Calculating HP with G

[Will McGonegal]

>           Just wondering how to calculate HP(torque?) with g .

Don't confuse up your Horsepower and torque.  If you measure Horsepower at the
wheels and want engine torque and can't simultaneously measure engine speed, you
have to know the radius of your drive wheels and the gear ratio from the engine to
the wheels (torque multiplication-speed reduction).  The Horsepower remains the
same through the system (assuming no loses!).  If you calculate wheel horsepower
and you know the equivalent engine RPM to vehicle speed then you can determine the
effective engine torque.G is a measurement of acceleration.  1 G of acceleration
is 9.80665 meters/(second squared).

Power = Force * velocity = Mass * acceleration * velocity.

If you're accelerating at 0.1 G in a 3000 lb car going through 25 mph........

0.1 G is 0.1 * 9.80665 = 0.980665 m/s2
3000 lbs = 3000 / 2.2046 kg = 1360.8 kg
25 mph = 25 * 0.44704 m/s = 11.176 m/s

Power = 1360.8 [kg] * 0.980665 [m/ss] * 11.176 [m/s] = 14 914.248 Watts
14 914.248 / 745.69 [Watt / Hp] = 20 Hp

Simplified......

Horsepower = Mass [lbs] * Acceleration [G] * Speed [mph] / 375.0

(the 375 factor comes from 3600 [seconds/hour] / 5280 [feet/mile] * 550
[ft.lbs/sec per horsepower]

Now if you happened to know that the engine was reving at 2000 rpm when the
vehicle was traveling 25 mph.......

Engine torque (assuming no drivetrain losses (HA!)) = 20 [Hp] * 5252.1131 / 2000
[rpm] = 52.5 ft-lbs.

(the 5252.1131 factor comes from 550 [ft.lbs/sec] * 60 [seconds/minute] / ( 2 * pi
[3.1415....] ) )

Will McGonegal
Project Engineer/Dynamometer Developer
Environment Canada