Underdrive pulleys

An underdrive pulley refers to a crankshaft or accessory pulley (a/c, alternator, power steering, water pump, etc.) designed to turn at a slower speed than stock. To Underdrive means to slow the rate of rotation in a system. Underdrive is achieved by either making the crank/main (drive) pulley smaller or the accessory (driven) pulley larger than the original diameter pulleys.

Underdrive pulleys increase engine output by reducing the draw of the engine's accessories by slowing them down and reducing the HP they use. Horsepower gains from underdrive pulleys can vary by vehicle, engine, number of accessories and the amount of underdrive (Improvements of up to 5-15 HP at the wheels have been seen). Additional and significant performance improvements can be seen by reducing the weight of the pulley versus the original pulley. Gains can range from 3-6 HP per pound of weight reduced.

Poorly engineered underdrive pulleys can cause unwanted side effects; this is due to not spinning the alternator, power steering, and/or air conditioning fast enough. This leads to low alternator voltage, weak/no power steering assist, and weak/no air conditioning effectiveness, especially at idle/low RPM. The most commonly seen result is lighting may dim, or the stereo may cut out. Too much underdrive for a race car is not much of a concern due to the high RPMs they run at, but for daily driven vehicles it can lead to a dead battery if too much time is spend at idle or low RPM.

In addition, changing the original crankshaft pulley can have negative effects if the replacement pulley is not manufactured properly. A crankshaft or accessory pulley not machined or balanced properly can cause severe damage leading to thousands of dollars in repairs.

References

Automotive engine

Part of the Automobile series

Basic terminology	Bore Compression ratio Crank Cylinder Dead centre Diesel engine Dry sump Engine balance Engine configuration Engine displacement Engine knocking Firing order Hydrolock Petrol engine Power band Redline Spark-ignition engine Stroke Stroke ratio Wet sump

Main components	Connecting rod Crankcase Crankpin Crankshaft Crossflow cylinder head Crossplane Cylinder bank Cylinder block Cylinder head Flywheel Head gasket Hypereutectic piston Main bearing Piston Piston ring Reverse-flow cylinder head Starter ring gear Sump

Valvetrain	Cam Cam follower Camshaft Desmodromic valve Hydraulic tappet Multi-valve Overhead camshaft Overhead valve Pneumatic valve springs Poppet valve Pushrod Rocker arm Sleeve valve Tappet Timing belt Timing mark Valve float Variable valve timing

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Ignition	Contact breaker Magneto Distributor Electrical ballast High tension leads Ignition coil Spark plug Wasted spark

Electrics and engine management	Air–fuel ratio meter Alternator Automatic Performance Control Car battery (lead–acid battery) Crankshaft position sensor Dynamo Drive by wire Electronic control unit Engine control unit Engine coolant temperature sensor Glow plug Idle air control actuator MAP sensor Mass flow sensor Oxygen sensor Starter motor Throttle position sensor

Exhaust system	Automobile emissions control Catalytic converter Diesel particulate filter Exhaust manifold Glasspack Muffler

Engine cooling	Air cooling Antifreeze (ethylene glycol) Core plug Electric fan Fan belt Radiator Thermostat Water cooling Viscous fan (fan clutch)

Other components	Balance shaft Block heater Combustion chamber Cylinder head porting Gasket Motor oil Oil filter Oil pump Oil sludge PCV valve Seal Synthetic oil Underdrive pulleys

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