Anatomy of a Blow Off Valve

[WhoaTed]

The following is by DSMtalk.com Staff Member 99mmcgsx

Greddy type S bov dissected, explained, cut-away with CAD graphics

Included here are some CAD images that I put together to help explain what is happening inside the Greddy Type S blow off valve. The CAD work was done from a sketch I put together, so the exact scale of things is not precise, though the general layout is correct. The text and graphics are meant to be fairly self-explanatory, but if you have any questions feel free to ask and discuss. I'm no bov expert, so if you have additional insights that I have overlooked here, please respond.

I tend to be a visual person, and when I intake leak tested my 99gsx and found leaks at my bov, I decided to put together this model to help diagnose my problems. It is our hope that if any of you are having leaks at your Greddy type S bov, you can use this graphic to help you pinpoint and fix the leak.

In my case, I had a major leak in the actual main bov vent, as well as leakage from the lower nipple of the bov. When the system was pressurized, anything over 5psi would leak directly out of the bov vent into the intake, no matter how much I tightened down the adjustment spring. Air would also escape the lower nipple of the bov.




By disassembling the bov and using this graphic, I was able to determine that the leak was due to a torn rubber membrane which separates the air at the top of the bov from air at the bottom. When under boost, air would travel from the intake manifold, through the torn seal inside the bov, and back inside the intake tract. Granted, a leak of this type is not as bad as one in which the leaked air is escaping into the atmostphere, but in this case the compressed, hot air was esentially "cycling" from my intake back to my turbo.




You can buy a replacement rubber membrane from any authorized greddy dealer, the part number is: 99900061 and it costs $80. The number for tech support at Greddy is 1-800-473-3392. I personally found them to be very helpful.




However, if you have a small tear like I had, you should be able to fix it using some high quality RTV sealant. I removed the membrane from the bov and coated both sides of the rubber with several thin coats of black RTV. It solved the problem, and now not only does my intake hold boost, I also felt an improvement in throttle response and overall the power feels much more solid than it did when I had the leak. This is with a Big 16G sized turbo running at 20 psi.

A quick note on disassembly: The top purple cap on the bov is under spring tension because of the dual spring pushing up on it. If you remove the allen bolts which go around the purple top cap, hold on to the cap or it will pop off, quite violently I imagine. To reassemble, you can push the springs down by hand and tighten the allen bolts back up.

-Jon
99mmcgsx

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