Parasitic Horsepower Losses
Home | Free Mods | Intake | Ported Heads & Manifold | Exhaust | Gears | Transmission | Brakes | Parasitic HP | Fuelling | Interior | Weight | Performance | HP Gains | PartsA car's engine does many things other than just make the car move. Power is used to run various accessories like the alternator, power steering pump, AC compressor, water pump, and smog pump. All of these use horsepower that could be used instead to make the car go faster.
Each accessory uses rotary power to perform it's function, whether it's an electrical component (the alternator) or a fluid displacing component (the water pump and power steering pump). They all have a pulley which takes power from the crankshaft pulley via a belt. Some of the engine accessories take more power than they really need, dramatically affecting engine performance.
An underdrive crank pulley is a replacement pulley that bolts up to the crankshaft like the stock unit but with a smaller diameter. This increases the ratio of the accessory pulley diameters to the crank pulley diameter, reducing the speed at which the accessories turn relative to the crankshaft and freeing up lost horsepower.
ASP Underdrive Crank Pulley
ASP offer an underdrive crank pulley (part no. ASP-523500) for the '94-'95 Mustang 5.0L V8. The ASP pulley is 5" in diameter compared to 6-3/4" for the stock crank pulley (part no. F4ZE 6A312AA), slowing down the rotation of other belt-driven accessories (alternator, PS pump, AC compressor, water pump, smog pump) by 26%. This reduces the parasitic horsepower loss caused by the crankshaft driving these accessories, resulting in a 3-5rwhp gain. Considering that the pulley costs a mere $39, this is a great bang per buck mod.
I installed the ASP crank pulley in less than 30 minutes using simple hand tools. The smaller diameter pulley required the use of a shorter drivebelt (Gates K060868, 2215mm) since I'd already removed the smog pump. This belt was a good tight fit.
Short Belt Smog Pump/AC Bypass
The smog pump and AC compressor can be bypassed by using a shorter drivebelt and modifying its routing. Though each accessory by itself takes very little power away from the engine when the pulleys are free-spinning, the effect of bypassing both is a further 3-5rwhp gain. The combined effect of the ASP crank pulley and short belt smog pump/AC bypass is a ~8rwhp gain.
The illustration shows the belt length that's required with the stock crank pulley. My engine has the smaller ASP crank pulley so it required a 6PK1993 (78.5") drivebelt.
Ford Racing Aluminium Driveshaft
The Ford Racing aluminium driveshaft M4602G fits on all Mustang models from 1979-1995. It weighs a mere 11lb (5kg) compared to the stock steel driveshaft's 20lb (9kg) but has the same 1.75" radius. The reduced mass requires 1.3lbft less engine torque to make it rotate, so that extra torque is transmitted to the rear wheels to make the car accelerate faster.
Since the rotational mass is almost halved, the driveshaft is less likely to vibrate in the useable road speed range and the driveshaft's critical speed is increased by 35%. This offsets the 36% increase in driveshaft speed caused by replacing the stock 2.73:1 ring & pinion with the Ford Racing 3.73:1 unit.