Originally posted by Bob H:
Holy crap that was a fast reply.
I was going back to edit my post to add that they may use the bypass valve for compressor surge, but the main benefit seems to be the economy, at least from Magnuson's marketing.
I'm still not convinced (or maybe I should say I don't quite understand) that the Roots bypass valve does anything for compressor surge though. On the 4th gen Magnuson blower, the bypass is an air path from directly before the compressor vanes to directly after. The throttle body is located well before the compressor. My understanding is that compressor surge happens when a downstream TB slams shut and the compressor continues piling air up behind the TB. If the TB is located before the compressor, there is no wall to slam the air into.
Now, whether the bypass is open or closed, the vanes are still going to be starved for air by the closed throttle body. Since any air that does not get sucked through the bypass and gets compressed by the blower has a relatively straight shot into the cylinders, there is no wall to create compressor surge. How does the bypass limit surge if there is nothing to cause the surge?
I'm not trying to be a dick by disagreeing with you, I'm just trying to show you my thinking so that you know how to approach an explanation that will make me understand. You seem to have a lot of experience with magnuson blowers, so please fill me in.
Hopefully this will make sense in the morning.
Oh wait, I think I might have it. I may be looking at the airflow through the bypass backwards. I assumed that when the bypass was open, the air went right through there into the engine and completely avoided the compressors. What you're saying is that the bypass valve allows air that comes out of the compressor to be rerouted directly in front of the compressor again, so that it is not starved of air, and thus prevents cavitation (assuming that is what causes the cavitation. And exactly what do you mean by cavitation, since the only meaning I'm familiar with has to do with bubble formation in a liquid from rotating parts)? The way it was explained on Magnuson's and whipple's websites makes it sound like all the air goes through the bypass, and none through the compressor. Was that written by the marketers to try to dispel the myth of superchargers causing huge amounts of parasitic drag in normal cruising?
Hopefully nothing in here sounds confrontational, I'm just trying to get this figured out. And get my posts up to 100. I think this thread might do it.
Bob
Edit: If roots bypass valves aren't there to help with fuel economy, please ask Magnuson to fix his website next time you talk to him, since his site clearly states otherwise. 
OK, I will make an attempt.[boy is this going to be long and drawn out]So bear with me. You are correct you were thinking the airflow went in the wrong direction, dont worry about it MOST people make the same mistake. Think about this for a minute. The supercharger is part of the air intake circuit,it lives between the throttle body and the intake valve. The bypass or blow off valve is in the circuit between the output of the blower and the intake valve. The only things that are generaly located between the throttle body and the intake of the blower are the EGR valve,vaccume takeoff points for such accesories as power brake booster, cruise controll and any other vaccume operated motors on the car and the connection for the bypass or blow off valve. Let me address the cavitation while you have this picture in your mind. A roots blower or a Vortech blower are driven by a belt that is turned by the crankshaft so as long as the engine is running the blower is pumping[of course the faster the engine runs the more the blower pumps]Now lets say you are accelerating, you reach the desired speed and let off the throttle. What happens? You create on huge vaccume drop in the section between the throttle body and the intake of the blower. This happens because the blower is still pumping by a great amount as long as the engine RPM is up. This is what causes cavitation, the rotors are still spining trying to pump air but they cannot because the supply of air[the throttle] has been closed. This is also refered to as dead heading a pump. Now this is where I will try to clear up another misunderstanding. The by pass or blow off valve will help in the MPG of the engine in this way. Keeping in mind the dead heading situation previously mentioned. There is a LARGE amount of vaccume on the inlet side of the blower and the blower is cavitating[because air supply is cut off by closed throttle] The output side of the blower[between the blower output and the intake valve] is HIGHLY pressurized, so the air is backed up and the blower is pumping against excess air. This puts alot of drag on the blower rotors and makes it hard for the crankshaft of the engine to turn the rotors. It is this parasitic drag that causes the engine to consume for fuel because this heavily loads the engine. The answer to both of these problems was to place a valve between the output of the blower and the intake valve and plumb it to the intake side of the blower between the throttle body and the blower intake. Now when you let off the throttle the bypass[blow off] valve opens and the excess air[pressure] flows to the intake side of the blower where it is recirculated through the blower[ending cavitation by supplying air to the intake side of the blower and releiving the dead heading situation by allowing the pressure to escape] and is finally consumed by the engine. This releives the parasitic drag on the engine and therefor helps the MPG because the blower is not dragging the engine down because it is dead headed. There are no untruth's on the Magnuson web site you just misunderstood what was meant and how it was acheived. The bypass[blow off] valve does the same thing in a turbo system in that it lets the backed up pressure escape to the intake side of the compressor on the turbo, same thing really! Boost is boost it doesnt matter if it comes from a turbo or a supercharger it is all the same thing. COMPRESSED AIR. The only difference is how it was compressed. By the way I didnt take any of your coments as confrontational. I think open discussion is good and should be done on a regular basis on this site so that more people can learn. Who knows what great things could be acheived if people would talk out there viewpoints rather than give smarta$$ed remarks and to ridicule someone for asking a question. I hope this helps, if not post more questions and we will try again.
