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with an underdrive crank pulley, the alternator and water pump will spin at a lower RPM in relation to the engine rpm. Worth noting that the alternator will start producing charge at a higher engine RPM as a result. e.g std charge starts at ~ 1500 engine rpm, when being driven via a undersize crank pulley, the charging will occur around the 2000 rpm there abouts. Ideally underdrive pulleys are suited to high RPM engines when you want your alternator and water pump to operate at more realistic speeds.