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Old 01-25-2017, 12:55 PM   #11
Scooby Specialist
Member#: 395882
Join Date: Jul 2014
Chapter/Region: MWSOC
Location: GR, Michigan
2016 WRX


Originally Posted by wadeh View Post
Think of it this way. Engine torque is a measure of cylinder pressure. Torque is a measure of force, and it has nothing to do with being averaged over anything. Power is the effect of torque over time. Consider applying a wrench to a stuck bolt. Applying force to the wrench is torque. But no work is done until the bolt starts to move. I agree that a dyno cannot measure torque directly. It measures power by looking at the effect of the pull on the dyno. It then calculates torque.
YOUR ENGINE IS MOVING. If you take POWER and DIVIDE it by TIME (RPM) you get WORK not FREAKING TORQUE. Torque is the lazy way to describe what's happening. Each piston is doing work and that's being translated through the crank mechanism into torque, but if your engine stops moving, no torque for you.

The reason low RPMs are bad are not because of higher BMEP (that's cylinder pressure averaged over the stroke, and gives you this "torque" figure that has nothing to do with rods blowing up), but because of higher PEAK pressures. When the volume stays small for longer, as in the piston moving slower at low RPM, the pressure goes up enormously for a time, until the piston starts moving down to allow for expansion. At higher RPM, the piston moves much sooner, in terms of time, and peak pressures don't get as high. The ~same amount of energy is added to the system over the stroke, but the high compression force in the rod isn't there because the burn doesn't complete until the piston has retreated.

If you are having trouble understanding how torque and power are truly related, this is a the best video I've found to explain it:

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