rotation (as the brake dyno does), the friction will increase on the gears, and, yes on the tires as well although I was not referring to them in my post.
So, physics wise, it is not proportional, as this excerpt shows, which is what I was referring to and, if I was unclear on this, I apologize, I understand there could have been some confusion since I should have posted this in the first place. As you can see, there are some components that have an increasing power consumption, which means that the percentage figure is appropriate, while there are others that don't, so for those the percentage figure doesn't apply. Since we have all been talking about overall losses, this excerpt actually throws away even the concept of the power loss being simply a fixed percentage (as an overall value).


"Quote:
Originally Posted by gio64
"False, check AWD cars. For RWD, 15% is commonly used as it's a "worst case guestiment"."

Now, this is borderline stupid. Obviously a 4WD car has higher driveline loss, it has twice as many halfshafts and three times as many differentials (and some other spinning components)!! What kind of an objection is this?


My objection was to your comment:

Quote:
Originally Posted by gio64
20/25% power loss through drivetrain is "impossible"


The statement as typed, is not correct. AWD cars typically see much more than 20%. So, your statement was false."

Well if this is the game you want to play, you're welcome. My statement is not false. You see, my statement is false if you talk about an AWD car, correct, but it is also false for an airplane or a tricicle or anything else motorized, which you didn't mention, BTW. But, if my statement were false, then it would be false for anything, while this is also false. My statement is true if referred to a RWD car (at least to the best of our knowledge and for the sake of this argument, anyway), which means that you should have said that the statement was either too general or not specific or perhaps unclear, but not false. False is something that is not real period (eg, plants are animals), while this is not always false.
Now, beside the little idiotic game you're playing, there is one consideration to be made: this post is about FDs, the arguing was in relation to power claims on an FD and the line you quoted was a little piece of a post that referred to a page I invited to go and read, where the same statement is present and is specified as referred to a RWD car.
Now, when I went to school, my teacher taught me that when you write, you need to "calibrate" your writing to the person/people you're writing to. This means you need to use an appropriate language, while you must also make the appropriate level of assumption when talking to somebody or else you would have to explain the history and the "how to" of each and every thing you mention (which includes the meaning of each word???). When you talk to an adolescent, you need to explain him/her how a piston engine works; if you are explaining a problem to your mechanic, you assume the mechanic knows how the engine works and move on to the problem.
I wrote that without specifying any further because I made the assumption that my audience was educated enough and intelligent enough to understand what it referred to, but I see that I was wrong, not with the percentage loss for RWD, but with the assumptions regarding the reader.
And I could go on asking you why would you even think that the statement referred to anything else but a RWD, but I'll just leave it like that. This is exactly what I meant when I was saying that it is stupid arguing.
Now that I have specified the (I guess not so) obvious to the less capable, the statement is still there waiting to be proven wrong. But as I said, you cannot do that, so you play little games. If that's what you want to do, that's fine with me.


"Quote:
Originally Posted by gio64
Note the part that says how even under acceleration, the percentage loss doesn't stay the same, but actually drops.


Yes, that can happens as the car squats which changes it's contact on the rollers, which can/will affect the total amount friction from the engine to the ground. However, there are many things which you can't measure which are going on. This is what I referred to above as "changing variables". The equations are set, they are real. It's the variables that change. However, the equations still have friction directly proportional to force. "

...OK, let me get this right. I say under acceleration, the percentage of power loss doesn't stay, but it drops. I am still referring to gears/bearings issues, if it was not clear, I apologize. If I understand what you're saying correctly, you suggest that the percentage of power loss is reduced by the fact that the car squats under acceleration.
Let me say this:
When a car "squats", the force that makes it squat is the torque of the engine that gets transferred to the springs through the increase in acceleration (please let's spare the details, I am aware).
What that does, is reduce the amount of power available to spin the rear wheels. This should actually increase the power loss, not reduce it!
Ferrari, in the Enzo has developed an electronic system that stiffens the rear shocks to prevent "squatting" (or reducing it) when the gears are upshifted (which is well known to be causing jerking/squatting). The system is supposed to basically reduce the amount of downtravel of the rearend, reducing the powerloss due to production of vertical movement of the rearend.
Second, the car's squatting transfers weight towards the back, which in turn, increases the pressure that the tires exert on to the rollers, hence increasing friction and power loss!
So, the way I understand this last "objection", you are totally off. Now, I might have misunderstood what you said, or I might have not understood what you were referring to on my post, so in that case, I apologize once again if that's not what you intend to say.
BTW, since you seem to not get the "implied and obvious", I will specify that the circumstances I mentioned above are only true for a RWD. In case of a FWD, the squatting happens anyway, but since the powered wheels are in the front, the effect of acceleration actually lightens the front of the car, thus reducing the pressure that the front wheels (hypothetically sitting on the roller) exert on the roller, while the loss through compressing springs remains unchanged. If you thought I was talking about an aiplane, no, I wasn't, I am not even going to bother with imagining how that would work. I'll leave the AWD to you, see what you can come up with.

Since this is (yet another) long post, I am going to say that I won't bother with responding to senseless arguing, since most of the times it is pointless.
I however will repeat what has transpired so far and what I will respond to and I'd like any input on:
(for those of you out there that need to be reminded every second word, all of this refers to RWD cars, it might apply to other cars in total or in part, all the times, some times or never, I don't know nor I care, it is not the purpose of this discussion.

Every engine power output is measured in BHP, steady rate (at the factory).

Power is calculated at the crank, with most components attached (at the factory).

Chassis dynos measure power at the wheels under acceleration (although this is true for all the data we see posted here, apparently there are brake chassis dynos, but they're not so far part of this discussion).

The power loss through drivetrain is somewhat proportional if calculated under acceleration, while it is somewhat constant if calculated under constant load (somewhat is indeed an awful term when we talk about engineering, but the level of complexity of calculations and/or the practical difficulty of generating exact experiments make its use necessary).

20/25% power loss through drivetrain is "impossible" (this refers, for those that don't have enough brain to understand the implied and obvious, that we're talking about drivetrain losses at the wheels on a RWD car; we are not considering tractors, firetrucks, cranes, electrical cars, airplanes, locomotors or whatever else moves, has an engine/motor and you can think of that I didn't mention)

Realistic worst case scenario for RWD power loss through drivetrain under acceleration is 12.5% (this is a realistic limit; it doesn't mean it is exact, it doesn't mean that every RWD car is going to lose that, they might lose less, but when you approach that value you are getting close to unrealistic figures, see links in previous posts on this thread; claiming otherwise would be fine, even great, if somebody can disprove what's written in the pages previously linked with sound science, of their own or "borrowed" from other pages).

I started saying that I couldn't understand why the crank to wheel power loss was proportional (because I was making the wrong assumptions).

I did not know at that time that the dyno measures the power under acceleration(this is valid for all the dynos I've heard of on this forum, for your average dyno, that is; apparently, there are chassis brake dynos, however nobody has come back from doing runs on those, so for the sake of this discussion, we'll always assume that we're talking about a regular chassis dyno).

Nobody ever posted that (the way a dyno works, that is), I managed to find out through my own research.

At that point, I posted it and I said that it was clear to me (and I thanked everyone, excusing myself for not being aware of the dyno's workings).

I also reiterated that under constant velocity the losses wouldn't be proportional.

I was then sent by you (Mahjik, that is) to a site that pretty much reiterates what I just wrote on the line above. This site also explains how measuring on dyno or on engine dyno give different aspects of an engine's performance.

I kept looking around and found out the pages I linked earlier, where there is an explanation of why drivetrain power losses cannot be as high as most people like to think they are (and somebody around here doesn't like that, but cannot prove it wrong, so they rebuttal my arguments phrase by phrase, to pick on the single statement instead of grasping the overall concept).

Mahjik

iTrader: (1)

You are obsessed aren't you?

First, you said:

Now, if you meant RWD cars only, you should have put that. I don't know you. I don't know if you are implying or that's what you believe. Since you didn't know what a dyno measured in the first place, I'm definitely not going to assume anything else with your writings.

So, as I said before; the statement as typed, is not correct.

If you agree there are AWD cars with much more loss, just say "OK, but I meant RWD cars only for that statment"; and then call it a day.

Yes, that is correct. That wasn't an example to demonstrate friction being less, but more of the variables of the tests which can change as the test is occurring. That was my mistake for not prefacing the statement with that.


However, the object was to show that you cannot predict all these changes with garage phyics. Friction is directly proportial to force. Physics tells us this (or you had a bad teacher). However, unlike a classroom lab; we have more than one or two varibles to deal with when it comes to automobile performance. This is the exact reason people pick percentages to use (which you agreed with)

As far as cars not being able to deal with 20% loss; there are quite a few cars (RWD ones) that are either there or close. Here's a 350Z driver who just estimated at 17%:

http://www.350zmotoring.com/forums/s...4&page=1&pp=15

and

http://300mclub.100megs42.com/forums...54ba5611#22865

and there are more around the web. I don't feel like linking them as you won't want to believe them.

I appreciate your "searching". You did find a lot of good stuff. If you found what you want to believe, good for you and good luck. But, I wouldn't put too much faith in "internet physics/mechanics".

gio64

iTrader: (1)

"You are obsessed aren't you?

First, you said:

Quote:
Originally Posted by gio64
20/25% power loss through drivetrain is "impossible"


Now, if you meant RWD cars only, you should have put that. I don't know you. I don't know if you are implying or that's what you believe. Since you didn't know what a dyno measured in the first place, I'm definitely not going to assume anything else with your writings.

So, as I said before; the statement as typed, is not correct.

If you agree there are AWD cars with much more loss, just say "OK, but I meant RWD cars only for that statment"; and then call it a day."


...Sure, I am obsessed. I wonder why you were able to grasp the fact that I was referring to cars instead of motorcycles (the statement doesn't mention the type of vehicle, right? You say you're not going to assume anything -because you don't know me, as if that has anything to do with what the thread is about or what my linked article talkes about-, but you assumed I was talking about cars, so...?), but you couldn't quite extend the assumption to RWD cars, which is the only thing we talked about from the first post in the thread; perfectly understandable, yes I am the one obsessed.
But anyway, I don't care because it doesn't change the substance of the statement for what we're talking about, which is RWD cars.
I won't go on with this nonsense, so I will say only this: posting a dyno means nothing as far as proving that the power loss is 17%.
First of all, you are the first one to say that there is approximation in the measurements of the dyno, so how do you go from that to using some guy's dyno to say that the car loses 17% is beyond me.
Second, the page I linked explains clearly that if a dyno tells you you're losing more than 12.5% (again RWHP in a RWD car, on an acceleration dyno) you have some problems (meaning, the dyno isn't telling you the truth, for whatever reason, or the power rating of the engine is not right).
Debunking what that page says takes another page that treats the matter the same way, but demonstrates that the number are different, the factors are higher, lower, whatever. Dyno sheets won't mean anything.
What I found was not influenced by what I wanted to find. You see, I believed the dynos worked one way (which is true for some, however, all dynos normally used do not work that way). I looked around until I found information on how the dynos work and I found the answer I didn't receive from the people on the forum (which are all experts, you included, but failed to inform me on this who knows why). I didn't care about being right or wrong, in fact, I found my assumptions were wrong, so I wrote it down.
When I went looking for powertrain losses, I found three types of information:
1) What people arbitrarily derived from two types of data that shouldn't even be compared:
2) Some values thrown out there based on "experience?" or "rule of thumb?"
3) Some disjointed formulas that didn't really come up with a final quantity or related to only gears, or only oil, or only driveshaft, or whatever.
The only page that gives some scientifically derived numbers and combines them into a generic value with a pretty sound justification, is the one I found and linked.
Did I like what I saw? yes, because primarily it is simple, straight and objective (unless you want to believe that whatever percentage for gear friction they use is wrong because they like to lie) and it justifies the impossibility of percentage numbers to climb with a pretty sound argument.
Your first link is pretty lame. Forget this, forget that, this is what I think I make. I quickly put those numbers together and interpolated them with the 37000 for a 200hp engine with 25% loss and it turns out that that Nissan's transmission and differential are dissipating 36000 watts. If you find that believable, great. If you believe that my linked page is wrong, then find something that shows that for an engine with those figures (200hp and 25% drivetrain loss) the actual energy that would have to be dissipated is actually lower than 37000 watts and it is instead a very "dissipable" number.
The second link is not very consistent and in any case, the point is, the values considered are once again skewed because there are uncertainties on what the original BHP is and the actual "right" conditions for the dyno, so who the hell knows what those numbers really are?

In conclusion, could you find some data that confirms or denys what that page says, without using dyno sheets?
Right now, crank to rear wheel power loss is 12.5% with the most dependable source. If we can find otherwise, it is great.
BTW, do you know those guys in the links you posted, or did you decide that it was OK to assume what they said was what you understood and it was true, even though you didn't know them just like you don't know me?

PS THe way you put that example of friction loss doesn't really add up. If you read your post, it sounds like you're saying that the squatting is decreasing friction.
According to your logic, I don't know you, so I should stay with what I understood the first time, which is that you feel that increasing pressure on the rear wheels would reduce the friction between those wheels and the dyno drums.
But since I am not you, I'll believe your second point and let you off with it

maxcooper

Some power is lost in the half shafts, too, which aren't covered in that assesment. I suppose you could triple the driveshaft losses for an estimate of the effect (if U-joints and CV-joints have similar losses).

And 12.5% isn't so far off what people often quote. I have a web page for calculating fuel system capacity, and I use 15% as the default drivetrain losses for RWD cars (but I also let you replace it with whatever percentage you want). Most people measure on inertial chassis dynos, where all the inertia stuff does come into play.

My guess is that wheels and tires play the biggest role in the inertial losses on the dyno. It would be interesting to estimate/measure the rotational inertia of a few different wheel/tire combinations, the time required to complete a 4th gear dyno run (and vary it to simulate diff power levels), and then calculate the HP required just to accelerate a pair of wheels/tires during the dyno run (wheel RPM1 to wheel RPM2 in T seconds). And then produce a table (or plot) with varying wheel inertias as X, varying time-to-complete on Y and HP required shown in the table cells or as the Z value in a plot. There is probably some more interesting stuff to look at when you start to account for the acceleration varying over the course of a single dyno run.

-Max
P.S. Forgive me if I am repeating earlier posts -- I haven't read this whole thread

gio64

iTrader: (1)

I agree with what you're saying, and yes, we didn't spend a lot of time on halfshaft.
If you look at that link I posted (in relation to your tire comment) I believe you're also right, as I think the author says that at least half or a bit more than that is power loss through tires.

gio64

iTrader: (1)

BTW, no problem with the repeating.
You see, my purpose here was to shed some light on some "strange statement".
I didn't mean to actually rebuttal anything per se, I wanted some "acceptable" values and not "rule of thumb" or "common practice", or "my dyno says, therefore...".
If you have time to read the thread (bit by bit), you'll see how it started.
Unfortunately, it turned in an idiotic discussion about me not having specified (2 pages in this RWD-FD thread and after having linked to a page that talks about power loss) that when i said 20/25% power loss, I meant, you guessed it, RWD cars.
if you have not read the linked page, the writer says that dissipating 20/25% through transmission and diff is physically impossible for a 200 hp car he also cites realistic figures for power loss due to other components, concluding, you guessed it right, that the loss through tires is very big.
Now, beside the arguing, I would not mind somebody showing me a page that says that common loss for RWD cannot be 12.5%, because "...". I don't expect to be right, in fact I don't expect anything, I am searching for answers. That number figure is not even my creation/calculation/thesis, it is only the best answer (at least in my opinion) that I have found so far.
Regarding dyno tests, I believe they are great tools. But some things must be kept in mind:
The purpose of the dyno is to measure the amount of power available at constant load or acceleration (depends on the type of dyno) available at the wheels (or to better say, available on the tire thread surface). If one is smart and rigorous enough to keep all "little-not-so-little" variables constant, comparing runs before and after mods will give a very good measure of the improvement someone manged.
Now, attempting to "backtrack" power at the crank, becomes a bit iffy, because of the variables involved.
According to the best page I could find so far, the 12.5% value is worst case scenario for a RWD car. The page explains why; I am in the process of a) finding more information like that which might confirm or deny what that page says and b) trying to work some physics out by myself (found an "automotive physics" book, and I am working on it).
The bottom line of that page is that your power losses exceed that figure, you must have something not going right. Maybe calibration on the dyno, maybe data plugged in, maybe your engine rating hp is less, maybe your tires are out of shape/pressure/whatever, but something is wrong. It might mean that your engine actually made less than what it was rated for by the manufacturer, or that your dyno result should have been higer than it was (so tires, again, should be checked along with rims).
My intentions are to find a decent number defined with acceptable accuracy (approximation is going to be in there regardless) and have it ready when somebody says that they make 300 rwhp and 600 at the crank (this is a paradox, I need to specify it or someone will come on arguing that nobody ever said that and blah blah blah).
I also would like to learn about this, for my own car(s) as well, so I can better understand what's going on.
If you have any other insight, especially on pages that deal with this issue in a practical and yet scientific manner, I'd appreciate reading them whether or they confirm/deny what I said.

maxcooper

I suppose I agree with some of the spirit of what the guy with the 12.5% paper is saying, but since he doesn't have any inertial losses in his 12.5% cap, he must either be talking about losses for brake dynos or his analysis is incomplete. Or, I suppose, he might think that none of the inertial losses are worth considering. But that doesn't seem right. Standing by the 12.5% cap on the grounds outlined in that paper is not a very sound position.

Also, I was not talking about the losses due to the rolling resistance of the tire (tire deformation). I am talking about simply accelerating the wheels and tires from a low RPM to a high RPM in the time it takes to do a dyno run. I suspect that it is not an insignificant number (and it is one that might be enlightening to calculate).

-Max

gio64

iTrader: (1)

I am going to go back to that page again, read it out again and get back. I agree with you with the necessity of calculating acceleration, at least for the commonly used dynos around North America. One thing though, I believe (if not, I apologize) it is written by an English guy and I think in either that page or another on those in the links under they talk about brake dyno, which would explain the missing acceleration factor. As far as what you were saying about the wheels, I agree, I think I read somewhere that measuring power at the "hubs" without the wheels yelds much higher values than with them on (which I guess anybody knew but maybe did not estimate as being so high). I'll see if I can find that again.