CC QuickiesPulley SpeedWhen calculating the speed of accessories such as alternators and superchargers, remember that even if the same belt is driving multiple accessories and idlers, you only need to consider the size of the drive pulley (crankshaft) and the one accessory pulley you are working on. Just divide the diameter of the drive pulley by the diameter of the driven pulley to get the ratio. For example, a 6-inch crank pulley and a 3.25-inch alternator pulley will spin the alternator at 1.85:1. Therefore, at 6,000 crank rpm, the alternator will turn 11,100 rpm (6,000 x 1.85).
CC QuickiesRearend RatiosHere's how we decide upon rear gears for a drag car. First, we estimate the mph the car should run in the quarter using horsepower and weight. For example, a 3,600-pound car with 750 hp usually runs about 130 mph. We want to cross the finish line just past the engine's peak horsepower rpm, say 6,800, so we need the combination of gears and tire size that deliver 130 mph at 7,000 rpm. With an automatic, we underestimate to account for converter slippage. In our example, 4.30 gears and 28-inch tires mean 6,708 rpm at 130 mph; with 5 percent slippage, that's about 7,000 rpm.
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