Steve Baur
Former Editor, Modified Mustangs & Fords
October 12, 2006
ProCharger's Pro Flo bypass valve (left) is a bit smaller in stature than its race-tested big brother, but it's still capable and sufficient for most blown street cars. Where it is important to vent the extra pressure from high-boost applications, the race valve is well suited with its larger frame and windowed construction.

The P-1SC can be spun harder with smaller pulleys and is rated to 800 hp, but you can sometimes run into belt slippage issues if you are still running the eight-rib serpentine setup. Smaller pulleys mean less surface area that the belts can grip, and if you use a small enough pulley, the belt will have a harder time overcoming the friction of the supercharger's drive system.

Planning ahead can help avoid this and save you money. If you have an idea at the time of purchase that you may well see 500 rwhp and beyond, you can upgrade the P-1SC kit to a D-1SC for just a little over $400. If need be, ATI can supply a larger pulley to detune and lower the boost of the D-1SC until your engine is ready for it.

ATI estimates the D-1SC is good for 3-4 pounds of boost over the P-1SC when using the same sized pulley. If you already purchased a P-1SC kit and major horsepower was a long time in the making, you can send your P-1 back to ATI, and for a tad over $1,200, the company will completely rebuild and upgrade your unit to D-1SC specs.

Beyond that, the F-1 series ProCharger is the next step, and is more than adequate for the rest of the street car crowd as it is rated to over 1,300 hp. If you're making 800-1,000 hp and drive your car a lot, the smaller F-1 models may be a better choice for such continuous duty.

When planning out your supercharged powerplant, intercooler size should also be considered as there are different sizes that may be better for your application, and choosing the right one from the get-go can save you even more.

To Cog or not to Cog?
Most guys think cog drives are cool. After all, they make loud noises and look racy, but they have their place, and it's not anywhere near a street car. "We don't offer cog drives for the P-1SC and D-1SCs," Comeau says. "We don't recommend them, and oftentimes people use them as Band-Aids for other problems."

This is the P-1SC's impeller. Note the straight vanes, which help produce a linear torque curve.

Deciding on what belt system to use depends on the application, the pulley sizes that are used, and the step-up ratio in the supercharger's drive system (step-up refers to the internal gearing that enables the compressor wheel to spin far faster than the engine rpm). A bigger step-up ratio equates to more resistance and a need for more ribs, i.e., a wider belt.

Cog drives are best suited for racing applications where the chances of missed gear changes are almost nonexistent. Over-revving the engine, whether from a missed upshift or wrong downshift selection is hard on the supercharger's transmission and drive system. Cog belts are expensive as well, retailing for around $125 versus $75 for a 12-rib serpentine belt.

Looking at the D-1SC's impeller here, it is easy to see the difference between the two. Check out the aggressive curve to the vanes. From the tip of the vane to where the slope meets the impeller's face is sized differently as well to alter the airflow characteristics.

With the proper pulley sizes, appropriate belt width, and gear ratios ranging from 4.10:1 to 5.40:1, ATI believes it can provide the street guy with a serpentine system that can get the job done. If you need more than that, you're going to have a bigger problem hooking up the tires

Bypass, Antisurge, or Blow-Off
All three of these terms refer to the same mechanical structure, which is a valve that vents the incoming compressed air when the throttle blade is closed. If this pressure is not released, it can damage the supercharger's drive system as well as the throttle body and other components. For now, we'll refer to it as the bypass valve.

ATI offers four different bypass valves beginning with its smallest, the Bosch valve. The Bosch unit features a 1-inch inlet and outlet and is used on some of the base kits. It is good to about 300-350 hp.

Our project car and the P-1SC system that it currently wears was equipped with the next valve up on the scale, the Pro Flo valve. With a 1.5-inch inlet and outlet, the Pro Flo can handle 600-650 hp. ATI is currently developing an intermediate valve as this article goes to print, but beyond 650 hp, you'll need to step up to the race valve which comes in recirculating and open designs.

The recirculating design can be replumbed to dump the air charge back in the intake stream or vented to the atmosphere in blow-through applications. If you're in need of the race valve for your street car, you'll be moving a lot of air through the motor, which will provide a loud discharge sound when the valve opens.

That being said, the recirculating valve will help quiet the issue, if that's your desire. Some people like loud noises though, and after all, that rush of air is what your average import poser considers the Holy Grail of performance.

"The best place to mount the bypass valve is between the supercharger and the intercooler," Comeau says. This protects the supercharger as well as the intercooler cores from the sudden increase in pressure when the throttle shuts.