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Having been away for the weekend, I came back to find everything straightened out on this thread. HP and torque explained. Great! Well almost.
"HP takes gearing out of the equation"????????????
Not sure that I am not taking this out of context, but it can certainly be misleading. If we are talking about the acceleration equation, gearing is still a key element and is really separate from the discussion of HP v torque curves.
The car that makes HP at a higher RPM will usually accelerate faster because it can use a higher gear ratio and the higher ratio gives it more mechanical advantage. Most people think that you want to gear a car and drive it so that
you shift at or near the power peak. But the concept of the mechanical advantage obtained from a higher gear ratio means that you will accelerate faster in a lower gear (higher ratio) well past the point at which you could shift into the next gear at an engine RPM where the HP would be much higher. The same is true whether we are talking about final gearing or transmission gears. All other things being equal, given similar HP numbers, the engine that makes the HP at higher RPM will accelerate a car faster as it can utilize gearing that provides a greater mechanical advantage. More turns of the engine per turn of the wheel produces more force at the wheel whether you look at it in terms of HP or torque.
An example related to this can be seen from my last visit to Watkins Glen.
About 1/2 way down the back straight I was up shifting at approx. 7,500.
The engine was tuned so that the torque peak was at 5300 RPM and the HP peak was at 6500. On shifting into the higher gear the rpm would drop to approx. 7,000 rpm but the car would only accelerate very slowly and only gain a couple of hundred rpm. On a subsequent lap I stayed in the lower gear and the car continued to accelerate past 8,000 rpm, attaining a faster top speed. It surprised me that being so far past the power peak there was still an advantage to staying in the lower gear.
This gearing and mechanical advantage concept is very imporant for the drivers of old Volvos. Stock,the power peak on a FI B20 is listed as 6,000 RPM. The rear end gearing is very high compared to most cars, usually 4.3. With a power peak at 6,000 RPM ( dyno testing has not shown it to be this high ) you would have to shift at over 7,000 RPM to get the best acceleration. Yet many owners persist in shifting at 4,500 rpm, or 5,000 rpm at the max, and then complain that their cars are not very fast. How would they know? They are not only shifting well before the designed power peak but are also wasting the mechanical advantage of staying in the lower gear for another 2,000 rpm. Despite stories that the B18/B20 engines are "tractor" engines, they are not low rpm engines, and need to be "wound out" to reach their potential.
John
VPD
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