Power vs Torque is the correct?

[SimonNOS]

This is a quote from a discussion with someone on another board as to whether torque or power is more important with regard to ET in drag racing.

Let me try and explain it to you with the following FACTUAL example;

1) Take the same bike / car with a fixed hit nitrous system and accelerate over a 1/4 mile. Do a run using the full rpm range therefore reaching peak bhp and then do another run changing gear say 1,000 rpm earlier. The 2nd run would be quicker than the first because the engine spent more time in the peak torque range rather than the peak power range.

In the above example there has been no change in the torque or power outputs, just how they are used. If your statement that "power wins races was true the 1st run would have been quickest.

The above scenario is based on 1,000s of actual results that I've had hands on experience of over the last 25 years and I've proven these results to a number of high end racers (and even using US made nitrous systems in some cases) who questioned my statement as you have.

There is no doubt that torque wins races and tuning a nitrous engine to achieve increased peak torque at lower rpm is more beneficial to 1/4 times than tuning to achieve peak power at high rpm.

By the way before you respond, just remember your first statement was "Very true. BHP wins races, not torque"

My final statement on the matter is this;
If you have the choice to make your engine make more torque at lower rpm at the expense of more power at high rpm, then go for the torque not the power.

Regards

Whilst shifting sooner would put you back into the higher toque rev range of an engine running nitrous, you will still be putting less power to the wheels, how can this make you produce quicker ETs? Is the quicker ET a product of something other than the increased torqued?

 

[patprimmer]

It won't.

Area under the POWER curve, peak power, transmission design and ratios and traction along the entire course all have an effect.

There have been detailed discussions on this before. Why not do a site search and save us all from going over it again, and again and again.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[icrman]

Torque is the measured force, and HP is time related.

33,000 lb ft per minute is a HP

So you basicly don't have one with out the other.

 

[samshen]

Torque is the measured force, and HP is time related.

33,000 lb ft per minute is a HP

So you basicly don't have one with out the other.
---------------------------------------------------
totally wrong!
"33,000 lb ft per minute is a HP"
in this sentence, lb-ft means the "work" not "torque", although they have the same measure.

for a engine, it will have a torque output graphic vs rpm, and also hp output graphic vs output. This two graphic are not same! And the peek hp and peek torque are normally not occurs at same rpm.








Sam Shen
Clutch and Driveline engineer
Drive for the future

 

[EddyC]

SimonNOS,

A certain measured value of torque (say 500 ft-lbs) doesn't care whether you get from one end of the track in 10 seconds or 10 hours. Torque is a form of Work (the length of the track), and Work doesn't care how fast it was done. But Power is Work/Time and is certainly dependent upon how long something (traversing the length of the track) took to do.

 

[crashbox455]

peak torque at the engine doesn't mean diddley.

measure it at the wheels. staying in 1st gear longer means more use of the 1st gear ratio, which will go leaps and bounds beyond what the engine is capable of in 2nd gear. the only way you'd lose enough torque at high rpm's is either by maxing out your airflow (redline) or having a fuel or ignition system that can't keep up.

if you have a torque graph of your engine, and the ratios of your gearbox, it's pretty easy to make a thrust graph.

 

[electrosmog]

The relation is P=M*w, where P is power(W), M is torque (Nm) and w is revolutions (radians/sec).

My intuition tells me that maximizing torque should give you the fastest lap time, because the torque is what turns into force between the wheels and the ground. I did a computer simulation to find out but got the answer that maximizing power minimizes time.

Hmm...not shure everything was correct though...

 

[NormPeterson]

The torque that you're trying to maximize is, as crashbox noted, drive wheel torque. You have to be way up the rev range, quite possibly beyond redline, to have fallen far enough off the peak engine torque value before you'd surpass the wheel torque in the lower gear with that in the next higher gear. Especially with the relatively wider spacing between 1st and 2nd gear ratios that's common practice.

Peak engine torque and its rpm only indicate the approximate speed where the maximum acceleration in any one gear occurs and its magnitude.

Norm

 

[electrosmog]

Norm & crashbox:

Sounds reasonable. The simulation shows exactly that.

But then I did another one where I changed the fixed gear ratios for a CVT-type transmission and got the same results (with shorter acceleration-time of course). Maximum power is still the best choise. Why is that?

 

[NormPeterson]

Try a little simplified derivation.

acceleration a = F/M, where F = RearWheelTorque/TireRadius, so

distance s = 0.5 * (F/M) * t^2, and therefore

elapsed time ET = SQRT(2 * M * s / F), where F = some F(t)

since you're looking to maximize F for every point in time over the entire time t that you're accelerating, this all comes back to RearWheelTorque over time, or 'area under the power curve' as bounded by adjacent transmission gear ratios.

Norm

 

[electrosmog]

Norm:

Hmm, I'm probably not getting this right, or I'm just tired.

When I aim for maximum power I get the largest power over time, in other words the "area under the curve". During this setup I also get the quickest acceleration.

But when I maximize torque, or force on the wheels, I get a slower acceleration even though I have a larger torque over time.

According to "...this all comes back to RearWheelTorque over time, or 'area under the power curve'...", my results don't quite add up.

(Note that when I write "maximize power" I mean "power over time" and the same thing for torque.)

 

[NormPeterson]

But when I maximize torque, or force on the wheels, I get a slower acceleration even though I have a larger torque over time.

Is that torque still engine torque, without regard for available gearing?


Perhaps instead of

F = RearWheelTorque/TireRadius

I should have written

F = Gearing * EngineTorque/TireRadius,

and instead of

RearWheelTorque over time

written

Gearing * EngineTorque over the range of road speed in each gear

That, and the post timestamped 14 Jul 05 11:58, should help clear things up.


sorry, pat

Norm

 

[jons999]

Regardless of all the math equations from my experiance in drag racing (which i do every week) Horsepower increases your trap speed, Torque has more of an effect on your ET. For instance i can change my rearend to a lower gear ratio say from 3.90 to 4.11 and my ET will go down a couple tenths but my trap speed will be the same or slightly slower. Increasing the engines horsepower will give me a much higher trap speed and only a slightly lower ET. Without having to put my car on a chassis dyno every time i change something with the engine i can look at my trap speeds to see if i gained or lost horsepower. Horsepower and torque go hand in hand but i tend to agree with the statement from the guy on the other board that torque is more important.

-Jon