how to mod stock blow off valve to hold boost

[someguy2800]

I thought some of the budget builders out there would appreciate this.

I used to have a test bench I built off an air compressor to test lost of different blow off valves, and would modify stock blow off valves and diverter valves for friends. I would bolt on bov and keep increasing the pressure till either it blew up, blew off, or leaked to the point where the air compressor couldn't generate any more pressure. Then I would modify it till it held. I had one plastic bosch valve which blew up into little pieces at 30 psi, but most they wouldn't hold over 10 psi without leaking horribly or tearing the diaphram. A 2g dsm valve was the worst I ever saw and would leak so badly you couldn't make over 3 psi without modifications. For my money the best are the metal mitsubishi valve like found of the stock 1g dsm and evo's. With this mod they will hold 50 psi with minimal or no leakage.

So here is how to fix stock dual chamber blow off valves to hold boost. Since they do not have the dual chamber, this does NOT apply to the short bosch diverter valves found on audi's and vw's. Here is a write up I did several years ago on DSM tuners.

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Okay so we all know that the 2g DSM blow off valve are notoriously crappy and hold no boost at all so I decided to get my hands on one and play around with it to see if I could make it hold more boost pressure without leaking. Here's my crappy drawing of a 2g BOV.



A DSM friend of mine gave me one for free to play with so I started to do some bench testing with an air compressor. I made an addapter so that I could hook the bottom port of the wastegate directly to the compressor tank and an extra line with a release valve to use for the top port of the diaphram. I started bench testing it just as it was and found that it was actually leaking at 3 psi of pressure!

Heres why, there is a hole drilled through the center of the valve which allows boost pressure to travel up the valve and inside the wastegate can under the diaphram. Under boost the pressure is equal above and below the diaphram so it relies totally on the spring pressure to hold it shut. With any amount of boost pressure at all the valve is forced open and it leaks.



So why is the hole there? When you let off the throttel manifold vacume sucks all the air out of the top of the wastegate can which combined with the boost pressure under the diaphram forces the valve open and venting the excess boost pressure.



So whats the fix? This is a trick that has been used on the 1g bov's for years, it just takes some creativity to addapt the trick to the 2g's. What you need to do is shut off the supply of boost going to the bottom port of the wastegate can. So just simply find something to plug the hole in the center of the valve. In my case I coated a nail with epoxy and simply stuck it in the hole. Then you need to make a hole to be vent the pressure out of the bottom part of the wastegate can when the valve moves. I drilled a small .100" hole in the can from the outlet port of the wastegate. Make sure you put the valve in in a vise or something to push the valve open to make sure you don't drill into the diaphram when you do it.



After you have done this the wastegate will work as follows





I again bench tested the valve to make sure it works so I filled the air tank with about 20 psi of air and the valve stayed tightly shut. Then I removed the pressure line from the top of the wastegate and the valve instantly vented the entire air tank even without a vacume source. The valve does not have a very good seal so it never seals up compleatly but the amount of leakage is minimal and won't affect performance, you can barely feel it on your hand. Next I started increasing the pressure to see what would happen. I eventually increased the pressure up to 50 psi and the valve still stayed firmly shut. Again I increased the pressure a bit and the valve blew compleatly off the connector from the pressure! So I made one final check to make sure the diaphram was still intact after seing 50+ lbs of pressure and it was. To test it out I put it on my BMW which runs 15 lbs of boost. It never leaked and vented quickly. Becouse I removed the boost source under the diaphram the valve doesn't open quite as easily as before, but it still opens redily at any pressure above 2 psi or so. So if your a cheap ass like me there is a way to make the bov on your 2g capable of holding some boost pressure without throwing it away and getting a new one.

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here is another write up on the 1g dsm valve. this covers the metal valves like the 1g dsm, evo's, subaru's, nissan's, and others.

http://www.thedodgegarage.com/turbo_bov_mod.html

This mod works on any stock blow off valve with the dual chamber design. The only variance really is where to plug off boost from the diaphram, and where to safely drill the vent hole.

As far as the plastic bosch diverter valves fitted to Porsche's, Audi's, VW's, Saab, Volvo, I personally think they are garbage. They don't seal worth a shit, the diaphragms tear and leak, and most of them are just too small. But if you happen to have one and don't want to change, you can make it operate as above with no modifications by simply turning it around backwards with the valve facing towards the boost. This way it will operate exactly as explained above. Until that is, the diaphragm ruptures and you have to throw it in the garbage anyway.

Note that if you are going to vent any type of valve to atmosphere you better put a filter on it. Enjoy

[MiniDude824]

wat

[SiGmA]

+1, what?

[MiniDude824]

this is how i modded mine

http://www.google.com/products/catal...ed=0CGIQ8wIwAQ

[TechnoViolin]

Que? I am running 10 pounds. Just tell me what Bypass valve to get that will work with my RMS stage 2.

[bawareca]

Great info.It could never come to my mind that someone would design and manufacture valve with such a design flaw.
In the past,I used old volvo metal BPV,either as a BOV or BPV.Worked fine without major leaks.

[someguy2800]

+1, what?

This is how to modify a junkyard blow off valve from another car to hold more boost. For the cheapo people out there.

Great info.It could never come to my mind that someone would design and manufacture valve with such a design flaw.
In the past,I used old volvo metal BPV,either as a BOV or BPV.Worked fine without major leaks.

The really shocking thing is how many different manufacturers use the same flawed design. The reason they do this is to make the valve open quicker to elimitate any possible compressor surge. They are only concerned with 200,000 mile reliability for the compressor wheels.

[whiteghost7]

slightly crushing the 1g DSM BOV in a vice will also increase it's holding capability to around 30psi.