Grimmspeed Lightweight Crank Pulley Red For Subaru FA/FB EnginesGRM 095024 Proven Performance. Your factory crank pulley is responsible for transferring energy from your crankshaft to your water pump, a c compressor and alternator. It does this by way of a rubber belt and it does it's job well. The drawback to the factory pulley is that it's quite heavy. Reasons for this include less expensive manufacturing and using the pulley's mass to smooth out 'noise' in the accessory system (NOT to be confused with