CAR MODIFIED WITH NITROUS OXIDE SYSTEM

The only supercharger I had ever owned briefly was a "Kompressor SLK230" with an automatic. Thankfully that was short lived.

So I am not an expert at supercharging and I am doing my homework. Thought I would share. But my initial gut check seems to be on right track. If your building for all out top end hp and top speed, centrifugal is prob the right path. if you want lower rpm grunt immediately, positive displacement. And when choosing the path to take, one shouldnt only look at peak whp/torque dyno numbers. Step back and look at the entire picture. Then I also you have to take into account the natural power curve of the engine being worked on. One thing's for sure, any supercharger is going to bring on the smiley face.

While a boost-to-boost shootout seems only natural, there are problems associated with choosing boost pressure to regulate the playing field. The main problem with using boost pressure is that since the four different forms produce decidedly different boost curves, where (in terms of rpm) do you take a boost reading? An example works well here. In the case of the Eaton supercharger, it provides a bell-shaped boost curve with boost being slightly lower at both extreme ends (high and low rpm) and highest in the middle. Thus the 11-psi max rule would have the Eaton blower spinning to produce a maximum boost pressure of 11 psi at roughly 4,500 rpm, where the pressure would have fallen to just 9 psi at the maximum rpm (for this test) of 6,500 rpm.

By contrast, the Kenne Bell twin-screw blower will make peak boost pressure at 6,500 rpm, not because the Roots blower produces better low-end response, but because the inefficiency of the Roots blower requires that it be spun so much faster (that the twin-screw) to produce the desired 11 psi. That the boost pressure offered by the Eaton falls off at a higher rpm is not a plus on the side of low-speed response but rather a negative on the side of keeping pace with the motor. Spun the same speed, the twin-screw design will offer more power and boost pressure (at any rpm), not to mention a lower charge temperature. To put this into perspective, the Eaton was spun over 2,000 rpm faster than the Kenne Bell blower to achieve 11 psi (at 4,500 rpm). That additional blower speed helped improve the low-speed power production where the airflow supplied by the Roots blower could keep up with the demand of the four-valve motor.

Obviously the centrifugal supercharger will also produce the desired peak boost level at the peak engine speed. The impressive power per pound of boost offered by the centrifugal supercharger (besting both the Roots and twin-screw designs) comes with a price. Though high-rpm (and peak) power is impressive with the centrifugal design, the low-speed boost response is significantly reduced compared to either of the positive-displacement blowers. Peak to peak, the Eaton is easily out-gunned by the Vortech, but Cobra owners do not live by peak power alone. Centrifugal superchargers (regardless of the nameplate attached to the supercharger), all behave in this manner. The boost curve increases with engine speed. In the case of our four-valve Cobra motor, the Vortech produced just 1.7 psi at 2,500 rpm before reaching a peak of 11.3 psi at 6,500 rpm. All the superior efficiency in the world won't overcome a boost pressure deficiency of nearly 8 psi at 3,000 rpm. It is at these lower engine speeds that the two positive-displacement blowers really shine. Note that neither of them matched the peak numbers offered by the Vortech at 6,500 rpm, but it is hard to argue with all that extra torque, especially on a street car.