Also, the huge jump in torque/power needs to be ignored here. That slope is not a true indication of what it going on. Some sort of intake butterfly or some other trickery pumps the torque up at that instant and it should show as a break and a jump in the curve, not connecting those two points. You need to look at the actual torque peak and look at the slope of the horsepower curve at that point only. It would be easier to see if you can get a more traditional dyno plot, especially one with a very peaky (non-flat) torque curve.

 

Actually, acceleration (rate of change in velocity) is at it's maximum at maximum hp and acceleration is still high once the hp starts dropping off. However, when hp begins to drop off, the "jerk" (rate of change of acceleration) is actually negative because acceleration, though still higher than points with less power, is becoming smaller and smaller.

If it were true that maximum acceleration was at max torque, then people would shift way before redline in order to have the highest average tq, not hp, which is not the case.

 

If possible you do want to shift so that your RPMs drop to where they are at or just before your peak torque. That is what vetteguy meant when he said it is better to make torque higher to take advantage of gearing.

 

This is incorrect. You seem to again be confusing acceleration with power generated.

Acceleration is at its maximum at the torque peak. That is not where the car makes the most power however. The point where the car makes the most power is the horsepower peak. The car has the most excess power (beyond what it takes to offset wind drag/other losses) at that point and that excess power will increase the speed of the vehicle. The best point to shift is when the RPM drop from the gearing will end up at a point on the horsepower curve that is equal to the horsepower at the instant before the shift. This gives the highest average horsepower (highest average power). Most cars do best at redline in first gear, but that may differ in that, or any other, gear. Depends on the power curve and the gearing. Even at redline, first gear is usually so wide that it drops it to a point where less power is available than at redline.

Perhaps it will help to think of it this way; The engine will rev fastest at/around its torque peak. Go drive a manual transmission car with a tach. Put in in second gear at 1500 RPM and stomp on the gas. You will see the tach needle slowly move (accelerate), note where it picks up speed and moves the fastest, then it will slow down (rate of increase will lessen) as you increase RPM. If it happens too quickly in second gear to notice, try 3rd.

Hopefully I have not muddied the waters further and it makes more sense.

edit below;
The horsepower rate of change is highest at the torque peak. That is when the car will 'feel' fastest as it is pushing the car the hardest. That is the highest acceleration, it is an instantaneous condition. The car will change velocity faster at the horsepower peak, because, even though there is less torque and it is being pushed less hard, it is being pushed more often (RPM). More total force is available to move the car.

I think that is the confusion between us. Acceleration is not over time, it is at one particular instant in time. The power generated is essentially how many times that acceleration is applied to the car.

 

Wow look at that peaky POS!

Is there any usuable power in that car? I'm assuming that is one of the older S2000's.

 

But here (knowing that (change in velocity) / (change in time) = acceleration), you are saying that "The car will ACCELERATE faster at the horsepower peak"


However, I do think I see what you are saying, and technically, we are both correct. You are correct in the fact that INSTANTANEOUS Acceleration is at a maximum when the peak torque turns the crank, but the greatest acceleration over a time period is at the range where hp is greatest.

Make sense?

 

And what if we are??

 

to add a little fuel to the fire....

Whats does a 500hp, 700hp, and 900hp Toyota Supra have in common? They all run 12's.

horsepower bench racing is great for dyno queens.

Torque wins races, pure & simple.

 

Torque only wins racing in tractor pulls. Pure & simple.

What are the two fastest drag radialed cars? Oh a supra and mustang 5.0. 7.8-9s on drag radials.

Have you ever watched tractor pulls??? SOOOO... awesome!

 

Tractor Pulls kick all ass! Farm boy here, So, Naturally a fan!

Finally something we can agree on!!!

 

False, pure and simple.

Torque isn't worth jack without RPMs, and RPMs aren't worth jack without Torque.


HP is what matters and...

HP = TQ*RPM / 5252

So, if you have a lot of torque, but you can't rev, you're still going to have no HP. And if you have enough RPM's you don't even need a lot of TQ to win races... like the S2000.

 

"horsepower is how fast you hit the wall, torque is how far you take it with you."

 

You are so close. I think we will get on the same page now that I can see where the communication broke down.

acˇcelˇerˇaˇtion
NOUN:

1.
1. The act of accelerating.
2. The process of being accelerated.
2. Abbr. a Physics The rate of change of velocity with respect to time.

When I am taking about acceleration, I am using the literal definition. (see #2. above) With that usage, the maximum acceleration is at the torque peak. That is the point where, per RPM, the power is increasing at its maximum rate.

I think we agree on that. The step that you were missing on increasing the velocity of the car is that horsepower is not a constant function of the torque output. The horsepower formula simplifies to HP= tq x RPM/5252.

The RPM/5252 is the part that concerns us. That number is not a constant, it changes based on RPM (obviously). That is why there is more power available beyond the peak torque value's RPM point.

You were using the word 'accelerate' to mean 'changing the velocity of' the car, I was guilty myself of using that a time or two and I apologize for the confusion it generated.

Even though there is more power above the torque peak, the rate of change is less. The power is greater (car changes velocity over a shorter period of time), but the acceleration is less. The use of the word 'acceleration' is confusing here, so we will go with; The rate of power change is less.

I tried an example earlier, let's do a similar one and see if it makes more sense now. A vehicle makes 200ft*lbs of torque at 3500 RPM. That would be 133 horsepower. Call it 5 ft*lbs less @ 250 RPM on either side of it. So at 3250 RPM you get 131 hp and at 3750 you get 139 hp. That means you have gained 8 horses in that 500 RPM range. Now, lets look at what happens near the horsepower peak. Peak is at 6500 RPM. Torque is in the toilet at 140 ft*lbs, but the power is at 173. Since the torque curve is on the way down, lets say that it was 5 tq higher at 6250 (172 hp) and 5 tq lower than the 140 tq @ 500 RPM above that (probably more as when tq falls off, it really falls off.), but anyway, that gets to 173 hp (actually a touch more, my estimate was too generous with the tq since I claimed the power peak was at 6500.)

So from 3250-3750 RPM you gained 8 horsepower and from 6250-6750 you gained 1 horsepower. That is 500 RPM in each case. That is the acceleration, that rate of change over the 500 RPM. You had 8 times the acceleration around the torque peak vs the horsepower peak. The engine is reving 8 times faster.

The power generated is quite different, 131 horses vs. 173 horses. The vehicle will use that extra 42 horsepower (minus whatever is needed to offset increased drag and other parasitic losses) and will increase the vehicle's speed. So (and I hate using the word 'accelerate' here) the car will accelerate faster because of the increased power, but the engine is accelerating less quickly above the torque peak. Actually it is experiencing a negative acceleration, but who need to open that can of worms?

The engine accelerates most quickly at the torque peak, the car accelerates most quickly at the horsepower peak.

Since there is no constant ratio between torque and horsepower, any discussion regarding torque and acceleration is intended to be limited to the rotational acceleration of the engine. There is no context to compare it directly to power generated. Hard to explain....but since they are not related in a linear way, well, read the next paragraph and see if that helps.

Accelerating the vehicle depends solely on the power generated and that depends on another variable (RPM) in addition to the torque available at that point. So you cannot compare torque and vehicle acceleration directly. That is where I believe the confusion arose after you quoted the vette guy (or whomever it was originally)

Now, here is the last part. If the torque curve were perfectly flat (and this one is pretty close), the rate of acceleration would be the same, you would gain the same power over the same amount of time. The power curve would have the same slope all the way up. He was incorrect about the acceleration being the same @3000 RPM as @8000 RPM, but only because the tq numbers are not the same, if they were, then the statement would have been correct. The rate of change (aka acceleration) would have stayed constant.

 

VERY nice post! Hopefully everyone that took basic physics will understand.

 

Thank you, however upon reflection, I did leave out just about the most important thing. It was so basic that I did not even consider writing it down, but it could be confusing.

The car will accelerate most quickly at the horsepower peak as that is the point where there is the most excess power above what is required to hold the current speed of the vehicle.

You are running the car in a gear that limits the top speed of the car by making it drag limited (like 5th) so, if the car accelerates and passes the point where it needs 172 horsepower to offset its drag/heat loss, it would be running (WOT) at 6250 RPM. Ignoring the extra wind drag at that speed, there is 1 extra horsepower available (excess power) to accelerate that car the next 500 RPM. Well, don't forget, you get more power during that 500 RPM window, a whopping 2 total hp to of excess power. You can imagine how long it takes the car to accelerate with those few horsepower.

Man...that took a long time. Now, think back to when you shifted into 5th and your RPM was at 3250. Engine is not guite lugging, but it is close and it takes 130 hp to maintain that speed. You floor the gas pedal and the RPM slowy climbs. You have the same 1 extra hp to accelerate the vehicle as you did at just below top speed, however during that same 500 RPM (3250-3750) you gain 8 horsepower. That means have more excess power available in that RPM range than at the first (6250-6750). The car will accelerate faster at the higher torque values (again, ignoring the extra drag from wind resistance).

The only reason a car will accelerate faster at the horsepower peak (unless running at close to top speed) is because gearing allows the engine to produce (and apply) the excess power to the wheels.

If you use the gearing (by shifting down to 3rd and getting to 6250 RPM ) to apply that 40 some-odd hp at the same speed as 3250 RPM in 5th gear, it is going to accelerate the vehicle a lot quicker than if you left it in the higher gear. It only gains 1 horsepower from the engine in the next 500 RPM instead of the 8 you would have in 5th, but you still have 40 excess horses straining at the bit accelerating the car. (ignoring the mechanical advantage change in the gearing)

 

I think we are ALMOST saying the same thing... but I think the use of the word acceleration is still screwing us up.

Technically, based on F=m*a, the maximum acceleration is @ the peak TQ, the force. When the crank turns @ peak torque, you are getting your maximum INDIVIDUAL acceleration.

Effectively, however, which is what we are actually interested in with regard to racing, maximum acceleration over the course of any significant time period is at @ maximum
hp.

Hopefully this will help...

Let's say you're at maximum tq of 400... the acceleration is maximum PER CRANK REVOLUTION. I will show this with 4 underscores...

____


Now, imagine that you are at your maximum hp, which happens to be only 3/4 ths of the maximum torque. (shown by 3 underscores)

___



Now, lets say that your maximum torque is @ 4k rpm, so scaled down, every minute, your acceleration will look like this (scaled down to 4 instead of 4000) [btw, the periods are just there as placeholders]

____
.......____
..............____
.....................____

So, total acceleration over that minute is 4 (4000) times your maximum individual acceleration, so your POWER, which is Work/Force with regard to time, would be 16 (4 rpm x 4 underscores of acceleration per minute). BTW, this is not in hp because it's much simpler if you don't use those arbitrary units of power. The actual hp at this point would be
304hp based on the formula for hp.

Now, let's look at the point of greatest power, which we will say is at 8000rpm, but only 3/4's of maximum torque (300). So, with 300tq and 8000rpm, the hp here is 457hp... or, in our simple form of power, 24 (8 rpm x 3 underscores of acceleration per minute). Now, for the representation.

___
.....___
..........___
...............___
....................___
.........................___
..............................___
...................................___



Now, lets see them next to each other...



Maximum TQ : 400 tq (4 underscores of acceleration) and 4000rpm (4 sets) = 304hp (16)
____
.......____
..............____
.....................____



Maximum HP :300 tq (3 underscores of acceleration) and 8000rpm (8 sets) = 457hp (24)

___
.....___
..........___
...............___
....................___
.........................___
..............................___
...................................___

As you can see, the individual acceleration PER CRANK ROTATION is greater at peak TQ, but the acceleration over a given time period, in this case, 1 minute, is significantly higher at maximum POWER.




Hopefully this clears up the confusion we were having I think the fact that there were 2 different 'kinds' of acceleration... actually just over different time periods... is what was throwing us off.


There are other things like when you should shift and what happens when hp is dropping off that I would like to try and clear up as well, but rather than try and put too much into this post, I'll just tackle that later.

 

As I was reading this I had a flashback to my 3 hour Physics Lectures. Man I dont miss those days!!

I am amazed everyday at the amount of extremely intelligent people on this website. This place is a gold mine of information!!!

 

We should increase the TORQUE on those cars ...

 

not the true defenition...but this is how i explain tq
tq is the energy of ONE revolution of a engine...whereas HP is that number times how many revolutions....

 

so many people miss the point it's crazy. hp doesn't even really exist. It's just a figure derived so a car salesman doesn't have to write a science equation on a sales tag. Tq never changes in a motor unless you modify it. Rather the eff. at which tq is applied goes up and so you get a tq curve and have a higher tq # at different rpms then others.

Tq is the actual measurement of power in ALL engines. Hp is just an acceptable term so salesmen don't have to give a crash course in physics to your average joe looking to buy a car.

 

Horsepower, horsepower, horsepower. That's what it's all about.

Torque does not equal acceleration. As someone pointed out already, if I put a one foot long wrench on the lug nut of my car and apply all my weight to it (Let's say 200 lbs.) and tighten that lug nut as tight as I can (without jumping) I'm applying 200 lbs/ft of torque.

Even if the lug nut doesn't spin anymore, and the car doesn't move (read the wheel doesn't spin either) then I'm still applying torque. Torque doesn't need motion to exist. HP does.

I don't care if you apply 2,000 lbs/ft of torque to any axis (drivetrain, crankshaft, axle, or whatever), if the vehicle doesn't move no HP is being produced. If no HP is being produced, the car doesn't move.

You can't have one without the other.

No HP = No motion and vise versa.

 

applying tq and making it are two different things

 

TQ gets it moving, HP keeps it moving.

 

horsepower is how fast you CAN go, torque is how long it takes to get there. You cant have one without the other

at least thats always been how ive seen it

 

horsepower is simply a measurement of torque. Torque gets it moving, torque and momentum keeps it moving.