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      12-16-2019, 04:17 PM   #1
makke
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N55 Zero Boost at High RPM

Hello,

I have recently installed an UltraGauge (http://www.ultra-gauge.com/ultragauge/) to keep an eye on my 2012 135i. One thing has jumped out at me that I don't understand. I am only getting boost when at WOT or close to it. With smaller throttle inputs, I get no boost regardless of engine RPM. Here is an example:

https://photos.app.goo.gl/VfQNbiWUTN32zTBG7

At 3,400 RPM, Boost PSI is showing -0.58ish which, I believe, is nominal vacuum in the intake. I can rev it up all the way to the 7K red-line and still have zero boost if I do it without large throttle input. The only way I can get boost (up to about 10 psi) is by stomping on the gas pedal...

Is this normal behavior? If so, how the heck is this accomplished?

To my understanding, there are two methods of controlling boost built into the system. Method 1 is via the wastegate valve on the exhaust side of the turbo and method 2 is via the diverter valve on the intake side. Looking at the latter first, here is BMW's description of the diverter valve in ST916:
The basic function of the diverter valve remains the same. The difference compared to the N54 engine is that the diverter valve is not operated pneumatically. The diverter valve on the N55 engine is an electric actuator that is controlled directly by the DME. The number of components has been greatly reduced by positioning the diverter valve on the turbocharger compressor housing.

The diverter valve is designed to release unwanted pressure in the intake by connecting the pressure side of the induction system to the inlet side under deceleration. The undesirable peaks in the boost pressure that can occur when the throttle valve is quickly closed are reduced. This means the diverter valve plays an important role in terms of the engine acoustics while protecting the components of the turbocharger.
Based on the specific mention of "under deceleration", emphasis mine, I don't think the diverter valve is relevant to my scenario of zero boost pressure under constant 3,400 RPM...

Turning to the wastegate, my understanding is that it is typically controlled by the pressure on the intake side. The wastegate opens when the boost pressure exceeds spring preload. In the case of the N55, the wastegate is electronically controlled, which I am assuming means that the "spring preload" can be adjusted?
The wastegate valve is used for the purpose of limiting the boost pressure and is already known from previous BMW turbo engines. It is vacuum operated and electronically controlled through a vacuum control solenoid by the DME (ECM).
Still, even if the spring preload can be adjusted, I am having trouble imagining that it can be open both under nominal intake vacuum and under high boost.

So, back to the original question... Is it normal for an N55 to produce zero boost at high RPM when throttle is not near wide open? And, if so, how the heck is this physically accomplished?

Thank you!
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      12-16-2019, 04:27 PM   #2
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Diverter Valve is known to rip over time. Could have a blown diaphragm in the DV.
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      12-16-2019, 05:44 PM   #3
makke
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How would it be able to build boost at WOT if the diaphragm on the DV is shot? Unless I am misunderstanding how this scenario would work, I expect it not to be able to build boost at all, regardless of throttle position. At least not the expected stock tune 7-10 psi...

But, am I correct in understanding that there is an implicit "this is not normal behavior for an N55" in your message? Yours builds boost with RPM at 30% throttle input, for example?

Last edited by makke; 12-16-2019 at 05:53 PM..
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      12-17-2019, 02:43 PM   #4
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It doesn't sound all that out of the ordinary to me. If you're only using 25% throttle, there's no need to build boost to provide extra acceleration so the ECM leaves the wastegate open for better efficiency. The Valvetronic variable valve actuation system on the N55 can also be used to control the airflow (and resulting boost pressure) through the engine by adjusting valve lift and duration. What happens at 50% or 75% throttle? I'd imagine you'll build some boost but not the full 7-10 psi.

I'm tuned with MHD, but I'll try to log some runs on my way home today at 25%, 50%, 75%, and 100% throttle to see what the target and actual boost pressure are for each. Hell, I guess I could flash back to stage 0 and see what the stock commanded values are...
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      12-17-2019, 04:20 PM   #5
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So, you can make approx 10 psi boost when you stomp the throttle.......this sounds very much correct for a stock n55.

When you apply throttle slowly to redline, you get zero boost.

The wastegate system fails (defaults) to full open when there's a problem on that end of the boost system. Since you can make 10 psi then that does not sound like a problem. You are also hitting expected max boost so the diverter is probably Ok, as well.

What about moderate throttle, highway overtaking maneuver situation?

I'm thinking you are not creating enough of a load when you ease the pedal down. Load is one of the factors in the DME boost level formula.

Just spit-balling here.
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      12-17-2019, 08:39 PM   #6
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Thank you both! BimmerAG, yes, I would be very interested in what you find out.

I will add throttle position to my UltraGauge and try to get better sense of how much input is needed to get boost. Wish I had a data logger...

I am still not sure how the ECU can leave the wastegate open if it is operated by a vacuum actuator? Isn't the fundamental principle that of pressure on the intake side overcoming a spring-loaded piston to open a valve on the exhaust side? The spring preload can be adjusted by the ECU, yes, but the ECU cannot simply command the gate open, can it? There is no electric actuator. Isn't pressure (some amount of boost on on the intake side) required to physically operate the vacuum actuator and open the gate?

Thank you again.
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      12-18-2019, 05:29 AM   #7
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Makke, the wastegate is controlled by vacuum but the DME drives the boost solenoid which controls the amount of vacuum getting to the WG. This is not a typical on-off type of solenoid control. There is a PWM signal that cycles the solenoid very quickly in order to get variable vacuum output to the solenoid, like a fuel injector.

The vacuum is used to push the WG closed, in varying amounts, from its normally full open position. If the vacuum pump fails, a vacuum line leaks, the boost solenoid fails, or the WG diaphragm fails then the WG will not operate and will default to open condition. This is opposite to how most boost control systems have worked before but it’s pretty smart because WG system failure lets the engine continue to operate as a NA engine and you’ll get the reduced power notice on the dash. To answer your question more directly: the spring tension on our WG diaphragm is used to keep the WG open and the vacuum works against the spring to close the WG.

Just drive it like you stole it and you’ll always have boost.

BTW you will soon want a tune after you get hooked on boost. Why run at 9 psi when 15 is just a tune away?

Last edited by drwillb; 12-18-2019 at 07:01 AM..
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      12-18-2019, 09:32 AM   #8
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Ok grabbed some logs last night after flashing back to Stage 0.

Full throttle: https://datazap.me/u/bimmerag/stage-...&data=2-3-4-13
Boost target and actual boost hold steady around 8psi for the whole pull

50-60% Throttle: https://datazap.me/u/bimmerag/stage-...13&zoom=22-129
Boost pressure initially spikes to 9 psi on tip-in, but settles to 4-5 psi for the rest of the run

Light Throttle: https://datazap.me/u/bimmerag/stage-...-13&zoom=0-154
Started at 20% and ramped up to ~40% throttle at higher RPM, but boost pressure stayed around 0-1 psi the whole time

Overall, looks like what you're seeing is pretty normal for a N55. drwillb gave a great explanation that while the wastegate in our cars vacuum actuated, there is a boost solenoid that controls the pressure to the actuator and gives the ECM full wastegate control at all engine speeds and boost pressures

p.s. This was all recorded using the MHD logging function. I have previously used a wireless OBD reader and RaceChrono Pro to log data during autocross/track days and have found that for some reason MHD reads ~1psi higher. For example, at idle with the OBD reader boost pressure read -0.7psi, but with MHD it shows 0.2 psi
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Last edited by BimmerAg; 12-18-2019 at 09:39 AM..
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      12-18-2019, 10:56 AM   #9
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drwillb and BimmerAg,

Thank you both very much! drwillb, your explanation makes perfect sense. I was thinking of it completely backwards, that the wastegate is nominally closed and requires intake pressure to open. Thank you for setting me straight! The world makes sense now!

BimmerAg, thank you for taking the time to log performance of your 135! You have both made me feel a lot better!

Quote:
BTW you will soon want a tune after you get hooked on boost. Why run at 9 psi when 15 is just a tune away?
Hehe... I may get there one day, but for now I value two things more than I do power. Linearity and driving my car instead of fixing it... My first love is a 2009 E90 325i with an N52. (187K miles -- I am the original owner.) I really love how rev-happy and linear that motor is. It feels like a turbine or a rotary -- just purrs its way from idle to red line. The N55 feels very different and not as linear. There is also what I presume is a VANOS change hick-up that some-times shows up around 3K RPM. When I bought the 135 a year ago, it had a JB4. I have removed it because I felt that it made the car feel more non-linear. I also have a philosophical problem with piggy-back type tunes. I feel that if you are going to tune, do it at the source. Don't put something into the loop to fool the computer... Just my personal opinion -- I know lots of people disagree. I have heard good things about MHD.

Anyway, great big thanks to both of you one more time!
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      12-18-2019, 03:49 PM   #10
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Yeah, cuz you need a piggyback to tune the car. Oh wait you don't. MHD, the go to choice is a full ECU flash. Very smooth and drivable like stock. JB4 sucks, I agree.

Nobody is blowing tuned stock turbo motors just driving them around and there are no maintenance consequences other than the additional mods you'll probably want to add for fun. If you're going to blow your motor on track it will happen due to oiling issues, nothing to do with the tune.

Tuned is way more fun, and nobody will disagree with that not even you once you do it.
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      12-24-2019, 12:09 PM   #11
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i LOL'ed pretty hard at this entire thread. (Thank you for that)

It always cracks me up reading the sage proclamations from those who lack a basic understanding of automotive engine dynamics.

Cool gauge. Happy motoring
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      12-30-2019, 09:55 AM   #12
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Quote:
Originally Posted by edisapimp View Post
i LOL'ed pretty hard at this entire thread. (Thank you for that)

It always cracks me up reading the sage proclamations from those who lack a basic understanding of automotive engine dynamics.

Cool gauge. Happy motoring
Care to share some of your wisdom? I'd love to know what's accurate and what is a misconception in this thread.
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      12-31-2019, 07:36 PM   #13
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Quote:
Originally Posted by edisapimp View Post
i LOL'ed pretty hard at this entire thread. (Thank you for that)

It always cracks me up reading the sage proclamations from those who lack a basic understanding of automotive engine dynamics.

Cool gauge. Happy motoring
Yes, please share your wisdom with us, your pimpness. I’m always open to anything that makes me wiser.
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      04-08-2020, 09:30 PM   #14
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Unhappy Similar issue!

Hey I just want to chime in here and say I'm experiencing a similar issue.
n55 135i 2010, FBO and MHD stage 2 - no boost pressure at all, high WGDC reading, pressure tested system and no leaks
Only thing I can think of is boost solenoid after reading every thread I can find.
I've ordered one so fingers crossed, any other suggestions?
I can't hear any turbo spool, throws half CEL and goes in limp mode above 4500rpm when trying to log the pull
Target boost is 19psi, actual is -4psi throughout the pull... WGDC 100%
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      04-09-2020, 09:51 AM   #15
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Quote:
Originally Posted by jake_nz18 View Post
Hey I just want to chime in here and say I'm experiencing a similar issue.
n55 135i 2010, FBO and MHD stage 2 - no boost pressure at all, high WGDC reading, pressure tested system and no leaks
Only thing I can think of is boost solenoid after reading every thread I can find.
I've ordered one so fingers crossed, any other suggestions?
I can't hear any turbo spool, throws half CEL and goes in limp mode above 4500rpm when trying to log the pull
Target boost is 19psi, actual is -4psi throughout the pull... WGDC 100%

+1 for solenoid
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      06-11-2020, 03:49 PM   #16
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I had the same exact problem and chased it for a month or so and it ended up being what everyone I talked to said it couldn't be. But soon as I replaced my valve cover to fix my pcv valve that was leaking and it some how caused the same issues. Once I fixed it and talked to a BMW tech at the dealership he confirmed that it would cause these issues. Hopefully this will help you. I started my car and put a lights flame up to the Pcv Barb and it sucked the flame in. Good luck I know this is a old Post but might help someone reading it oneday
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