I recently installed a boost gauge in my car, I also have a 15% pulley on it... In first gear I top out at 8 or so psi. Second gear is a few psi higher, 12-13. Third gets me to a steady 15-16psi, same with fifth and sixth. Is it normal for the boost to be progressive through gears like that? I wouldn't think this would be a boost leak issue or it would never reach 15+psi no matter what gear. I've had a 350WHP turbo'd bmw and I always hit my target psi. Not having owned a supercharged car before my r53, is this just the way supercharged motors act?
I'm thinking possible belt slipping?

 

should have the same boost through out all gears, if your throttle position changes, so does the amount of boost, since there will be a difference in vaccum and the bypass valve will vent some boost away. first gear loves to roast tires, and 2nd is nearly the same. the DSC could have kicked in and lowered the throttle % to 60% and in turn gives you less boost

 

So bad bypass valve possibly? This being an 2006 car I thought I had the better designed one

 

Springs may still wear out, but yes a better fitted one for 05/06. I have a Detroit Tuned one. Much like yours, better fitting butterfly but stronger spring. Been in there for last 145K miles....so far so good.

 

I read somewhere that's the DT bpv isn't recommend on the '06 cars

 

the 05 and 06 will have better bypass valves. and some people don't enjoy the DT bypass valve, some think its too invasive. unlikely its bad, but you can ziptie it and see if its the same, but since you have full boost in higher gears, I would say its more throttle related or DSC

 

Will I jumped on the DT bypass, had planned on getting one eventually so whatever. I did a pull with the dsc off and still the same issue. I plan on doing the bypass valve this weekend and check. If no fix still, I'll throw a new belt on even though I don't think it's gonna be the issue. It's a cheap piece. After that I guess I'll start looking for vacuum leaks

 

Here is the updated bypass valve part number:
https://www.northamericanmotoring.co...rt-number.html

 

Did u find your problem? I was having a similar issue but mine actually had about 8psi in 1st and 2nd and then about 5psi the rest of the way.... I ZIPTIEd mine shut and that fixed my problem.... Now showing about 11 or 12psi in every gear so..... New bypass valve on the way....

 

My boost gauge does work but at normal speed, it's around -10. Is that normal? If I step on it, I can hear (and feel) it spool up and it looks like I might get +15 before I shift it. Normal too? Taking this further - can we / do we / should we have blow off valves?

 

You cannot put a functioning blowoff valve on a supercharged car. Turbos have them because the boost is built up before the throttle body. Supercharged cars build the boost after the throttle body. When you let off the gas on supercharged car, the bypass valve opens, thus shutting the air supply to the supercharger and allowing the air intake to continue on through the regular intake. This way, the supercharger can continue to spin as it does all the time. A turbo forces the air THROUGH the throttle body so when you take your foot off the gas the throttle-body closes but the turbo is still pushing air, this is when the blowoff valve comes into affect. The blowoff valve just releases this air out of the intake system making that ppssshh sound.... Yes, -10 is normal while driving. That is the vacuum of the engine that keeps the bypass valve open besides the many other things that the vacuum is used for... Your engine should have close to -18 when idling. You will notice when you hit the gas that the needle on your gauge immediately jumps up into the boost range of the gauge and then when you let off it drops back down to the vacuum side. My boost gauge says about 12psi max in any given gear when revved out. That is the best way I know to explain it....

 

You can put a blow off valve on a supercharged car. But not with this type of supercharger

 

My bad.... I stand corrected. apparently I misunderstood....

 

Great info guys. As much as I learned before I bought him - learning lots more now.

 

One of the items I have planned for this car is the M7 strut tower plates. Should I go with the brace bar too or no?

 

I would, the amount of performance increase it gives you is some what debatable, but it protects the towers, isn't terribly expensive, and can't hurt performance.

 

Craven STD'S...
strut tower defenders are nice too...similar to the m7 but IMO they got better...there was a cable that the m7 would rub on sometimes if I recall right...but I got mine nearly 10 years ago...so maybe it was updated.