Zach McAfee

I know the 15% rule is generally accepted so I'll probably get hell for this.

I've seen a few people pose a similar point, but nobody really recognizes it. I'm questioning the 15% loss theory with the following:

RWHP is less than FWHP due to the loss of power in the drivetrain. Since energy can neither be created nor destroyed, it's going somewhere. The horsepower lost is converted into heat due to the friction in the drivetrain components. The friction is the same in your drivetrain no matter how much horsepower you have.

Engineers, physicists, mathematicians, please explain to me why more horsepower is lost when you add more horsepower. I'm thinking that a certain (constant) amount of power is required to overcome the drivetrain friction. Give me some energy equations to challenge this new idea of mine.

Edit: I thought I'd clear this up with an example:
Rather than a 15% loss in horsepower, a constant 30hp loss as follows:

15% rule:
150fwhp=127.5rwhp
300fwhp=255rwhp

30hp loss rule:
150fwhp=120rwhp
300fwhp=270rwhp

KevinK2

"The friction is the same in your drivetrain no matter how much horsepower you have."

some loss is load related, some isn't. I had a direct shift linkage on prior car ... shift arm would get hot as a pistol at the road circuit track, but stayed cool at similar ave speed on hi way.

industry practice for gear boxes is to assume around 2% efficiency loss per gear mesh, ie proportional to input hp. bevel gears loose more. dyno 'coast down' output will give mostly the loss that is not load related, but includes some tire hysterisis heat due to weight load.

my rational approach is assume 50-50, with oem output as a calibration point. FD ex:

255 hp, typ dyno 222 rwhp for 1.15 multiplier, 13% loss of 33 hp. half loss is constant, 16.5 hp, multiplier is 1.075
so FD rwhp __ fwhp __ fw/rw

222 __ 255 __ 1.15
285 __ 323 __ 1.13
300 __ 339 __ 1.13
350 __ 393 __ 1.12
400 __ 446 __ 1.12
500 __ 554 __ 1.11
600 __ 662 __ 1.10

not exact but closer than full or no load proportional loss assumption.

Evil Aviator

Drivetrain losses are:

1) Inertial (varies with acceleration and power)
2) Frictional (varies with speed and torque)

Correct, you can't accurately apply a constant percentage for drivetrain loss. If you really want to know the engine's performance at the flywheel, then just throw it on an engine dyno.

protlewski

your right it isn't always true.

But the 30Hp thing i'm not so sure about.

take for example a chevy corvair that has had a v8 dropped in it. Now (this is accutal acount) this v8 was a dyno tested chevy 350 that made 300hp it was brand new and the transaxle was completly rebuilt and it on a rw dyno made 245rwhp. thats like a 55hp loss! but they don't lose that much at lower hp levels.

the stock engine is 110 gross hp (remember the '60's) and I have seen them dyno around 85-95rwhp aow thats around your 30Hp theroy but then why didn't it lose only 30hp at higher levels of hp?

That still eludes me.

rx7tt95

I have a pretty good indication of what my FWHP is thanks to a new M5 that ran on the dyno right behind me. If I remember, the M5 makes 394hp or so SAE. We both put down 330hp even to the wheels.
Michel

RICE RACING

Which comes pretty close to the dynojet std multiplication factor of 1.17 * rwhp = fwhp

Dynapack is 1.20 * rwhp = fwhp

Dyno dynamics is 1.3 * rwhp = fwhp

All for 2wd rear manual cars, remember this is an estimate, but it is very close for the listed dyno types.

Zach McAfee

This is probably the best response I've had since I joined the thread. Thanks guys!
I'm sticking with the 15%/1.17 multiplier since I use a Dynojet 248. The sad thing is, my first dyno run only gave me 6hp over stock fwhp (146). Not exactly what I consider money well spent. I'm going back tomorrow for another shot, cleaned up some ignition breakup and a few other changes. FYI details are here:
https://www.rx7club.com/forum/showth...hreadid=106366

Evil Aviator

Doesn't that Dynojet have a drivetrain loss calculation function which activates when you press in the clutch at the end of your dyno run?

BTW, I don't see the 6-ports opening on that dyno chart. Did you disable them?

DriftinJim

iTrader: (24)

BTW, FWD cars use a transaxle..where the tranny and diff are one unit...which minimizes loss. RWD uses 2 seperate gear units, the tranny and diff, so that is why FWd loses less hp. There are some exceptions like some Porsches(944 offhand) and whatnot that use a rear transaxle.

Zach McAfee

As far as I know they work, but I'm still running off exhaust backpressure activation. What should I see on the graph to indicate they are opening?

turbostreetfighter

let me throw something at y'all about the FWD calculations. i purchased the third dynojet in texas back in 1997 so i do have some background!! if you take a stock integra GSR ( which i also had the first 10 second one in TX) which is rated at 180 FWHP a typical pull would only put out around 136-140 WHP. a type-r rated a 195 FWHP would only put out around 150 WHP. ifor some reason i always had to use a very large mutiplier to get the numbers to come out right. is the ratio diff on a FWD vehicle is some way? last week i dyno tuned a camaro that made 525 RWHP with a automatic, we went to the track and the car ran a 9.60@140 mph which calculates to around 675 FWHP the car weighs 3100 lbs and is naturally aspirated, that comes out to around a 1.26 multiplier!! and this car uses a powerglide tranny.


MWW

Evil Aviator

The stock 6-port NA 13B's that I have seen on the dyno display a burble in the graphs.

For example, there is definitely something going on around 3600rpm on this particular dyno chart attachment. I don't know if it's the aux ports or the secondary injectors causing it, but it's obviously there. I don't see this on your chart, which may be good, LOL.

Zach McAfee

Looks to me like this person has the notorious 3800RPM hesitation and his six ports are not opening up because his power curve starts shooting down after 5.5K. Based on my experience fixing my 6 ports, without the extra intake volume the top end seems to flatten out, according to the butt dyno.

I had an air leak for a long time where the actuators bolt to the motor. Once I fixed that there was a definate improvement in high end. Nothing to write home about, but instead of flat horsepower above 6K, it seemed to pull harder no matter high the RPMs. I'm still not confident about their opperation. I'd like to put a pressure gauge (0-4 psi) in line so I can see how much backpressure is generated.

Zach McAfee

These are some pretty hefty drops. A friend of mine has a stock S5 TII (200 rated HP, but you already knew that ) and pulled 170 (15% again). It does make sense intuitively that a FWD car would have less drivetrain loss with a transaxle setup.

von

Heres my graph. Notice how its smooth and linear. I took out my rods to my 6th ports completly and jb welded my sleeves with pineapple inserts inside the block. I also have ported manifolds at the flanges.

The first blue run is base without headers. Just streight pipe bonez exaust. The very top one is with headers the next day. It was a speed to rpm conversion... The linearness is due to no sleeves and wired VDI. Notice how I didnt really lose any low end as well...

RETed

Most 5-speed, RWD will be about 15%.
Lowest thing I saw was an FD running all Redline in the trans and rear diff pop out a 12.5% loss reading.
Automatic transmissions, RWD will typically be between 15% - 25%; that 525/675 vehicle comes in at 22.3%, which is within range.
Manual trans, FWD is typically lower at around 10%; I have no idea why those Hondas are running such low numbers - did they all run to their indicated redline?&nbsp Most GS-R's I've seen run about 160hp at the wheels, and Type-R's running around 175's, which is close enough (but way off your numbers).
These are all off DynoJet 248C/E models.


-Ted

maxcooper

Front drive has no ring and pinion 90-degree gear setup. That's worth about 5% of loss, which is in line with the reports of 10% and 15% for FWD and RWD manuals, respectively.

And yes, we all know that a straight percentage is not an accurate portrayal of the losses. But its just a rule of thumb and doesn't matter much anyway. It is way more touble than it is worth to figure out the real equations, even though it would be kind of interesting.

-Max

von

If 125rwp S5 car has 160bhp then why cant anyone figure out the loss??? I believe it comes out to around 17%. why go off on all these rule of thumbs?