Boost tapering off after 4k rpms 2011 bmw 135i n55

[Milo Jacob]

Hi, so I have a 2011 bmw 135i and I have been chasing a boost leak for months. The issue is, I make around 21 lbs at 4k and then as I approach redline, I taper down to 13 lbs. my waste gate duty cycle also hits 100% after 4k and stays till redline. I have done a smoke test, leak test, checked waste gate vacuum line. Thought it was a diverter valve so I got a DV + but still seems to be having the same issues. Anyone have any ideas as to what might be causing this issue?

[tracer bullet]

There's a chance it's "normal". I don't recall the #'s but want to say the N55 gets 20-ish psi at a lower RPM and tapers towards 15 or so at high RPM. Don't quote me on those numbers but I am pretty sure it's not linear through the rev range and that it does drop off. It's basically that the size of the turbo is such that at high RPM it can't keep up and the engine is sucking in so much air it ends up making the pressure drop.

Someone else would know more but, again, what you're seeing may be normal. That you cna't find any leaks and such is indicating that perhaps as well.

[Milo Jacob]

So here’s the thing tho, I’m running a stage one Vargas turbo that should allow for maybe a 1-3 psi taper, with a stock turbo that would make sense but with a stage one I shouldn’t be having this issue. I am running FBO also.

[wheela]

Do you have any logs to share? Also, are you still on the original boost solenoid/pressure converter? Another thing to check is the vac line for your exhaust flap - it pulls from the same vac reservoir in your valve cover as the boost solenoid, so a leak in that line will reduce available vac to your boost solenoid and cause low boost issues.

[Milo Jacob]

I am on original boost solenoid and pressure converter I do have some logs I’ll post one down below.

https://datazap.me/u/milojacob/switc...og=0&data=3-13

[Afam28]

Did this boost leak happen after your Vargas turbo install?

And are you running a tune?

[Milo Jacob]

I never hit this much boost before I was always around 12-13 and yes I am running a custom tune from wedge. So to answer your question about leak before install, I’m honestly not 100% sure.

[Milo Jacob]

After some further diagnosis testing vacuum leaks, I believe I found some. Pulling the very short hose from the boost solenoid that connects to the vacuum reservoir and attaching a pump to it, there is a small leak. I started with 15 inHg and it went down to 10 after a few minutes. I also checked the exhaust flap vacuum line and it seems that it also has a leak as well

[GoRomeo]

Originally Posted by Milo Jacob

After some further diagnosis testing vacuum leaks, I believe I found some. Pulling the very short hose from the boost solenoid that connects to the vacuum reservoir and attaching a pump to it, there is a small leak. I started with 15 inHg and it went down to 10 after a few minutes. I also checked the exhaust flap vacuum line and it seems that it also has a leak as well

Cool thanks for sharing.

Where is this exhaust flap vacuum line? Is it in the back of the car by the exhaust muffler or engine bay? I am courius since I never have checked that vacuum line.

[GoRomeo]

Originally Posted by GoRomeo

Cool thanks for sharing.

Where is this exhaust flap vacuum line? Is it in the back of the car by the exhaust muffler or engine bay? I am courius since I never have checked that vacuum line.

Duh 🙄 if course I know whre is that valve. I ttied to the T Golf mod on it but decided to put it back. Sorry 😜

[Houranimd49]

In my experience with several N55s, that’s just how they are based on the butt dyno.

[Milo Jacob]

Ended up being the boost solenoid, $90 to replace and I hold 21lb till redline