Nihilanthic

Because, SPOautos, a big powerband (AKA torquey motors) will let it be making the power at basically all speeds. A peaky motor will not.

With constant power acceleration can be drawn as basically an asymptote of the Y and X axes with one as speed, one as time. A motor with a big powerband (aka a torque downslope) would look like little wedges of that curve for each gear... whereas a peaky car would be a bunch of 'steps' of straight lines, or a curve or straight line going the OTHER way from the asymptote.

If you match the asymptote given a constant power curve then yeah the ET calculator can predict it, but if its a peaky car its the same as having a lower average power.

Power DOES matter, but nobody knows how to just take it at face value "you want to make that power at more rpms than with a peaky powerband", so people try all these lengthy explanations to make people get it, yet ironically it is power that determines wheel torque.

The ricers just dont realize the whole constant power = good arguement.

SPOautos

Ahhhh, except that there is no connection between power and tq when you dont know the rpm so they are still basing the formula strictly from horsepower to weight ratio with no tq or gears involved. So if acceleration cant be calculated with hp how could this be?

Its ok, I just want Jim to explain it, he probably knows all the math behind it and can explain it pretty well.

Stephen

Nihilanthic

The "why dont you still get it" answer is this:

If a engine made X hp from idle to WHATEVER rpms and it was geared properly, it would be ideal... it would match the time/speed curve from an ET calculator given a computed power/weight.

It comes down to average power, and calculus is apparently to ohard to 'get'. Youre accelerating from the rpm you launch im to shiftpoint in 1st gear, then from the rpm you dropped to in second to your shiftpoint in second, and so on and so forth - and you want to have all the power (and thus TORQUE) you can at every rpm you will be pulling at.

So, you want flat power. It means you get negative jerk, but boo hoo, its faster than flat torque cars with peaky power that FEEL faster.

Nihilanthic

If you got constant power in all gears from a big powerband I can find out the torque at a given road speed if I know the tire diameter.

GtoRx7

iTrader: (5)

According to Jimlab, and me, I think you are wrong. I am not talking of a little 4-banger vs. a V8! I speaking of the same engine vs. itself. Both make the SAME peak torque, just one holds it flat almost to redline, the other drops off sharply. The one holding its torque longer, will accelerate HARDER with more G's and for a longer time than the one with downward sloping torque. It dosent just "feel" faster, it is faster. What am I missing here?

Orr89rocz

no, sorry.....drag racers dont launch at Peak HP rpm but at or around peak TORQUE rpm..... get to the track man

there is nothing else...yoru right.... LT1's have flatter torque curves than the mild bolt on L98s. even tho the L98 makes more peak torque..its torque curve drops off drastically while the Lt1 holds it longer...thus the lt1 is usually faster even tho the two cars will be close to similar hp numbers. Lt1 has a solid pull thru out the rpm range...more constant cuz the torque is flat

Nihilanthic

The fact that if it has flat torque it makes MORE POWER!?!??!!??!?!?!

Take two motors with identical peak power ratings. Say one makes that peak power for a large part of the rev range due to 'torque dropoff', and the other has flat torque (and less of it) so it only makes its peak power for a short duration over the rpm range.

The former one will be faster than the previous one.

Let me repeat: We are again assuming CONSTANT/SAME POWER! Obviously a car with much more power can defeat one that makes less even if it makes it at any rpm (assume an electric motor and a Mr Fusion under the hood, for the sake of arguement) also assuming its properly geared and isn't overly peaky.

Anywho, it boils down to average power over the acceleration in each gear - average power can give you a better indication of that than just peak figures. But in simple terms something that makes "max" power the entire time its in every gear but first is gonna be faster than one that doesn't, assuming power/weight is the same.

Nihilanthic

Only for first gear One youre in higher gears its about the powerband.



Consistency in terms of acceleration is actually the opposite of what youd expect to accelerate the hardest - negative jerk from constant power would be the fastest assuming you can put the power down.

Also I really doubt a LT1 would beat a L98 if power and weight were the same, becuase.. uh.. the L98 would make that power sooner and longer as a function of the rpm range than a LT1, mostly because of the LT1's short intake runners making torque FLAT vs the huge low and midrange of the L98, but lack of top end.

There's a reason LT1s with dual plane intakes and a carb are so much 'torquier'.

Orr89rocz

it be a good race, but i seen stock LT1 swapped thirdgens running better times than bolt on L98s making about the same power...cuz about full boltons will make about as muchpower as a LT1. also seen 330hp carbed Gm crate motors running about similar times if not worse than stock LT1 cars. its that flat torque curve and the nice horsepower curve.

jimlab

And distance traveled...

SPOautos

well i figured that was a given since the distance is in the name of the calculator lol

So anyway, since hp has nothing to do with acceleration and its all based on tq curve and gearing how do those crazy calculators work? They dont know your rpm, gearing, tire size or anything that they would need to compute the tq.

All they know is the weight of the vehicle and how much hp is applied to that weight over a certain distance. With that can conclude how long it will take to travel that distance.

So given that, I'm confused how only tq and gears creates acceleration and that you cant figure acceleration on hp.

But dont worry Jim, I really want to know, so I'll wait.

Stephen

Orr89rocz

those calculators give u a rough idea but they are not that accurate....they assume acceleration is constant

slo

iTrader: (1)

1/4 mile calculators are actually pretty accurate. Try http://robrobinette.com/et.htm

Put in the specs for an 06 z06 including a 180 pound weight for the driver, and you get 11.45.

Motor trend got 11.5 in their test vehice.

Next try an FD using stock 95 curb weight plus driver weight gives you around 3000 LBS

255 HP and you get an estimate of 13.96, which is right on for a bone stock FD.

Orr89rocz

yeah, and that site has my car at 3600lbs race weight(which is on the high side i think) and about 230whp doing 14.57....i been 13.7's already and should be 13.6's now.

like i said, they aint that accurate

FD is faster than 13.9's by the way... and new z06 has been 11.2's

SPOautos

No they dont....they assume that hp is constant which means you should really use your average hp during your shaft range to be more accurate but no one knows that stuff so they use peak to get a rough estimate. Also, the amount of time it take you to shift gears as well as your 60 foot makes a difference. Im sure they just base those on a built in number for the average driver. Its also going to assume perfectly flat pavement, no additional head winds, ect.

On a side note, I find it hard to believe that a 3600lbs vehicle with 230rwhp is running 13.6's.....I think your dyno chart, weight, or combo of both is off some. Im pretty sure the curb weight (which includes fuel being topped off) of a stock Iroc was a little less than 3300lbs, then you replace the exhaust, run less gas in teh tank, ect and you just shaved prob 150-200lbs off it. What kind of 60 foot time are you cutting?

And it is VERY rare to see bone stock FD's running faster than 13.9 on street tires without weight reduction. Im sure on slicks with a very low 60 foot you can get down into the mid 13's but I've never seen one on street tires get below real high 13's.

Orr89rocz

thats what i ment to say.

what u dont see is what i believe is important....TORQUE.. i make nearly 330lbft at the ground. i cut 1.7x 60 foots. and my car is a L98 with all the options which puts it at 3400-3450lbs or so...plus my 160 and 20ish lbs extra of beefier crossmember/torquearm/suspension parts. and maybe more in exhaust weight since i went to 3inch over stock 2.5. closing in on probly 3600lbs race weight with full tank of gas. and from what i heard and seen, curb weight does not include gas...but maybe i read it wrong or w/e

and i have always heard that rx7s are mid higher 13's cars stock.

SPOautos

have you actually dynoed your car?

Curb Weight - The weight of an unloaded vehicle (no driver/passenger/cargo) with all standard features, a full tank of fuel and or fluids necessary for the functioning of the vehicle. These include gas, oil, transmission fluid, Freon, anti-freeze, etc. and all factory installed equipment (such as the weight of the spare tire and the jack, etc.)

How many gallons does your car hold? I used to have a Iroc and it seems like that thing held nearly 20 gallons of fuel lmao. Gas is roughly 6.5lbs a gallon so it adds up fast, if your racing with a full tank of fuel you should consider trimming it down to about a third or half tank. It saves a lot of weight.

Stephen

DamonB

Torque and Power

Read Myths #3 and #4. Repeatedly.

Read them as many times as it takes to understand the calculations used to derive the shift points.

SPOautos

Damon, shame on you....you aren't supposed to be helping Jim with his answers!!!

lol

Stephen

Orr89rocz

yes i have....dyno only started reading 3900rpms so i didnt get all my torque...but i made 200whp even at 4100rpms with the converter unlocked..... locked up, it shoudl be no more than 230.
261lbft at 4000rpms which is WAY above my peak. at 3000rpms i'll make well over 300 at the ground. this is a dynojet by the way

i usually race light as possible.... that is 1/8 tank or less. stock tank is 15.9 gallons i think.... 87 L98 irocz is 3341 curb unoptioned. so lets say i'm atleast 3400 optioned but now with less tank fuel and no spare, so drop down to 3300lbs or so....added atleast 40lbs in mods from beefier stuff....so thats up to 3340..my 160lbs so thats 3500. even so at that weight, the calculator shows me being 14.3's or so. it doesnt take into account my awesome off the line torque that makes my car as fast as it is.

jimlab

No, they're fairly accurate for trap speed, since the distance traveled is known and with a constant power to weight ratio, you can calculate terminal speed by also assuming a constant rate of acceleration.

(note the constants used for conversion of units in the equations, Stephen...)

Roger Huntington's original first order theory from the early 1960s...
MPH = 225 (hp/weight)^1/3

Patrick Hale's 1986 variation...
MPH = 234 (hp/weight)^1/3

Geoffrey T. Fox's 2001 variation...
MPH = 230 (hp/weight)^1/3

For E.T., the formulas don't work as well for cars at either end of the spectrum (exceptionally quick or exceptionally slow) or for cars that are turbocharged (non-linear power delivery), and they cannot even begin to compensate for changes in DA (density altitude) or traction factors (both suspension and tires), but once again you're calculating time elapsed over a specific distance using an assumed constant rate of acceleration based on a constant weight to power ratio.

Roger Huntington's original first order theory...
ET = 6.290 (weight/hp)^1/3

Patrick Hale's 1986 variation...
ET = 5.825 (weight/hp)^1/3

Geoffrey T. Fox's 2001 variation...
ET = 6.269 (weight/hp)^1/3

The membership of this forum alone should be proof enough that just having enough power to run a number on paper doesn't necessarily mean you can run that number at the track. Hell, Eric Cheatham alone is proof enough.

jimlab

What he starts off by saying in #4 is that if you had an exceptionally flat curve around peak torque, the maximum rate of acceleration could potentially be maintained past the rpm at which peak torque occurs, which apparently is supposed to prove #3 a myth. OK, whatever. Theoretical hair-splitting and infinite gear selections aside, this statement was interesting...

"the one with the more powerful engine will exhibit the greatest maximum acceleration, regardless of which one produces the most torque"

What he's saying is that the engine with the greatest average torque will (all other factors being equal) accelerate quicker than one which produces more peak torque. In other words, maximize the area under the torque curve to maximize acceleration.

No news there, and a g-meter will quickly prove that there is a peak in acceleration at the torque peak no matter how flat the torque curve around peak torque appears to be on a dyno chart after smoothing...

SPOautos

What I think the main jist of myth 4 is getting at, is what I was also getting at. Its that you can figure acceleration from horsepower or by using tq, rpms, and gear ratios. This is the paragraph that I found to be the most important to that arguement......


"The simplest way to assess what the acceleration can be at any given wheel speed, is to convert that wheel speed to the equivalent engine speed for each gear, and then look at the power curve to find which of those engine speeds yields the most power. You can also answer this question from the standpoint of rear wheel torque, but then after looking up the engine torque for each of the engine speeds, you have to turn back around and multiply those engine torque values by their corresponding reduction ratios in order to find the rear wheel torque for each gear. You'll get the same result either way; if you don't, then at least one of the two graphs is in error. However, the torque method requires more computational work as compared to simply looking at the power curve, so why would anyone want to do that? "



Basically when you take the tq and run it thru all of the reduction ratios what your going to come up with is that the rpms best suited for faster acceleration and proper rpms to change gears are the ones that your hp curve follows so you might as well use the hp curve instead of going thru all the hassle of using the tq method.

I was just trying to make the point that hp isnt meaningless and it can definitly be used to calculate acceleration as well as proper rpm to change gears, 1/4 mile calculators, ect.

Of course there are many many variables, but that is true in either method of calculating acceleration it doesnt matter if you use the hp method or the tq and gears method.

So what it boils down to is that no matter if you think its all about tq or if you think its all about hp.....both are right so we might as well all have a big group hug!!!! lmao

Stephen

SPOautos

I dont think thats what he is saying. It says the engine with the most POWER (horsepower), will have the most acceleration, regardless of which one produces the most tq. This is because; as you know, one engine could have huge low end tq with little horsepower but the other have low high rpm tq with big horsepower. He is saying the one with the most power is the fastest....and NOT the one with the highest tq. Yes, typically miximizing the area under the tq curve will make the car faster, but its also creating far more horsepower.....which I think is the point....meaning that you might as well just look at the hp curve because its telling which where your tq is the most effective.

Anyway, I better get back to work!!!
Stephen

jimlab

It's what he meant to say.

This is nothing more than stating that you're going to accelerate more quickly by selecting a gear which puts you in your powerband. So if I downshift, I'll accelerate more quickly? Well, duh.

Of course, what most of these theoretical web expositions don't take into consideration is that there's nowhere to downshift from first gear. So where are you accelerating the quickest in first gear? At the engine's torque peak... not the horsepower peak.

However, the shape of the vehicle's acceleration curve matches the engine's torque curve, not the horsepower curve, so you might as well go through "all the hassle" and do it right.

You can calculate shift points from the horsepower curve. You cannot calculate acceleration in g's without dealing with the gearing and/or converting horsepower to torque.

No one said it was. However, there are (or were) several people in this thread apparently insinuating that torque is meaningless.

Sure it can... with a few unit conversions.

Actually, it does. Fnet takes into consideration all of those factors, which includes drivetrain losses, road surface frictional losses, aerodynamic losses, coefficient of friction at the contact patch and so on, all of which can be calculated or approximated if a few other variables are known.

This is the method that any decent vehicle performance simulation software (e.g. Car Test 2000) uses as opposed to the rough approximations offered by the formulas listed above which rely solely on horsepower and weight ratios.

My vote is for clubbing a few well-deserving people like baby seals, but whatever...