ZZP Underdrive Water Pump Pulley Testing

Underdrive Water Pump Pulley Modification

One traditional modification to street and strip engines is to underdrive the engine water pump. Theoretically, this is suppose to reduce the amount of power (horsepower) consumed by the water pump and allow that freed up power to accelerate the car. Thus, the engine is able to develop more crankshaft and wheel horsepower. The amount of horsepower lost to the water pump does vary from engine to engine. Also, the amount of power lost is proportional to the engine speed. This means more horepower is lost at higher RPMs than at lower RPMs. The testing done here was to obtain a popular Underdrive (UD) water pump pulley and test the benefits it would bring to a GM 3800 Liter V6 (Naturally Aspirated L36 and Supercharged L67).

Test Setup

To help resolve the question of how much power is freed up by an UD pulley, I obtained a test vehicle (Pontiac Grand Prix GT, L36 engine) and performed before and after testing. The engine had a stock water pump pulley and an UD water pump pulley was obtained from ZZPerformance.

Testing Performed

Prior to the installation, I collected performance data to baseline the car’s performance. After this data was collected, I replaced the water pump pulley with the ZZP aftermarket unit in about 10 minutes. Once this swap was competed, I repeated the performance tests. The weather conditions for the day were cool with a normal humidity. During the testing, the temperature was 72ºF, Humidity of 75% and a barometer of 29.85“of Hg. The engine was thoroughly heat soaked and running stable at 182ºF by the PCM’s Engine Coolant Temperature values. A total of 14 passes were used to collect all the performance data. The results for replicated runs of each parameter were averaged together in the final analysis. Since the pulley swap took such a short time, the test conditions with the UD pulley install were identical. The GT was running on 93 octane and had 14,000 miles on the engine. The vehicle had other modifications done to it, but those are not a factor in the test since nothing else was change on the car's test setup.


As can be seen in the charts below, the gains in peak HP and Torque were ~2 to 3 WHP and ~4 ft-lbs respectively. Also noteworthy is the improvements to the HP curve were relized in the entire upper RPM range. In fact, this data supports the theory that the higher the engine is revved, the greater the gains from an underdrive pulley. The performance gains from this modification were small but significant none the less. My conclusion is this modification does have a worthy performance benefit. The only downside that would be possible is the reduced coolant flow rate through the engine at lower speeds. If this pulley were put on a car where the overall engine cooling system was marginal for keeping the engine cool during hot weather, then this reduced coolant flow rate may present a problem. If the vehicle has an adequate radiator, I'd say a problem should not be presented.

Stock Water Pump Pulley


ZZP UD Water Pump Pulley



While these results were obtained on a Grand Prix GT with a naturally aspirated L36 3800 V6 Series II engine, they are representative of the gains made on the supercharged L67 version engine. The cooling systems on between the two engines are practially identical. The L67 models will have a larger radiator. If cooling problems are not close at hand, I'd recommend this modification if the couple of horsepower are worth the price of the pulley.