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      07-21-2008, 09:37 AM   #24
bruce.augenstein@comcast.
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Quote:
Originally Posted by footie View Post
Maybe Audis are different because every single one I have owned accelerate as quickly if not quicker when shift occurs at peak power, revving the extra 500rpm has not benefit what so ever.

You want to be shifting into that thick torque band and until your torque is also far up the rev range then there is no benefit holding on to the gear.

I can only gauge from experience and that is what I have found on every occasion.
footie, I think we can agree on the basics. Then we'll go from there.

The basic idea that I think we can agree on in this context is that, at any given vehicle speed, more power means more acceleration. No ifs, ands or buts involved. Gearing doesn't matter. Torque doesn't matter. It's just power and weight. As a follow on to that, any car will accelerate more quickly from one speed to another speed if it's making more average power between those two speeds than if it's making less average power.

You can work out torque at the drive wheels if you like, but you'll find that, 100% of the time, more power equals more torque at the drive wheels, which then equates to more acceleration at any given speed.

If we can't agree on this, then just stop reading here, and we'll agree to disagree. In fact, even if we agree, you can probably just stop reading here, since, if you think about it, you can see that revving past the power peak to some degree will give you more average power at the wheels. This is because you'll be making more power after the shift than if you shifted earlier.

However, from a driver's perspective, the problem is that, in any given gear, more torque means more acceleration, so any car will accelerate the hardest in that gear while at the torque peak, and will accelerate with less urgency at the power peak than at the torque peak while in that gear.

Furthermore, there is a physiological component to this as well. Human G receptors are lousy at differentiating actual acceleration forces, but are very sensitive to rates of change. Therefore, when accelerating in a given gear, torque production is falling off precipitously past the power peak (it's dropping even faster than rpm is climbing), and as a driver, you can feel that drop off in acceleration. Then, when you grab the next gear, the car gets that jump from the engine's rotating inertia (it slows down to normalize to the next gear, expending that energy to momentarily accelerate the car), and the car is near the torque peak in that next gear, pulling about as well as it can in that gear. In other words, it feels healthy in that gear, as opposed to the dying swan feeling in the previous gear.

So footie, when you say that there's no benefit to revving past the power peak, you're relying on what torque on your backside tells you, and not a stopwatch or time slip at the drags.

"Torque is what you feel, but horsepower rules."

For the reasons mentioned, an M3 would be even quicker than it is now if it could rev another 1000 rpm or so past the power peak before the shift.

Bruce

Edit: PS - I am ignoring dynamic rotational inertia factors which vary gear by gear, since I don't feel like arguing the point anymore with lucid and Swamp. Suffice it to say that, this factor included, you still need to maximize average power at the wheels to get best acceleration.

Last edited by bruce.augenstein@comcast.; 07-21-2008 at 10:21 AM.
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