Where is the point (RPM) of maximum acceleration

[Stan Weiss]

Just a one question multiple choice quiz.

I have a stick shift car that has just moved away from a traffic light and is rolling at 5 mph. I floor the gas and accelerate up to the red line. If I data captured RPM and acceleration in G's, where would the point of maximum acceleration be?

1) can not tell
2) peak torque
3) peak hp
4) red line

Stan

[PhilD]

Maximum acceleration in in a single gear occurs at peak torque, and maximum acceleration at a certain mph occurs at peak horsepower.

So the answer is 2)Peak Torque

[Procision-Auto]

I'll agree slightly with Phil and say it's somewhere *around* peak TQ...I'll say slightly west of the peak.

I've actually logged this sort of thing with my street car and had the following results:



First gear was a loss due to wheel spin, but 2nd gear reprsents a close follow to the torque curve on this dyno graph.



Is this going to be another TQ. vs HP debate?

[Stan Weiss]

I'll agree slightly with Phil and say it's somewhere *around* peak TQ...I'll say slightly west of the peak.

I've actually logged this sort of thing with my street car and had the following results:

First gear was a loss due to wheel spin, but 2nd gear reprsents a close follow to the torque curve on this dyno graph.

Is this going to be another TQ. vs HP debate?

Not by me

Oval track semi fixed MPH is another story.

The easy part.

In a 1/4 mile drag maximum acceleration in each gear is at peak torque.

This is because you have the most wheel thrust / torque at that point.

The vehicle that accelerates the fastest has the higher average wheel thrust / torque


The hard part.

The engine that makes the higher average torque may not make the higher average wheel thrust / torque.

In "simple terms" wheel thrust / torque = (engine torque * transmission gear rate * rear end gear / tire radius in feet) - transmission and rear end power losses

Stan

[Procision-Auto]

Can we expand on that and say:

The greatest accleration as a change in velocity will happen from

Slightly lower Peak TQ RPM to Peak TQ,

as opposed to just Peak torque (static RPM value)?

[motormonkey]

Having driven turbo cars, you really feel the diference when boost comes in. So my uneducated guess would be when the HP starts to multiply the tourque.
Having driven alot of NA stick cars I would say the same as above, like a diesel will move with lots of torque, but wont get out of its own way untill the HP starts to climb.

[PhilD]

Thanks for posting those graphs Procision.

But I don't agree with "it's somewhere *around* peak TQ...I'll say slightly west of the peak."
It's exactly at peak torque.

You have to remember that in your graphs the car is gaining speed, that means the drag isn't constant and the acceleration figures aren't directly comparable.
Say in 3rd gear peak torque occurs at 70mph.
This directly implies max acceleration in 3rd gear occurs 70mph.
However if you could change to a lower gear that puts you at peak horsepower at 70mph the car will accelerate faster at that point.

The torque at the wheels is multiplied by the lower gear ratio.

[Troy Patterson]

Why isn't this question in Tech?

[F1Fever]

IDK but I 100% agree with philD (assuming you have the gear available)

[Procision-Auto]

Thanks for posting those graphs Procision.

But I don't agree with "it's somewhere *around* peak TQ...I'll say slightly west of the peak."
It's exactly at peak torque.

No problem. I'm actually getting ready for a track session and will be logging more data, so if anyone has any requests let me know and I"ll do my
best to graph it.

I'm not disputing any of the science, or whether the acceleration is/or is not the greatest. I'm being 'nit-picky' about the area at which
the greatest acceleration occurs.

It cannot be one RPM; it has to be a change in velocity. When I say "slightly West of peak TQ" am I stating that the acceleration is greater
from point x below peak up to peak TQ and then anything after will be less in comparison.

IE: The acceleration is greater from 3990 to 4000, as opposed to 4000 to 4010 if the peak TQ is 4000 RPM

I'm with you, it's all good.

[Stan Weiss]

Why isn't this question in Tech?

When I was looking to post this question I looked at Other Tech and General. I felt more people would see it in General. Where would you have posted it.

Stan

[Stan Weiss]

Thanks for posting those graphs Procision.

But I don't agree with "it's somewhere *around* peak TQ...I'll say slightly west of the peak."
It's exactly at peak torque.

No problem. I'm actually getting ready for a track session and will be logging more data, so if anyone has any requests let me know and I"ll do my
best to graph it.

I'm not disputing any of the science, or whether the acceleration is/or is not the greatest. I'm being 'nit-picky' about the area at which
the greatest acceleration occurs.

It cannot be one RPM; it has to be a change in velocity. When I say "slightly West of peak TQ" am I stating that the acceleration is greater
from point x below peak up to peak TQ and then anything after will be less in comparison.

IE: The acceleration is greater from 3990 to 4000, as opposed to 4000 to 4010 if the peak TQ is 4000 RPM

I'm with you, it's all good.

Maybe we could call it instantaneous acceleration.

Stan

[A Atwood]

Just a one question multiple choice quiz.

I have a stick shift car that has just moved away from a traffic light and is rolling at 5 mph. I floor the gas and accelerate up to the red line. If I data captured RPM and acceleration in G's, where would the point of maximum acceleration be?

1) can not tell
2) peak torque
3) peak hp
4) red line

Stan

The correct answer is 2.

With that said, do not let this lead you into thinking that the goal of acceleration is based on engine torque. Acceleration is and always has everything to do with engine horsepower. As been already said, it is rear wheel torque, not engine torque that determines acceleration.

Consider a car with a CVT (continuously variable transmission) that has the ability to continually adjust gearing in the trans to keep the engine at one continuous rpm all the way down the track. Run the car down the track at a continuous rpm two times. Once at peak torque and once at peak horsepower. Now I ask, which rpm will the car accelerate the quickest at. Do a little math and the answer is obvious.

An engine has the greatest potential to do work in the shortest time at peak horsepower, not peak torque.

What gets confusing to people is when you do the math, you will find the the highest wheel torque in "one gear" always happens at peak torque, but if you re-gear, such as a downshift, that takes you to peak power, you will find a higher rear wheel torque figure.

If you are in a gear, such as 4th, and you nail it at peak torque, then you do a downshift that gets you right to peak horsepower, acceleration is increased.

ARN

[MadBill]

Although the simple answer is indeed "2", converting this answer into an optimum gearing set up is a little more complex, as the goal in most race applications is arriving soonest at the finish line or braking point, not attaining maximum speed before same. A car could be geared to rocket off the line or previous corner to the detriment of top speed as the power nosed over, yet still arrive at the finish line/braking point first, as the greater initial acceleration means higher speed/less time down the entire straight, not just in the last few car lengths...

[mbrooks]

i'm just guessing, but i have to say tq is the measure of the power potential of the engine, whereas acceleration involves time which would involve horsepower, so max hp would for me would seem to be the point of maximum acceleration. kind of like the diesel example earlier, tq will sustain the pull, but hp makes it go faster.