FWIW, torque is a measurement of force, with no regard for time. HP is a measurement of power, which is force applied over a given period of time. The formula to calculate HP at a given RPM from torque would be HP = RPM * torque / 5252. Measuring torque at the engine says nothing by itself of potential for acceleration. HP (not just peak, at any RPM) is much more relevant, since relatively low engine torque at a high RPM geared down may produce more torque at the wheels than relatively high engine torque at a lower RPM. A torque curve will show you at what points in an engine RPM continuum you will accelerate more if the gearing is a constant.
There is a measurement BSFC (Brake Specific Fuel Consumption; units are lbs of fuel per HP * Hour) which may be relevant to this discussion. BSFC does not necessarily correspond with Volumetric Efficiency (a measure of the air pumping efficiency of an engine). Peak VE in a well tuned engine does generally occur near peak torque, which makes sense when you think about it. However, BSFC does not seem to have such a correlation. Even if it did (in theory), there are a lot of competing factors and concessions being made during design and production of engines which prevent them from being delivered in an optimally tuned state.
I'm not really qualified to speculate much here but I believe internal losses will go up proportionately if not exponentially with RPM (friction, fluid dynamics on bearing surfaces, and other stuff that I have no clue about). So I would think peak (low) BSFC would occur at a trade off point of when the engine is beginning to get into an efficient combustion range (ie; moderately high dynamic compression ratio, decent swirl/turbulence in the combustion chamber making for a uniform fuel dispersion, and of course reasonable fuel metering, ignition timing), while being at as low an RPM as possible in order to lessen internal losses.
If you guys are really interested in this, there is a program called Engine Analyzer which I believe has a free demo you can download. You can plug in all kinds of data for engines, and it will model the engine in software and you can experiment with different with modifications, like lowering the CFM on the intake to mimic a lower throttle opening. I ran a few simulations through with a 2003 Ford Focus Stock engine model provided by performance trends. The peak (low) BSFC appears to occur around 1500 rpm, while peak torque occurs around 4500 rpm and HP around 6000 rpm.
It would be great if someone had an engine definition for our boxers...
Rob
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