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LIGHTENED AND UNDERDRIVE CRANKSHAFT PULLEYS
Lightened and Underdrive pulleys are supposed to serve three purposes. They reduce rotating mass and increase the horsepower available from the engine, they increase accessory life by lowering the operating RPM of the accessories and increase available horsepower by reducing the horsepower used by the accessories. All of these claims are undeniably true. The real question is how much horsepower do you really gain.
Let's assume that you are autocrossing, you are accelerating in 2nd gear, your engine RPM is 6000, your rate of acceleration is 800 RPM/sec, your air conditioning is "off" and your battery is near full charge. The "original" crankshaft pulley was 6.25" in pitch diameter and weighed 5.0 lbs. The lighted underdrive pulley is aluminum and is 4.5" in pitch diameter and weights 0.91lbs. The alternator driven pulley is 2.5" in pitch diameter, the alternator armature is 3.5" in diameter and the pulley and armature weigh a total of 4.0 lbs. These values are hypothetical but are close to the actual values for a 1991 Honda Prelude SI.
Now let's forget about claims and calculate the theoretical horsepower which can be gained for accelerating the car.
Original Crankshaft Pulley
Power for producing alternator electrical output:
hp = [Amps * Voltage] / [746*0.9]
Assume: alternator output = 35 Amps at 14.7 Volts
Then: hp = [35*14.7] / [746*0.9] = 0.766 hp
Power to rotate the alternator armature and driven pulley:
Alternator RPM = 6000*6.25/2.5 = 15,000 RPM
Alternator RPM rate of change = a = 800*6.25/2.5 = 2000 RPM/sec
Inertia of alternator armature = I = 0.5 (1.75)2 (3.0)/386 = 0.0119
hp = [rpm * (a) * I] / 601,848 = 15000*2000*0.0119 / 601,848 = 0.593 hp
Power to rotate the crankshaft pulley
Engine RPM = 6000
Engine RPM rate of change = a = 800 RPM/sec
Inertia of crankshaft pulley = I = [(0.5)(3.125)2 (5.0)] / 386 = 0.0632
hp = [rpm * (a) * I] / 601,848 = (6000)(800)(.0632) / 601,848 = 0.504 hp
Total power consumed = 0.766 + 0.593 + 0.504 = 1.863 hp
Lightened Underdrive Crankshaft Pulley
Power for producing alternator electrical output:
hp = [Amps * Voltage] / [746*0.9]
Assume: alternator output = 30 Amps at 14.7 Volts
Then: hp = {30*14.7] / {746*0.9] = 0.657 hp
Power to rotate the alternator armature and driven pulley:
Alternator RPM = 6000*4.5/2.5 = 10,800 RPM
Alternator RPM rate of change = a = 800*4.5/2.5 = 1440 RPM/sec
Inertia of alternator armature = I = 0.5 (1.75)2 (3.0)/386 = 0.0119
hp = [rpm (a) I] / 601,848 = 10800*1440*0.0119 / 601,848 = 0..308 hp
Power to rotate the crankshaft pulley
Engine RPM = 6000
Engine RPM rate of change = a = 800 RPM/sec
Inertia of crankshaft pulley = I = [(0.5)(2.25)2 (0.91)] / 386 = 0.00597
hp = [rpm (a) I] / 601,848 = (6000)(800)(.00597) / 601,848 = 0.0476 hp
Total power consumed = 0.657 + 0.308 + 0.0476 = 1.013 hp
Power saved by using a lightened underdrive crank pulley
Power saved = 1.863 - 1.013 = 0.85 hp
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