by Patrick Rall
I have seen some discussions online about how Hellcat Challenger and Charger owners are looking to increase the output of their supercharged Hemi engines by increasing the boost. The two “simple” ways to do this are to replace the lower pulley on the crankshaft, or the upper pulley on the supercharger itself.
Changing the upper pulley is a common means of boosting boost levels, but unlike some engines, the Hellcat Hemi uses a clutch system in the upper pulley. That makes it harder to simply swap to a new pulley for increased boost levels.
Some people on Web forums insist that you can just swap to a pulley without the clutches without any short-term problems. They might be right, and swapping to a clutchless upper pulley on the Hellcat engine might not cause any short term problems, but before changing such a crucial component of your Mopar muscle car’s supercharger, you should understand why the clutch system is there in the first place. Automakers don’t spend money on unnecessary systems, after all.
Before the Hellcat Challenger hit the market, I had the opportunity to visit the Chrysler Headquarters in Auburn Hills for a close-up look at the most powerful American muscle car engine of all time. The engineering team was on hand to walk the small group of media through the most intricate details of the supercharged Hemi, and in addition to slides chock full of information, they had a Hellcat Hemi engine torn down into individual components.
The engineers discussed the development process of the Challenger and Charger with the IHI supercharger. The Hellcat Hemi supercharger requires around 80 horsepower of driving force from the crankshaft to achieve the internal revolutions-per-minute (RPMs) needed for full boost; that force causes the engine RPMs to drop dramatically when the driver lifts off of the throttle. In other words, the supercharger has a braking effect on the engine.
Engine RPM loss hinders performance, particularly when you lift off the throttle to bang from one gear to the next. What’s more, in some cases, the change in the supercharger pulley speed would be too violent for the belt system, so when it didn’t slow down the engine RPM, it would cause the drive belt to spin on the pulleys. That leads to increased belt wear and, over time, lower performance when the belt begins to slip under boost as well.
To prevent having the supercharger’s rapid internal deceleration cause belt slippage or sudden engine RPM drops, the engineering team added the one-way clutched upper pulley system. This way, when you let off of the throttle in either of the Hellcat cars, the supercharger can slow down quickly without having any impact on the engine RPM or the belt system, since the clutch system allows the pulley and the blower internals to spin freely at different speeds.
Swapping to a new upper pulley without the clutch system will most certainly lead to increased boost pressures and increased power levels, but it will also lead to increased internal supercharger RPMs. Because of this, the deceleration of the supercharger could be even more aggressive, leading to sudden RPM drops between gears or belt slippage and increased belt wear.
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