E92 M3 Project Part 3, Underdrive Pulley
There are many ways to increase the power output of your engine to make you car go faster. Conversely, there are also ways to use less power to make you car go faster. It's all about how the power is utilized. The engine not only drives the wheels, but it also powers accessories that do nothing to motivate the car (they run the AC, power the lights, and pump fluids to the steering rack). This is often called parasitic loss. Reduce the drag and use less power to drive the accessories; you have more power left over to motivate the car. Enter the “underdrive” pulley...
By changing the diameter of the crank pulley, which drives all the other pulleys, you're mathematically changing the ratios-- it's like shifting gears.
We went with Turner Motorsport's custom pulley for the S65 engine in the E92 M3. According to Turner:
"The Power Pulley reduces rotational mass and safely under drives your belt driven auxiliaries for reduced parasitic power loss... We've found this pulley alone adds 10 to 14 HP and 7 to 9 Ft/lbs Torque on a stock M3 with no other modifications"
Installation requires a good bit of disassembly. Since we were also changing out the stock intake (see Part 1) we combined tasks, reduced labor hours for our customer and changed the pulley at the same time. Here you see the stock airbox removed:
Next we had to remove the electric radiator fan and the various shrouds surrounding it in order to access the front of the engine where the pulleys are located:
Once the surrounding components are out of the way, the pulley is relatively easy to get to. The key to the installation is using proper torques and "Loctite" on the bolts holding the pulley to the crank, and remembering how the belts are routed.
Next up, engine tuning...