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Talk the torque...

In simplest terms, torque is a measure of the force created by the combustion process, as transmitted via the crankshaft through the transmission and ultimately to the rear wheel. Horsepower is a measure of how quickly the engine and drivetrain can deliver that force.

It is possible to have an engine that creates an enormous amount of torque, and yet have relatively low horsepower. Consider one of the earliest forms of "engines": a waterwheel. Say you see a waterwheel that has a diameter of 12 feet; with a rotating mass of 400 lbs; and that is rotating at a maximum velocity of 12 RPM. Such an "engine" has a torque output of: 6' (radius) x 400 lbs mass = 2400 lbs/ft of torque. (By way of comparison, a Twin Cam SE Stroker 103CI engine puts out a peak of about 110 lbs/ft of torque). You'd think that with this much torque you'd have a pretty good horsepower figure, right? But according to the formula (torque x RPM)/5250 you only end up with 5.48 HP.. not very impressive. A bike powered by an engine with similar torque and HP characteristics would accelerate like hell until it got to about 6 mph or wherever the 12 RPM "redline" occured (provided you could prevent wheel spin..but thats another story) and then it would max out. The secret then is being able to produce torque at high RPMs...

You also asked the question why do torque and horsepower both drop off after a certain point ..

As an engine increases the number of RPMs it is doing, several things are happening. The most obvious is that more fuel and air are being combined per unit of time.. this is what leads to the steady increase in horsepower output in the early part of the range. Torque also increases because the pistons are being hurled downwards at higher speed (Force = mass x acceleration). But as RPMs increase greater inefficiencies start to creep in. As engine speed rises, there is more friction loss, not only between the pistons and the cylinders, but also in the crankshaft and cam bearings. These are mechanical inefficiencies. You also lose more energy to heat loss - thermal inefficiencies. Lastly you will reach bottlenecks in the intake and exhaust systems. The engine cannot combine the fuel and air as efficiently, so more is wasted. Ultimately you reach a point of diminishing returns - putting more fuel and air into the engine creates more friction than the energy its combustion provides. It may be possible for the engine to keep increasing its speed (RPMs), but the resulting torque is so low that net horsepower also begins to drop.

Succesful engine design therefore seeks to achieve two aims: as high a torque figure as possible, across the widest possible range of engine speeds. Engine parts are designed to minimize friction losses at high speeds, but also utilize high compression ratios and large cylinder volumes to create the torque.
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