8th Generation Honda Civic Forum - View Single Post - Everything You Wanted To Know About Car Measurements(HP, TQ, Gear Ratios etc.)

ryker · 12-14-2007, 08:30 AM

Units of Shaft Power

When using pound-feet as units of torque, revolutions per minute (RPM) for shaft speed, and horsepower for power, shaft power can be expressed with the following formula:

Shaft power in horsepower.

The above power formula is often misinterpreted as showing that power and torque are the same thing, or that they somehow trade hands with each other at 5252RPM. This mistake is from the fact that a graph of torque in pound-feet and power in horsepower versus engine RPM has crossing lines at 5252RPM. Torque and power play the same role whether the engine is revving below, at, or above 5252RPM. Many diesel engines, and even some gas engines, are not even capable of revving that high at all.
The above statements can be proven by changing the units for power and torque. Let's use kilowatts for power, and Newton-metres for torque, just like the Aussies do. With that, we get:

Shaft power when using metric units.

The constant used for calculation is now 9549, not 5252 like it was when we were using pound-feet and horsepower. This means that a graph of power and torque versus revs using metric units would have crossing curves at 9549RPM instead of 5252RPM.

12-14-2007, 08:30 AM	#17 (permalink)
ryker Senior Member Join Date: Sep 2007 Posts: 10,065	Units of Shaft Power When using pound-feet as units of torque, revolutions per minute (RPM) for shaft speed, and horsepower for power, shaft power can be expressed with the following formula: Shaft power in horsepower. The above power formula is often misinterpreted as showing that power and torque are the same thing, or that they somehow trade hands with each other at 5252RPM. This mistake is from the fact that a graph of torque in pound-feet and power in horsepower versus engine RPM has crossing lines at 5252RPM. Torque and power play the same role whether the engine is revving below, at, or above 5252RPM. Many diesel engines, and even some gas engines, are not even capable of revving that high at all. The above statements can be proven by changing the units for power and torque. Let's use kilowatts for power, and Newton-metres for torque, just like the Aussies do. With that, we get: Shaft power when using metric units. The constant used for calculation is now 9549, not 5252 like it was when we were using pound-feet and horsepower. This means that a graph of power and torque versus revs using metric units would have crossing curves at 9549RPM instead of 5252RPM. Last edited by ryker; 12-14-2007 at 01:17 PM.