I recently picked up a supercharged S, so I'm relatively new to supercharging. It's a high boost SOS kit running ~10psi.

Over the weekend I decided to vent the diverter valve to atmosphere. In partial throttle/low boost, it's a clean whoosh sound when I back off the throttle, but it has a strange farty/purring note at higher loads. Am I now experiencing compressor surge? I didn't notice it when it was still recirculated.

I'll get a video uploaded later.

 

If you're venting to atmosphere without retuning, you're running extra rich in moments when you purge. Maybe the exhaust sound is changing due to too much fuel?

 

LMFAO @ "farty"!!!

In for videos, but it sounds like you may have some compressor surge. Every time I've had a 1" diameter BOV/BPV venting to atmosphere I've heard the same thing at high RPM shifts. I heard it on the stock Bosch BPV, I heard it on the Turbosmart Kompact BOV, and I've heard it on the Tial QR. The only other BOV I've had was the Tial Q, and I didn't hear it with that one. That could be because it vents better, or it could be because the valve is so damn loud that I just couldn't hear anything else.

 

Can you explain why this would be? Unless I’m missing something, thinking about what’s going on with this scenario, it shouldn’t make a bit of difference. The TB is closed when blowing off to atmosphere or recirculation, so tuning isnt part of the equation. The only thing recirculating the BOV does is maintain hot spent charge air in your intake system, which again is isolated between your closed TB and blower. Venting lets that hot compressed air out. The only thing that would effect tune possibly in this scenario is a slightly lower AIT, however I would be surprised if it warranted any fuel correction beyond normal day to day atmospheric conditions. If the car was tuned when heat soaked or not, this will have a wider impact overall.

 

I thought this was only an issue with MAF sensor vehicles?

 

My 1" dv valve did this on my N1200....go bigger. I think the issue is the 1" DV valve is too small and when its hooked to the intake the sound is either muffled or the vacuum helps pull the air from the charged pipes.

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You are correct. MAF-based vehicles exhibit this problem, not Speed Denisty-based. In addition, "retuning" will not solve a rich off-throttle condition on a MAF-based car due to the fact that you are venting metered air. You can't "tune" around that. To some degree, you will always have that issue with VTA on a MAF car.

You may or may not be experiencing compressor surge. It may just be the valve in operation. On my Supercharged SHO, I had an HKS BPV that had a second reference port under the diaphragm that could be used as a pop-off valve (vent at spring pressure) and could also be used to eliminate the fluttering you're hearing by using the blower reference boost port to help push the valve open from bottom of the diaphragm.

 

Alright thanks - this is exactly along the lines of what I was thinking.

 

Now that I think about it I am mistaken. I was thinking about venting my Kraftwerks kit to atmosphere a while back and I found that there would be tuning required to due so properly. I believe it was due to the air that would be missing in PART throttle conditions that would lead to being rich, not during off throttle. Maybe I'll dig up the links later on.

 

I think you guys are reading into this a little too much. Venting to atmosphere is better for one main reason, to not recirculate hot air into the inlet of the blower. I did this on my old SOS N1200 kit in addition to the custom cai I made. I suppose their is truth to needing a new tune, but the difference will not be noticeable.

The bosch diverter with the SOS kit is probably working just fine. Even at 400whp I ran the stock diverter to the atmosphere with no issues. It did flutter a bit but I took it as working properly. The bosch valve is very quiet compared to any other valve available. If you want to be annoyed with always hearing air moving while part throttle driving, go ahead and "upgrade it" to a different brand. The only way to "solve" compressor surge is to weld a larger oriface on the aftercooler and run a larger bov/diverter. You can only move so much air with a 1 inch hole.

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