I have a 540i with the ERT s/c kit. I recently replaced the supercharger and the new one has a much smaller pulley (2.92 in vs. the old 4.02 in.), but for some reason it is making the exact same boost and at the same rpms (a max of 5.5 psi at 6200) the only thing I can think of is that the bypass valve opens up when it shouldn't. Anyone know the part number of one that would allow me to run 7.5psi or perhaps an adjustable one, or maybe just tell me Im wrong :)
Thanks,
Chris

I have a 540i with the ERT s/c kit. I recently replaced the supercharger and the new one has a much smaller pulley (2.92 in vs. the old 4.02 in.), but for some reason it is making the exact same boost and at the same rpms (a max of 5.5 psi at 6200) the only thing I can think of is that the bypass valve opens up when it shouldn't. Anyone know the part number of one that would allow me to run 7.5psi or perhaps an adjustable one, or maybe just tell me Im wrong :)
Thanks,
Chris

Why not remove your bpv and perform a desk check of it? Hook up a handheld mighty-vac and see where it opens and closes? See, since the valve's 'normal' condition in the absence of vacuum is to be shut tight, under boost, it is supposed to be shut if its operating as it should, and the valve should be out of the equation when feeding it full boost. Vacuum pulls it open. Therefore, . To me, its indeed a mystery as to why you're seeing no more boost with a 2.92" pulley than your 4.02" pulley. Id expect to see several pounds more boost!

But wait. You say you replaced the ERT supercharger with another.. Did you replace with same model, style, orientation, etc, etc? Different blowers have different efficiencies, with a resulting difference in output.. Make sure thats not the cause of your problem first... .

It is the same model supercharger, both are powerdyne bd11a's. the owner of powerdyne told me when I bought the new one that this pulley should make 9lbs at 6500 with my 6 inch crank. So if i'm understanding you correctly, the model of bypass valve does NOT matter? They only perform one task and that is to open when under vaccuum. Not to vent when a certain boost is reached as a wastegate valve would. I pull 22 in of vac at idle so i think the valve is sealing properly, but maybe i'm wrong!?!
Thanks again!

If the number on the valve is -102 or -103, it is a weak springed valve. It may hold 7 psi or it may not. A better Bosch valve is the -108 or -110. It will hold all the boost the Powerdyne could deliver. You can also spend twice as much or more for an aftermarket valve that does not flow any more, work any better, and that will eventually leak and require rebuilding. The Bosch valve will also eventually fail, and be unrebuildable, but it reliable and durable enough to be used by Porsche, VW, Audi, and others.

Philip Bradley

I thought most bypass valves around today can handle whats considered the minimal amount of boost that we typically see on the powerdyne units.

it is a 108 series, I'm just going to replace it and see what happens. they aren't that expensive.

A -108 will not leak because of the boost you are running. They are good for more than that. An old one can develop a diaphragm tear. You can test for this using a Mityvac vacuum/boost tester on the nipple. If it holds, it is working properly.

If it is good, your problem is elsewhere. Idle valve, hose, clamp, etc. Belt slip is also a possibility. Sometimes you can test by watching the boost on the gauge throughout the rpm range. If boost stops climbing at 6000 but your redline is 7000, it may be because the belt is slipping. More tension is required.

Philip Bradley

is belt slip common? i remember hearing a noise that sounded like slipping when the windows were down and I hammered it. the boost builds gradually all of the way to redline, it really seems like it just isn't making that much to begin with. guess its time to do some digging :(