None really. It just has to do with how much air the bypass can handle. If it's not up to the job your going to get reversion, heat build up, and problems. The small plastic one that comes with all the blower kits is good up to around 12 psi. After that you need to look at going with a billet aluminum race bypass. I run 10 psi for my street set-up, but for the track this year I'll be running 18 lbs of boost and I'll need the race bypass.
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Dan McClain, Editor
The Mustang Works Magazine
1991 Mustang GT - NOVI Supercharged 377 Stroker
1999 Ford Lightning SVT - 2000 Prototype updated by Roush