The Pros & Cons of an Underdrive Pulley

When the diameter of an accessory pulley is increased, the revolutions per minute of the pulley decreases, which cuts down on parasitic drag of the component's demands. As much as 15 horsepower gains can be realized at the wheels with an underdrive pulley kit, depending upon the number of accessories involved. Underdrive pulleys typically replace the stock water pump, power steering, AC unit and alternator. Except for the AC unit, which can be periodic in use, the other gains can be noticeable and even add to acceleration.

Belt wear is substantially reduced with the replacement of an underdrive pulley, since the belt makes less frequent contact with the pulley guides as a result of reduced rotation. Belt flex, durability and weathering is reduced, with the possibility of extending belt life past the manufacturer's specifications.

Since the underdrive pulley turns less -- in some cases, 30 percent less -- the wear and tear on bushings and sealed bearings is reduced, extending the life of the bearings, shafts, seats and races. Accessory bearings which would benefit include the AC unit, water pump, alternator and power steering pump bearings. Clutch rotation on the AC compressor would also decrease with an underdrive pulley.

Most manufacturers provide a buffer zone for the performance parameters of their belt-driven components. For instance, alternators have the capability of additional output, even past maximum specifications. However, if the vehicle has an underdrive alternator pulley and all the components and accessories are running at the same time, it causes a significant voltage drop in the charging system. The battery can drain faster; the engine electrical system will not get the voltage it needs to fire the spark plugs, and the headlights can dim. The power loss would be compounded at vehicle idle, where the engine rpms are at their lowest threshold. The alternator would not spin fast enough to provide adequate current for all of the accessories.

If the water pump pulley is replaced with an underdrive pulley, and the cooling system passages have clogs and leaks, the reduced speed of the water pump impeller will slow the coolant flow to the engine. This could lead to higher than normal operating temperatures, and if the underdrive pulley is radically oversized, overheating could result.

If the vehicle owner has installed aftermarket modifications that are of the high-performance design, the possibility exists that an under-driven alternator pulley will not produce enough voltage to activate and run the add-on system or component. Such systems would include high-output stereo amplifiers or multiple high-performance speakers. A dual battery setup would suffer since it would not receive adequate charging voltage. Modified ignition, such as high-output coils, racing spark plugs and aftermarket turbo chargers, would lack the high voltages needed to run them. The addition of an electric fuel pump or multiple electric aftermarket cooling fans would put too much of a voltage draw on an underdrive pulley.