Here's my dumb question. Factory turbos peak at 8 to 10 psi, chipped ones go to 14 psi and higher. But I've never seen a supercharger, factory or aftermarket, that exceeds 7 psi. How come?

...on Mustangs and Camaros.

The main reason that you see superchargers turn low boost numbers relates to what they are bolted to. Most superchargers from the aftermarket are bolted to a high compression stock engine. 10.5:1 isn't conducive to 15psi of boost.

On stock (OEM) engines, they limit the boost to only several psi because they don't want to compromise longevity and because they want to use the same components that the standard production engine uses.

Mike O.

Centrifugal compressors like on turbosuperchargers, or Vortech style (belt driven) superchargers only produce signifcant boost when they are spinning fast (100k rpm or so like 'WhatYouSee' said). Positive displacement superchargers, like the Eaton used by GM and Ford, and Jackson Racing, pump the same amount of air every time they rotate. Therefore, boost is available right off idle and all the way through the rpm range. There isn't any lag. The engine basically acts like it has a larger displacement.

5-8 psi boost with positive displacement blowers throughout the rpm band usually gives the effect the manufacturer wants; as much as 40-50% more torque and hp. If you size this type of supercharger to give say 14 psi boost at high engine rpm, (where the TT engine gets it), it'll also give about that throughout the range, which the TT doesn't.

This will be hard on internal engine parts, and drivelines. In other words, 'managing the boost' like the TT ECU program does with our turbo isn't as practical with the positive displacement 'charger. Remember, a 225 TT is getting 125 hp per liter, while a torquey supercharged 3.8L GM V6 is geting about half that. Hmmm, I wonder which engine is stressed the least?

BTW, you can get tons of boost with positive displacement blowers, especially if you are looking to make upwards of 5000 hp on a Top Fuel engine!

have the supercharger run at 20psi or whatever at max RPM, and have the hypothetical design include a controlled blow off to regulate the pressure. With these engine computers, somebody could make a car where the ECU decided boost pressure and got it by adjusting the servoed blow off. And the ECU then could calculate air-flow (for injector and lambda control) with an inbound MAF and an "outbound" one on the blow-off. All figured in with engine load, pedal position, and gear selected.

Positive displacement pumps make about as much boost as you'd ever like them to...

In the case of the PES kit (which I have and like) it's worth noting that the 2.8's compression ratio is kind of high to go a run huge boost with pump gas. And all that previous babble about a blow off wouldn't work unless you were designing the ECU and other bits of the engine yourself. Which PES was not.

You see stock cars from 7-14 PSI maybe, even turbos for the most part. Mfg, seem to want to avoid blowing engines up (what smallTT said)... unlike us...

- Z

back into the inlet under part-throttle (cruise) conditions. This actually lowers the parasitic losses of the blower drive because the higher pressure air is blowing down on the rotors helping them.

You could probably use this system to manage the boost. It may already have been done on some of GM's concept cars.