Gearing

When choosing the gear for max acceleration we keep the RPM close to max HP so why do people say that with a race engine

the torque number is the one to look at even though the RPM never goes down to where the engine makes max torque.

Like most things it's a mis-understanding that people repeat without thinking about it - there are torque figures throughout the rpm range, just as there is power.

Where I believe it originated is when engines were very 'peaky', with high power but over a very narrow rpm range, and "the area under the curve" was not just lower, but drastically lower when changing up.

Might help to think about it as a turbocharged engine with a too big turbo' - it might make huge power on boost between 8 and 9k, but when changing up the rpm drops to 7500 and it drops off boost and is sluggish until it gets to 8k and takes off again.

Many who don't understand the torque, rpm and power relationship think "torque" is like low rpm power and so they say "torque" is the important thing, when it's actually the power spread/band below and above the power peak.

It is possible for the torque peak, and the power peak, to occur at the same rpm, but it's impossible to have the torque peak after - I don't know of any engine that does that, but some do come close, like motorcycle and race engines.

Great stuff Gord

Since the formula for Power (hp) is Torque (lb-ft) * RPM / 5252, Torque (lb-ft) and Power (hp) will always be equal at 5252 RPM. In general above this RPM, HP will be numerically higher than torque.

The really cool part, is we can always increase the torque at the wheels by using gearing. But the RPM limitation is what stops this from being practical.

Proper gearing analysis is done with wheel torque vs. vehicle speed. Increase the gear ratio as long as you can hit the maximum speed on the longest straight to have the maximum acceleration.