I began to hear slight blowoff valve type noises after a track event several weeks ago. It occurred just as boost was coming on and just as it was going off.

I finally got around to investigating the "bypass" valve which a vacuum actuated valve located behind the throttle body. It serves to recirculate boost so that you are not loading the cylinders unless you draw low manifold vacuum. It is connected to a plastic runner by a short rubber hose and two BMW trick hose clamps. The runner travels across the engine and dumps back into the supercharger.

One of the hose clamps (top) was loose even though the clip was secure. These things make US spring clamps look good. The vacuum actuator hose is not clamped on either side and pulls off much too easily. I replaced the two clamps with screw type clamps and used ties to secure the vacuum hose. No more leaks.

The plastic runner has significant flashing on the outside (looks like it has been melted) and the inside. I am told that this is normal. Probably doesn't hurt performance as this chamber is not functioning under boost.

However, the vacuum actuated valve allows boost to fluctuate between 0-5 lbs under light throttle with a flutter feel. A more positive actuation or an adjustable valve would probably take this out.

Any ideas?

 

Good work on your part. Sounds like you may have been losing a little boost.

To be honest, I'm more than a little curious about the whole bypass thing. It seems to me like there might be the potential to change the threshold at which it ceases to operate from 2800 rpm to something lower, say 2000 rpm and get a bit more low end torque. On the other hand, if it were that simple, we'd already be doing it, right? Maybe it's driveability thing. Still, I'd like to hear a good explanation of the bypass, how it works and whether there's any opportunity for a perfromance increase.

Your idea of an adjustable valve might be the ticket. Can anyone out there help us further the development of this idea?

eMINI

 

It seems to me that most superchargers have this bypass to function at low loads. When not loaded, the engine is more than capable of operating without the supercharger. Therefore, running all the intake air through the SC costs HP and efficiency. How much I don't know. So, the solution is that under part throttle conditions, you connect the two sides of the SC with a "bypass" pipe. This bypasses the intake air around the SC, so the SC is doing very little work - i.e., it is not pushing any air. Yes, the SC still takes HP, but less than if it were actually doing work - i.e., pushing all the air to the engine.

As I understand it, the bypass closes under low vacuum - meaning higher loads. It has nothing to do with power, and closing the bypass earlier would not increase power (the engine is only lightly loaded, remember). Given that a MCS makes more HP across the entire range than a MC, it appears the engine is using the SC at almost all engine speeds - if it needs it..

I could be wrong, but my fading memory seems to recall that this is the purpose of the bypass....

Allan

 

Oh yes, positive displacement air pumps (Superchargers) can "surge" under certian flow and pressure conditions. This is pretty much a disaster if it occurs. Don't know if the SC operates above or below this condition - but if it happens it will probably over pressure the engine, and create holes all over tha place. Surge on a large compressor will shake the ground and scare you silly. (Been There, Done That!) So you do not want to change th operating parameters to get near this "surge" condition.

Allan