Originally Posted by Freon
On a Subaru or the N55, the fueling is MAF based. The air is "metered" as it passes the MAF sensor, so the engine computer injects fuel as if that air was going into the cylinders. If it is instead dumped back out to the atmosphere instead of recirculated into the intake, the car still injects that fuel and the car goes rich. It has absolutely no way to "know" that air was dumped. There's no "tune" fix for that either. It is a system design error.
The worst part is that after the oxygen sensor detects that rich spike, the ECU try to pull fuel, and then the car goes lean. This can be bad. You shift, go rich, ECU tries to compensate, you punch the gas, car goes lean. Not good. In situations such as full throttle, part throttle, full throttle you can easily cause the engine to knock due to the lean condition at high throttle. Oxygen sensors are not fast enough to deal with this. If they were, you wouldn't need any MAF or even speed density.
There's really no way to tune this out on a Subaru or N55, but people love their ridiculous noise making external dump blow off valves, so there is a falsehood spread that you are ok as long as it is tuned for so tuners can take money in to "fix" it. But there's no actual way to tune this problem out, and I'd be more than happy to have an open debate with anyone who disagrees. It's a system design flaw to dump metered air. The real fixes would be to relocate the MAF between the blow off valve and throttle (which is a huge undertaking and really only done for big turbo cars, and comes with its own host of problems), or switch to speed-density fueling like the N54, which again is not always simple. I see no circumstance where you should do this just because you want a noise making device so the car sounds cool.
I highly recommend against ever using a blow off valve that dumps air out of the system on any car with a mass airflow sensor. Not even 50/50 valves that recirculate at first, but open dump on larger throttle reductions. You've just made the flaw smaller, but it is still a flaw.
N54, good to go, doesn't really matter as it uses speed density fueling. Use whatever blow off valve style you want. You may want to adjust the spring on the blow off valve itself to give you the best behavior. There are pros and cons to tighter or looser release. N55, stick with a full recirculating setup where the air released from the charge pipe is routed back into the intake.
I still tend to think the tune itself really doesn't have too much effect.
For the record, I tuned dozens if not hundreds of Subarus for years from around 2004-2008, and was heavily involved in OpenECU, and wrote an actual speed-density patch for the 32bit Renesas CPU that is now widely used in many EJ257 turbo that are tuned using OpenECU.
Thanks for the great info, you definitely seem quite knowledgeable in the subject and it all makes sense. I remember on my Subaru I never really had any issues with the BOV besides when it was "unntuned." It wouldn't idle. I always did open-source as well (bugeye 2003 w/ EJ257). But do you think the jerk from releasing throttle midboost would be due to the BOV? I remember it always did that but I always assumed it was backpressure buildup from the turbo.
After reading this, I think I may stick with the BPV