Recently, I purchased a used Greddy Type S bov and noticed that the lower nipple is missing, I have never purchased or used an aftermarket BOV before so was wondering where does that lower nipple connect to in the engine bay.

 

ah the Type S. That's oldschool now. It brings back memories of the Brian Spilner era.



It's two-chamber diaphragm system. A source of air from the intake plenum should be applied to the top port. As you release the throttle, a vacuum is generated behind the throttle plates due to the restriction of the closed throttle valve. That vacuum pulls on the top side of the diaphragm in the BOV and lifts the valve inside the BOV, provided that there isn't too much pre load in the system. Attach a boost pressure source from before the throttle plate to the bottom nipple and technically it will assist in opening the valve. You have manifold vacuum on one side of the diaphragm and boost pressure from before the throttle plates operating on the other side of the diaphragm inside the BOV. That's the same basic idea behind the turbo control valve btw (it has a pressure and a vacuum side to help it open quickly).

In practice the bottom port is normally not needed as long as you don't have too much preload in the spring. The pre load is set with an allen wrench after you loosen the lock nut. If it's too tight (righty tighty lefty loosey), the valve won't open (except perhaps with the assistance of the bottom nipple). If it's too loose your BOV will leak under boost. Be careful playing around with the adjuster too much though. It can eventually fail, causing a leak. There's a reason why Greddy modified this design on their later blowoff valves.

 

sounds like a reason for me to return it, I personally like the sound of the Type S guess I'll have to adjust to the newer version of it.

 

no it'll probably be just fine. like 99% of people don't use that bottom nipple on their type S

 

I didn't even realize that it was supposed to have that lower port. That is why I offered him the option to return it.

 

I was under the impression that this would be determined by the preload amount from the what I read above. So how much preload is too much? I will be running 400hp on a street ported single turbo.

 

Well the bigger concern is too little preload. The boost pressure would push open the BOV and you will have a big boost leak. A quick way to test is to do a boost pressure leak test by pressurizing the inlet of the turbo with an air compressor. You can make a tester out of home depot parts or buy one at www.boostpro.net/prodtester.html (universal tester at bottom)

 

ty for the informative tip