I'm posting this here because all the technically advanced members seem to hang out here as opposed to general automotive. Here is my question.

Say you have a roots blower on an engine and it is boosting like it should, revving up to say 6000rpm. When you let off the throttle (and the intake is basically blocked from the engine) and the engine is still spinning at 6000rpm before spinning back down to idle (with the blower pumping out PSI like normal), wouldn't there be a boost spike? So you would need a BOV wouldn't you? I don't see them installed on the Nelson/Paxton SC installations on NA FC's.

Also, is a fuel riser used for richening up the fuel mixture under boost?

Thx to anyone with SC experience.

 

Yes, superchargers may have BOV's. Superchargers are usually fitted with a "bypass valve". A bypass valve is a BOV which recirculates the air back to the inlet, as opposed to venting it to the atmosphere. Turbocharged RX-7's come from the factory with a bypass valve.
http://www.superchargersonline.com/content.asp?ID=85

A rising rate fuel pressure regulator is a common addition to a supercharged car.

 

Yeah - like Evil Aviator said ... Its usually a bypass valve, or something built into the compressor. -JinX

 

Whether you need one is determined with the location of the compressor (turbo or supercharger) in relation to the throttleplate. If the turbo or supercharger is after the throttleplate then you don't need one. If the compressor is before the throttleplate then you do. This is where the excess pressure is when the plate is closed. There is nowhere for the compressed air to go but back towards the turbo or supercharger and this will put alot of stress on the compressor wheel as well as everything in the assembly. Paxton/Vortec supercharged vehicles should use them as well as turbo'd cars. The RX-7 kits don't have them though but many of the kits for other cars do. The bypass valve on Eaton types of superchargers are for fuel economy when cruising. When not under a load, the bypass valve is open which recirculates boosted air back to before the supercharger. This allows the engine to function as an n/a. When under a load the valve closes and provides boost to the engine. Remember that more air needs more fuel. A roots style blower is always providing boost regardless of rpm. This will hurt fuel economy without a bypass valve. On a racecar this is of no concern. Some big supercharged dragsters use popoff valves much like on some aftermarket turbo cars. Their use however is different. It is used to let off excess pressure in the event of a backfire through the intake from nitrous use. A popoff valve on a turbo car is a safetey device that releaseas any excess pressure in the event of boost spike or wastegate failure.

 

I have this crazy idea to install a Mercedes roots compressor (the one with the electromagnetic clutch) after my throttle plate but before the surge tank of my S4. I wonder if the clutch (on the pulley) is for the same reason as recirculation valves: fuel economy. These compressors are pretty cheap on Ebay with low miles. There was even an intercooler for one up for auction.

So I would need brackets, adapter pipes, ducting, and the fuel riser and I would be set? I know that there is a boost sensor on the NA fuel injection apparatus . . . would this sense the boost as opposed to vacuum levels of normal NA operation and function correctly on the stock ECU? Maybe I should just do research and stop asking here.

 

Corky Bell's "Supercharged!" book is available from most online book stores.
http://www.bentleypublishers.com/pro...SUP&subject=24