A boost controller fools the wastegate into opening sooner/later on a turbo application thus letting you control the psi without havin to swap springs. Superchargers dont have wastegate since they're powered via a pulley. So you'd have to have some sort of clutch pulley like AC compressors have, but even then it would just work as an off/on switch and not variable like a boost controller.
If you're using a centrifugal supercharger you can use a wastegate just like a turbo. Using a roots type blower it could be done, but would take some pretty creative and complex work. Also it wouldn't really be necessary. having the proper pulley will keep you from having to worry about any overboost situation.
Centrifugal, Screw and Roots type superchargers use a closed throttle bypass valve to relieve boost pressure at closed throttle and limited operation strategy in the event of electronic faults. It is usually part of the blower and is controlled with a vacuum diaphragm. A turbo boost controller allows either manual or electronic control of the wastegate which is on the exhaust side of the turbo. They can be set up to control a bypass valve also but in most boosted applications, bypass valve just prevent the pressure build up when the throttle is snapped closed. Turbo boost controllers can continuously control the actual boost pressure by varying how much exhaust pressure bypasses the turbine reducing boost pressure where supercharged engines are fixed based on rpm and engine design due to the relationship of engine speed vs supercharger speed.
Wastegates are for TURBO's only Blowers are controlled by pulley ratio Blow off valves and bypass's are not for boost control
The OP wasn't specific in the type he was talking about and I corrected the way that I worded that first statement, my brain moves faster than I type sometimes, oops. I was speaking of the Ford factory supercharged cars with EFI and most of the upgrade kits available at present. I also have never seen a carb application with a bypass valve, I wouldn't think you would need it since they are usually installed under the carb(s), hence the throttle plate. The Vortec and Procharger kits have bypasses as well and as Joe Dirt stated, are NOT for boost control. They are more for drivability to prevent surge due to the supercharger(whichever type) pumping more air than the engine can use at idle and to prevent damage from high pressures created when the throttle is snapped closed. These systems have intercoolers and piping that would be damaged or blown off due to the pressure spike, but boost pressure on a supercharger is only controlled by controlling the rotor speed with pulley ratio.
This type of "closed throttle bypass system" is only found on OEM systems or the newest types like Whipple and others. You can not do this with any type of 371/471/671/871 type blower. The only way to change acutal boost on a "Roots/Screw" is to change pulleys. I believe what Injectedmav means to say is that OEM types use a close throttle bypass. When I had my Flathead with 471 I used an MSD distributor, Barry Grant Blower carb, then tapped into the intake and ran the line to boost signal in the car...............used an MSD 6BTM Boost Retard with control nob on the dash.................but this just controls the ignition, not boost and ofcourse we all know the only way to change boost is to change the pulley, at least on real blowers...................................IMHO
