Engine physics question...

[hcoles]

a fairly interesting excercise is to develop/understand for yourself the equation that converts torque to horsepower...

step 1 - convert torque (foot pounds) to energy per crankshaft revolution also foot pounds - this is sometimes a point of confusion
setp 2 - multiply by crankshaft speed (revolutions per minute) this gives foot pounds per minute
setp 3 - divide by 550 * 60 foot pounds per minute per hp.
result = hp at that engine speed

Example:
rpm = 5000, torque = 100 ft - pounds
force at 1 foot = 100 pounds force
distance the force is applied = 1 foot * 2 * pi = 6.283 feet (the force is applied around the circumference of a circle)
so
the energy put out per revolution = 6.283 feet * 100 pounds = 628.3 foot pounds

the energy put out per minute is 628.3 foot pounds per rev * 5000 rpm (revs per minute) = 3,141,590 foot pounds per minute

the definition of a hp = 550 foot pounds per second
or

33,000 foot pounds per minute

the horse power at this rpm is therefore

3,141,590 foot pounds per minute / 33,000 foot pounds per minute per hp = 95.2 horse power

[masraum]

Originally Posted by Robert Buchanan

After reading all of the "technical replies" I thought that a common sense one should be offered. In the example given...."car #1" simply has 30% more power and torque.
30% is a lot in terms of just about anything....30% more money, 30% more property, 30% more power....it's a common sense thing.
The one I like the best is 30% more 'time'....
If you have ever stepped out of a 3.2 Carrera and jumped into a 3.3 Turbo, you would understand power and torque easily. Going from 205 HP to 282 HP is a vast difference.
911_pilot

Robert, the OP example was 200hp vs 300hp. That means that the 300hp model has 50% more power than the 200hp model.

edit-- I see, maybe you are saying that the 200hp model has 33% less power than the 300hp model.

[Robert Buchanan]

Steve,
Your math is correct. The actual "paper" calculations are 50% more power. How does this relate to a 911. Easy, drive a 3.2L and then borrow your brother-in-laws Turbo. It won't take math to obtain a clear understanding of the principle of "more power".