Peak in gear acceleration is not quite at peak torque...

[swamp2]

Originally Posted by e1000

Wheel torque is what moves a car. PERIOD. Engine TQ x gearing = wheel torque. Whether an engine does this at 500 rpm or 5000 rpm, as long as the resultant wheel torque figure is the same, the (point) acceleration will be identical. Small detail with this is, due to gearing, the speed at which this acceleration is happening may be different.

The "problem" of vehicle acceleration peformance can be stated equivalently by power or by torque+gearing. Each representation via formulae are useful for different purposes. In my view power is a more fundamental and powerful (no pun intended) quantity but for many equations such as a basic force balance (F=ma) rpm by rpm, torque is a more useful quantity. One can not really exist without the other in a rotating system.

What you are missing is that power IS torque x rpm. Well technically the formula in SI units is P = T * ω (ω is just the SI equivalent of rpm, different only by a constant). If a car has low torque (relatively) and large gear ratios (again relatively) it WILL also have high power (relatively). Power is the ultimate capability of the engine without having to worry about gearing!

Lastly in your example about a fixed wheel torque at either 500 or 5000 rpm. In this you are missing the point that any two such real cars would absolutely have radically different gearing, thus they would also have radically different power! Thus that particular example is not very insightful. It is always better to make torque at high rpm than at low rpm!

[swamp2]

Originally Posted by bruce.augenstein@comcast.

the "small correction" is obviously gear dependent, as you've stated, but also may be not-so-small based on the shape of the torque curve. I'd postulate that an S2000 in fifth would have a significant difference between theoretical and actual, based on the fact that the peak is so "soft", with gentle ramps before and after.

You are absolutely correct. Is your insight that good without having actually run the numbers? Do you have an actual factory dyno? I had to use a somewhat hacked version of a dyno somewhere between meeting factory stated values for peaks and the curve available at rri.se.

I went ahead and ran the numbers. Could not do it with CarTest since it does not allow results across all rpm in a fixed gear. Not entirely happy with the tire loss model I used, but close enough for sure. For the model year 2000 S2000 in 5th gear the "correction" is indeed quite large. The rpm of peak engine torque is 7500, the rpm of peak in gear acceleration is way down at 3000 rpm. The odd thing is that for 1st - 4th the formula with no correction works either precisely (within the working 500 rpm "bin resolution") or to the point where there is a double and almost identical peak in acceleration, one at high and one at low rpm (due to the soft midrange I would say). At 5th then the formula with no correction simply goes a bit haywire.

Of course as you said it is not damning to the overall point that for maximum sustained acceleration you need to spend as much time as possible making as much power as possible. It does however, give those who argue that torque is "more important" some small ammo against the "gospel"...

[e1000]

Originally Posted by swamp2

The "problem" of vehicle acceleration peformance can be stated equivalently by power or by torque+gearing. Each representation via formulae are useful for different purposes. In my view power is a more fundamental and powerful (no pun intended) quantity but for many equations such as a basic force balance (F=ma) rpm by rpm, torque is a more useful quantity. One can not really exist without the other in a rotating system.

What you are missing is that power IS torque x rpm. Well technically the formula in SI units is P = T * ω (ω is just the SI equivalent of rpm, different only by a constant). If a car has low torque (relatively) and large gear ratios (again relatively) it WILL also have high power (relatively). Power is the ultimate capability of the engine without having to worry about gearing!

Lastly in your example about a fixed wheel torque at either 500 or 5000 rpm. In this you are missing the point that any two such real cars would absolutely have radically different gearing, thus they would also have radically different power! Thus that particular example is not very insightful. It is always better to make torque at high rpm than at low rpm!

I'm absolutely correct. The RPM and gearing relationship describes speed, not power. Power (tq) multiplied through the gearbox describs the resultant wheel power.

http://www.car-videos.net/articles/h...wer_torque.asp

[bruce.augenstein@comcast.]

Originally Posted by swamp2

...You are absolutely correct. Is your insight that good without having actually run the numbers? Do you have an actual factory dyno? I had to use a somewhat hacked version of a dyno somewhere between meeting factory stated values for peaks and the curve available at rri.se...

I suppose - but for me, using the S2000 as an example was a fairly simple choice. With such a flat torque curve in a high gear, you could very easily envision a scenario where you are at, say, 90% of peak torque, 20 mph below the peak torque rpm, pushing into only 85% of the total drag you'd experience at the higher speed.

Bruce

PS - I suppose I shouldn't be surprised, but did you envision the amount of sheer crap your two simple and completely true statements would generate?

[swamp2]

Originally Posted by e1000

I'm absolutely correct. The RPM and gearing relationship describes speed, not power. Power (tq) multiplied through the gearbox describs the resultant wheel power.

http://www.car-videos.net/articles/h...wer_torque.asp

Sorry, I will not have my basic physics corrected by some crap web site. Most of what they say is in fact correct but this does not support your points. Just FYI I do have a Masters degree in Physics from a top US school... Not that this means I am always right (just the vast majority of the time ). Sure rpm and gearing (and tire size!) will provide speed. I do not disagree with that. What I am arguing with is that somehow torque moves a car but power does not. You can not have one without the other (for rotating devices). In fact, the site you reference also confirms my point that power is vastly more important than torque!

Also my third paragraph above is also absolutely correct which makes your example I was replying to perhaps correct in some sort of very odd theoretical way, but in practice it makes it entirely incorrect. Such cars would have radically different gearing and power but you have artificially chosen them to have the same torque. Revisit you "favorite" and trusted web site as the confirm my point above in bold!

[e1000]

Originally Posted by swamp2

Sorry, I will not have my basic physics corrected by some crap web site. Most of what they say is in fact correct but this does not support your points. Just FYI I do have a Masters degree in Physics from a top US school... Not that this means I am always right (just the vast majority of the time ). Sure rpm and gearing (and tire size!) will provide speed. I do not disagree with that. What I am arguing with is that somehow torque moves a car but power does not. You can not have one without the other (for rotating devices). In fact, the site you reference also confirms my point that power is vastly more important than torque!

Also my third paragraph above is also absolutely correct which makes your example I was replying to perhaps correct in some sort of very odd theoretical way, but in practice it makes it entirely incorrect. Such cars would have radically different gearing and power but you have artificially chosen them to have the same torque. Revisit you "favorite" and trusted web site as the confirm my point above in bold!

Absolutely they were arbitrary and extreme choices to illustrate a point. Engines of those characteristics may not even be possible. Again, it was to illustrate a point.

Even if assuming you are correct and max in-gear acceleration is much lower than peak torque, it's still a moot point other than first gear. It would always be more beneficial to downshift due to the larger multiplication factor of the shorter gear.

[JCtx]

Maximum acceleration occurs when revs are between peak torque and peak HP, otherwise known as the 'power band'. Yes, peak acceleration occurs at peak torque, but you're only there a fraction of a second . That's the reason why most sports cars have 'close ratio transmisions', to get to that range as quickly as possible and keep rpm there after every gear shift.

[arter]

I though I might input a graph I made here of my estimate of power at the wheels on the M3 going through the gearing and torque estimates I have seen.
Yeah I know I made assumptions and I was converting it to a theoretical crank Hp applied.

Shows the big drop-off in power right after you shift into 2nd ( and again into 3rd).

I also compared this to my SLK55 which has less max Hp but much higher engine torque which allows it to not drop off as much as you shift. Can really feel this difference between the two cars when accelerating after a slow turn.

We need more gears ( or never slow down below 80 mph).

[swamp2]

Originally Posted by elp_jc

Maximum acceleration occurs when revs are between peak torque and peak HP, otherwise known as the 'power band'. Yes, peak acceleration occurs at peak torque, but you're only there a fraction of a second . That's the reason why most sports cars have 'close ratio transmisions', to get to that range as quickly as possible and keep rpm there after every gear shift.

Just to be clear you mean overall (i.e. across gears) in your first sentence, correct?

Also assuming you do mean that your statement is very roughly true but is not any sort of truly general statement. We could also say that for maximum acceleration you spend your time between idle and redline, true but not useful . In many cars during a maximum acceleration run, you do not get anywhere near as low as the rpm of peak in gear torque (engine and wheel torque), unless you begin from a stop and then only in 1st gear. Maximum overall acceleration occurs by being as close to peak power for as long as possible. From there the only other "rules" are stay below redline and shift according to the drive power curves.

I know this is obvious, but watch the video (skip to 17 seconds in). Peak torque and peak in gear acceleration are around 3000-3500 rpm, peak power of course is just a hair below redline. Except 1st gear the car is mostly above 5000 rpm and in many gears above 6000 rpm the entire time. Now granted the M3 produces engine torque levels very close to peak torque well beyond 3500 rpm but the same conditions hold for engines with more peaky torque curves.

The real "power band" on any car is simply from as low as the rpms get post shift all the way to the next shift point, often at or near redline.

hp, hp, hp, period.