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!