Picking The Stick
As we often do when undertaking this kind of project, we consulted with one of our favorite cam gurus, Comp Cams' Billy Godbold, to pick a suitable bumpstick to go with our force-inducted 383. We added the appropriate lifters, pushrods, and Ultra Gold rocker arms to go with it. Godbold suggested a piece from the company's newest line, the XFI series. Specifically, we went with PN 08-468-8, grind CS 292XFI HR-13 for our blower application. Why an Xtreme Fuel Injection (XFI) cam? "You want to take overlap out in a fuel injection setting," Godbold explained. "You don't need the signal to pull fuel out, since the injectors squirt it in. You're only concerned about moving air. It's similar to a positive-displacement supercharger in this respect. It doesn't need the vacuum signal provided by overlap to move air, as you have the pressure in the manifold." Godbold also explained that the forced charge would "blow right through" if a cam with a tighter lobe separation angle (LSA) were used.
According to Godbold, the duration specs for this cam also work well with a positive-displacement supercharger. "You have 6 degrees more exhaust duration than intake, which is enough to help scavenge exhaust gasses," he continued. "With a centrifugal supercharger, you need less intake duration and longer exhaust duration so it makes torque down low and works well up high." With positive-displacement, he told us, "You can just put a bigger cam in it." And finally there's the fact that the XFI features Comp's newest, optimized intake and exhaust lobe designs. "It's a great all-around cam," he summed up. "It's as good as we can do." We're not inclined to argue.
An Insider Look
We've mentioned that the Whipple supercharger is a positive-displacement compressor based on a twin-screw design. The rotors, or screws, which never touch each other or the case, produce compression through intermeshing helical lobes. A male rotor with usually three bulbous lobes is complemented by a female rotor with usually five valleys. The valleys are similar in contour to the lobes, but different. In this typical three-five rotor combination, the male rotor turns 1.66 times for every turn of the female rotor. These numbers can vary, but the female rotor will always have more valleys than the male rotor has lobes. The number of lobes dictates three compression cycles per revolution. This means that the compressed air has pulsations when compared to a reciprocating compressor, though Whipple says the air is pulsation-free by the time it leaves the compressor unit. The male and female rotors go through three phases: inlet, compression, and discharge. The compression sequence continues on all rotor segments simultaneously, and the compressor we used can be spun up to 18,000 rpm. This high-efficiency, positive-displacement design is intended to deliver large power gains throughout the entire rpm range-and it did just that for us.