Factory experimental

[Tim Kish]

Originally Posted by Jared Jordan

One aspect that hasn't been discussed in this thread is the effect the supercharger will have on those cars at an altitude track. Those of you in the Eastern U.S. don't have nearly as many of those as we do out West. If you're getting blown off at sea level by one of these cars how much further behind are you going to be in thin-air against a forced-induction vehicle?

Main difference being that if drive ratio is controlled then you are still pumping the same volume displacement of air at sea level or at altitude. And that air volume is less dense at the same ratio as the NA cars have to deal with. Where you can benefit from a boost device (supercharger or turbo) is if you pump more air volume at higher altitude to keep the charge density at the same power level you can achieve at sea level. The only way to do that with a crank driven supercharger is to change the drive ratio - the pro guys can and do this at the altitude tracks. In stock this will not be allowed.

The boost level achieved by a roots blower is a function of its physical displacement, drive ratio, blower efficiency (mechanical and thermal) and inlet air conditions. If only the inlet air condition varies you get the following results:

A setup to run 10psig boost at sea level with an atmospheric pressure of 14.696psia (29.92 baro) as measured at the supercharger inlet is said to be operating at a 1.68 pressure ratio.

As the pressure ratio performance of this setup is fixed (fixed drive ratio, blower displacement, etc), as inlet pressure changes, the measured gauge pressure in the outlet (Boost) will change as well.

At 2500ft above sea level atmospheric pressure drops to 13.41 psia (27.32 baro), the resultant measured boost level will be 9.12 psig

Likewise for those so fortunate to run at 1000ft below sea level (and not drowned), boost level would be 10.35psig.

So help me understand how a tell tale boost gauge will work, unless of course it has an altitude correction factor which we all know are so accurate.

Another huge factor in intake manfold pressure changes is cam timing. Ex: Less overlap = more boost at fixed drive ratio. So again within the legal mods for stock, how do you maintain some "stock" boost level.

Not trying to be critical of anyones concerns or ideas, just trying to help educate on how supercharged engines function.