Big Bore Meets Soft Clutch

AIM's Variable Pressure Clutch

1. We started the installation by loosening the cable adjuster until it was fully collapsed.

2. After this derby cover was removed, we adjusted the clutch adjustment screw with the stock retainer spring in place. We turned the screw until it bottomed out, then backed it out 3/4 of a turn.

3. After loosening the retainer spring screws evenly using a 10mm socket, we were able to remove it.

4. The six base plate tangs were bent outward slightly at the same angle as the pressure plate, then the base plate was inserted inside the clutch spring.

5. After installing the VPC unit, we tightened the bolts to 100-inch pounds with a torque wrench.

6. A new gasket was installed, and then we adjusted the clutch cable to 1/8-inch of free play at the lever pivot.

7. Finally, the derby cover bolts were tightened to 90-inch pounds with a torque wrench.

In the quest for more horsepower, we all encounter things that seem to get in our way-things like burned up tires, speeding tickets, and hard clutches. While we really can’t help you with the first two, we have you covered on the third. There are more than a few options for a softer clutch on a big bore Twin Cam, but many just seem to not do much for the performance of the clutch itself.

After checking out a few good alternatives, we decided to go with a Variable Pressure Clutch (VPC) kit from Advanced Innovation Machinery (AIM) in Huntington Beach, CA, on our 103ci-outfitted engine. The kit uses a technology that acts like a lock-up clutch, providing a much better feel as well as providing a consistently strong clutch engagement with 40 percent more pressure than stock. If you have a stock Evo or Twin Cam motor, there are VPC kits available that will fit your application as well.

Another effect of the VPC is that at idle, the system reduces the force it takes to actuate the clutch lever by up to 50 percent. With no extra parts needed in less than a half hour, you’ll know why we chose the VPC.

Here’s how the VPC kit works. Using centrifugal force, the pressure plate actuator kit does a few things. At low rpm, when the clutch is most frequently engaged, it provides a very light clutch pull due to its spring configuration. As rpm goes up and the engine produces more torque, the VPC’s centrifugal flyweights apply more force to the clutch plate, eliminating slippage across the whole powerband.