The belt wouldnt slip under idle or you'd be having charging problems. I still think you have a boost leak. Try brushing a little soapy water along all your sealing surfaces with the engine running first.

 

Whoops. Sorry. Don't mind me. Carry on. :smile:

 

Problem solved.

I spent 5 whole minutes zip-tying the bypass valve closed and the motor runs the full boost (16+ lbs on a 40 degree night!!) with my 19% pulley. Though the motor feels revitalized, I need some closure to my worries about doing the "fix."

Is this an example of a broken / defective bypass valve? Was the bypass valve faulty, and therefore, by zip-tying it closed and not allowing it the opportunity to regulate the pressure actually FIXING the issue? How safe is this for the long term - the bypass valve was meant for a reason, and previous posting on this subject has resulted in members trying to modify the springs and change set screw positions, etc, not JUST zip-tie it closed and be done with it.

BMW put it there for a reason, just as we use blow-off valves on intercooler piping for turbo cars to regulate back pressure and increase the longevity of the turbo. I had purchased a new NAPA 060539 belt, but wanted to try this "fix" first to see if it was just the bypass valve all along. Don't get me wrong, I am THOROUGHLY enjoying the boost I once had as soon as the 19% pulley was installed, but I want to make sure it is safe and something I could advise others on in the future.

Of note, the whine from the supercharger after performing the "fix" is much quieter than before - estimation as to why that is revolves around the fact that though there is much more compressed air going to the system, the bypass valve isn't able to control it, and thus the intake (which was responsible for making the majority of the noise in the first place) wasn't getting any of the effects of the increase in airflow. I just cannot figure out why 6lbs of boost makes more of a while with a defective bypass valve than 16+ lbs!

If I'm worrying over nothing just give me the and tell me "hey, its not a turbo! Its different!" Either way, this was one well deserved surprise I gave myself on my birthday!

 

the bypass valve is operated by vacuum sourced from the area between the throttle body and the blower intake. the only time this area sees negative pressure is when you have the throttle closed, at which point, you don't want any boost. so the valve opens shunting the blower output back to the input. It is an economy measure. the biggest drawback is lag time for the valve to close when you stomp on the gas, or a leaky valve. I would get a new one and examine it to see that it closes completely (check the fit of the butterfly in the openiong and the setting of the screwstop.

this is not a pressure regulation system like a pressure operated wastegate for a turbo

 

Yes but jlm, doesn't having the bypass open or partially open at no or low throttle save the supercharger b/c it isn't always compressing air? It seems to me that permanently closing the valve would make your supercharger work hard all of the time instead of only some of the time. Wouldn't this decrease the life of the supercharger???? Is this correct???

 

seems right to me; how much it matters in reality is an unknown. I'm not usggesting you leave it tied closed, just make sure it works OK.

 

Yes I understand you're not suggesting that you tie it closed but many people have - I believe it is one of Ryephile's fix-its. I haven't done it and don't plan on doing it (my throttle response and boost levels are great) but it's always bothered me that people ARE doing it and it just doesn't seem like good supercharger-sense to me. I think it could prematurely wear it out.

 

That's correct, it is one of the "fix-it" solutions, and many people are leaving the bypass valve zip-tied shut. The theory is that it will create premature failure of the supercharger as it is always compressing air? Interesting...

In this case, what should be done for my bypass valve? The system, as far as I know, isn't leaky, its defective. Its not working properly, and submitting the extra compressed air courtesy of the 19% pulley back into the input. I was told by BMW that 02 models had a defective bypass valve, but mine is a 7/03 build!

Thoughts on keeping the bypass valve zip-tied shut for everyone who has it done? Thoughts on what should be done about my defective bypass valve?

 

IMHO, the bypass valve is touted as primarily a tool for fuel economy. There are thousands of vehicles with Roots-style blowers that do not have bypass valves. I haven't seen any evidence to suggest that the supercharger will fail prematurely if it does not have a bypass. Let's say you tie the bypass shut or remove it from the system altogether. The times when the bypass would normally open are times when airflow through the system and air pressure are low, since the throttle is partly or mostly closed.

 

Do before and after gas mileage tests: miles per tankful, not the ECU values.

 

Kinda hard to do considering I'm enjoying the result of having 10+ lbs more boost right now....

I hear what you're saying though, I'll start on that as soon as I run through this first tank of gas after being able to achieve the right boost from the 19% reduction since before September!