ok, I copied this bit on torque from my 400 hp V8 vs. 400 hp 4 cylinder report (in which I didn't copy from another place):
if anyone's curious on that V8 vs I4 topic:https://www.celicahobby.com/ubbthrea...part=1&vc=1
barefoott4 gave the correct mechanics (physics) definition but here is a mechanical/automotive explanation
maybe a bad assumption, but this assumes knowledge of the 4-stroke cycle and basic engine components like what a cylinder, piston, connecting rod and crank are.
Torque is the twisting force developed at the crank by the power strokes of the cylinders at a certain rpm. How can we increase the force? Many ways: Add more cylinders or more displacement (volume of air and fuel to be ignited). We can up the compression in the cylinders (more potential energy). We can increase the volumetric efficiency (VE) for a particular engine. There is a direct relationship between torque and VE, which is a measure of how well an engine is able to fill and discharge its cylinders with air and fuel at a certain rpm. The greater the VE the greater the force imparted on the pistons by the ignited air/fuel mixture, leading to more torque on the crank. Many factors influence VE, and it is possible to increase VE by using Forced Induction (F/I). The higher an engine revs the less air it is able to take in and discharge on each 4-stroke cycle. This is why all torque curves trail off on the upper rpm range.
Torque is what is real and measurable. Dynamometers measure torque and calculate horsepower. Power or horsepower is a concept. It is a concept of work output over a certain amount of time. The more torque applied over a distance (revolution), the more work, and the more work over a certain amount of time, the more horsepower. It does no good if a force is generated and work is done, if it is done slowly. It is the horsepower curve that is the best description of how well a car accelerates. Torque matters because that is the measurable force we use to calculate horsepower. We also learn much from the torque curve of an engine as it indicates the engines volumetric efficiency according to rpm, and this info is useful for tuning purposes. And more peak torque generally means more low-end horsepower.
Horsepower = (Torque x RPM)/5252
The 5252 is the factor to make everything end up correctly in the right units.
Torque is in lb-ft.