Richard Holdener
May 1, 2002

This is one story I've wanted to do for years, but just never seemed to get around to. I have tested camshafts on both normally aspirated and supercharged motors, but my supercharged cam testing has been limited to centrifugal superchargers. As luck would have it, 5.0 owners have been blessed with the fact that performance cams that offer power improvements for their normally aspirated motors will do so on supercharged applications, at least those equipped with centrifugal superchargers. Basically what this means is that the performance cam you installed in your 5.0 will probably work well after you add that Paxton, Vortech or PowerDyne centrifugal supercharger. There are of course some cam profiles that work better with superchargers than others, but for the most part, cam choices for centrifugal supercharged motors mirror those for normally aspirated applications.

To establish a baseline, we decided to run the test motor first with a stock 5.0 H.O. cam.

The question now is how does this cam information apply to positive displacement superchargers? Is there really a difference between positive displacement superchargers and centrifugal superchargers? Don't they both just supply more air to the motor? The answer to this last question is yes, sort of. True enough that any type of forced induction supplies additional air to a motor that it could not otherwise ingest of its own accord.

In the very near future, we will be experimenting with cam timing for turbo applications, which is definitely different than those required for a supercharger, but back to the differences between the positive displacement and the centrifugal. While both types of superchargers supply excess air to the motor, their method of supply differs greatly. Before getting to the cam test, let's take a brief look at how they differ.

After establishing a baseline, we planned to swap in an Xtreme Energy 258HR hydraulic roller cam. The dual-pattern XE258HR offered a .533/.544 lift split and a 208/216 split in duration. Note that the lift and duration were skewed in favor of the exhaust. Dual pattern cams seem to work best on blower motors.

The positive displacement supercharger offers a fixed amount of air per revolution. The actual amount of air supplied per revolution is determined by the size or more accurately the displacement of the supercharger. The twin-screw Autorotor superchargers offered by Kenne Bell are available in a wide variety of different displacements to meet the needs of different engine sizes and efficiencies. The airflow to the motor is determined by the size of the blower and the number of revolutions in relation to engine speed. Basically, spinning the blower faster will provide more revolutions of that fixed amount of air. A paddle wheel on a Mississippi river boat is a good example. Each revolution pushes a given amount of water based on the size of the individual paddle. The only way to increase the speed of the boat is to increase the speed of the wheel as the displacement of each paddle is fixed. Such is the case of a positive displacement supercharger.

Being much smaller physically, the impeller of a centrifugal supercharger requires much more speed to produce a similar amount of airflow. One revolution of the impeller will not provide the same amount of air as one revolution of a comparable positive displacement supercharger. Accordingly, centrifugal superchargers require significant rpm in order to function properly. While the efficient operating (rotor) speed of a typical positive displacement supercharger is around 12,000 rpm, the impeller on a centrifugal supercharger might need 40,000, 50,000 or even 60,000 rpm to produce a like amount of flow. In order to achieve this high impeller speed, centrifugal superchargers rely on an internal step ratio (the ratio is 3.45:1 on most Vortech models). This step speeds the blower up relative to the engine. Additional ratio is obviously needed to reach 40,000 rpm or more, so the centrifugal superchargers rely on a drive pulley ratio to further speed the impeller. Installing a larger crank pulley relative to the blower pulley greatly increases the impeller speed. This method is also employed by manufacturers of positive displacement superchargers.