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A performance pulley is an engine drive pulley that is specifically designed to give superior performance under the harshest of competition conditions. While a stock or inexpensive racing pulley will often retain the V-belt grooves or a Gilmer®-type belt configuration, a performance pulley will usually be designed to use a high torque drive or HTD belt. The HTD belt and performance pulley system use a rubber belt with deep-round ribs that fit inside of deep recessions in the pulley. This design has been proven to withstand much greater power without slipping and also performs well at high engine speeds.
Often, a racing engine is designed to accelerate so quickly that anything short of a performance pulley and belt will slip and fail under the extreme acceleration. Every time a belt slips, it becomes a slight bit looser until the belt is continually slipping at all engine speeds. Cooling systems and charging systems are the first to suffer the effects of a non-performance pulley-related belt failure due to the extreme amount of torque the two systems place upon the belt drive. The design of the HTD belt requires less retention force to keep it on the pulleys where it is intended to be.
The effectiveness of the HTD belt and performance pulley system is so renowned that the most powerful racing engines now utilize this belt drive system in place of a timing chain or gear drive to drive the camshaft. The belt requires less power to spin and is more accurate in retaining the initial setting than either the chain or gear drive systems. The performance pulley can also be manufactured in a plethora of sizes to make subtle drive speed changes possible with any accessory the system drives. The ability to change the driven speed of any engine accessory is tantamount to adding several horsepower to the engine. This free horsepower can be the difference between a winning or losing racing vehicle.
When installing a performance pulley, it is critical that all pulleys in the system line up exactly. Using shims and taking several measurements, an engine builder is able to ensure that the drive belt is subjected to no unnecessary side loading caused from a slightly out-of-line pulley. Both the performance pulley and the mounting surface it attaches to is ground or machined perfectly flat to eliminate any unwanted wobble that could result in a thrown belt at high engine speeds.