Horsepower vs. Torque ?

[ChipBeck_imp]

Horsepower does work and is what accelerates the vehicle. Torque is a simple force and can be increased or decreased without a change in horsepower. Torque X speed = horsepower. Neither one is energy.

I don't understand why anybody even talks about horsepower. To elaborate a little bit on what Dakota said:

Horsepower = Torque x RPM

If you know the torque curve, then you know the horsepower curve automatically. The confusion between torque and energy is that they share the same unit, the newton-meter (http://en.wikipedia.org/wiki/Torque#Units). Torque is a measure of how much force is transferred to the crank each rotation, so you multiply by how fast the rotations are to get the total power.

Gentlemen,

DBXXX nailed it in the post quoted above. Forget everything else that has been guessed or theorized about in this thread. Torque is turning force at the crank. Horsepower is that turning force multiplied by the number of times that force is generated in one minute. If torque is 10 and RPM is 10 that is the same amount of horsepower as another engine where torque is 5 and RPM is 20.

To create a mental picture think of an engine that is built to lift weight straight up on a pulley. If one rotation of the crank would lift one pound of weight one foot high then the torque is 1 foot pound. But that doesn't tell us squat about the work that engine will do without knowing how many rotations per minute the engine will turn. If a motor with 1 foot pound of torque only turns 10 times a minute (10 RPM) it will lift that 1 pound weight 10 feet in a minute. Another engine with the exact same torque that spins at 30 rotations per minute will lift that 1 pound weight 30 feet in one minute. Horsepower is a measure of how much work an engine will do. Torque is only a measure of turning force.

Now if you cut the torque in half but double the RPM, horsepower remains constant. In the last example above we have an engine with 1 foot pound of torque turning 30 RPM lifting a 1 pound weight 30 feet in a minute. A different engine with half the torque that spins at 100 RPM will lift that 1 pound weight 50 feet in a minute.

This is a simplified example for engines that run at constant RPM but you get the idea. Torque is meaningless as a measure of power without multiplying it by RPM. Given optimum gearing and holding other variables constant, two motorcycles with 100 HP engines will accelerate at the same rate even if one of them has only half the peak torque of the other. The lower torque engine must turn twice the RPM to do the same amount of work.

In short, it is horsepower, not torque, that determines how fast a motorcycle will accelerate or how much total work any engine will do.

Some people like the feel of slow turning engines that chug along at 2000 RPM like Harley Davidsons. High torque per turn is important here as that turning force is only being generated 2000 times a minute in 6th gear at 50 mph. A 600cc sport bike that is turning 6000 times a minute in 6th gear at 50 mph may only have half the torque per revolution but that turning force is being generated 6000 times per minute or 3x as often. So that sport bike will have 1.5 times the power of the Harley and will accelerate much faster. Comprende'?

Cheers.

Chip