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Alex Roy · Posted: Wed Apr 09, 2003 1:45 am Post subject: Blow off valve

Has anybody installed a blow off valve to reduce spool up time between shifts? I am planning on installing one at some point.

The hard part about putting a blow off valve on the 931 is that the type that vents to the atmosphere will not work with the 931 as it will greatly mess up the fuel metering when it is open, making clouds of black smoke when the blow off valve is active because it will dump too much gas into the engine.

I am thinking of getting the Greddy Type S blow off valve as it has input and output flanges so I can re-route the air from after the turbo, to before the turbo, but after the fuel metering, making the extra air go in a loop around the turbo, but not effecting the fuel metering (I hope that makes sense).

I'll try to draw a picture and post it here later, but do any of you have any ideas on this? It would cost about $100-$130 for the BOV, would it be worth it?.

Smoothie · Posted: Wed Apr 09, 2003 3:11 am Post subject:

Here's a past discussion on it:
http://www.924board.org/viewtopic.php?t=2842
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

Alex Roy · Posted: Wed Apr 09, 2003 3:53 am Post subject:

There is a little bit of miss information in that post.

A pop-off valve serves a different purpose (though similar) than a blow off valve. The best explaination I found was here: http://www.rx7.com/cgi-local/2catalog.cgi?cat=9&part=5

The correct info in there is that the blow off valve re-circulates the already boosted air so you don't slow down the compressor wheel by forcing air back through the turbo. For the cost of the part(s), I think it might be an interesting thing to do, since from the looks of it, nobody here has actually used one.

numbbers · Posted: Wed Apr 09, 2003 9:36 am Post subject:

Well, if you can figure anyway to keep the boost up between the first and second gear upshift, be sure to post it. Those damn synchro's just won't let me shift fast enough.
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1980 924 Turbo

-nick · Joined: 16 Nov 2002 Posts: 2699 Location: Cambridge, MA

seems that the pop-off somewhat serves the same purpose, though not as efficiently. when the throttle slams shut while shifting the excess pressure activates the pop-off. same as the blow-off, except the BOV automatically opens when it sees intake vacuum thus is much more responsive.

if you use a BOV then what is the point of the pop-off? i guess if the wastegate and the boost cut-out switch both fail then the pop-off will keep the boost from soaring too high. other than that i'm not sure that i understand how they are so different.

sounds like a good plan. the BOV's are not too expensive, keep your boost up better, and sound really cool when they release the pressure (a buddy of mine had a modded Skyline GTR that would make some pretty neat sounds

).

-nick

Alex Roy · Posted: Wed Apr 09, 2003 4:11 pm Post subject:

The pop off valve shunts off dangerous over boost while the blow off valve opens when pressure after the throttle body is lower than in the output of the turbo to keep the pressure from spinning down the turbo unnecessarily. It keeps the turbo spinning as fast as possible with the least pressure resistance, the pop off is merely a safety feature, while the blow off valve is for turbo performance between shifts. The Blow off valve can serve the same purpose as the pop off valve, but the pop off valve doesn't do the same thing as the blow off valve.

924 turbo · Posted: Wed Apr 09, 2003 9:52 pm Post subject:

The car already sort of has a blow-off valve, in the form of the compressor bypass valve. A modern blow-off valve may help the turbo live a little longer, but I doubt you'd find much in the way of performance. It might be worth a shot, though! I'm certainly not against using an external blow-off valve if it proves to be significantly more efficient than the compressor bypass.

The 931's fuel-cutoff switch in the pressure pipe essentially serves the same purpose that a pop-off valve would, by shutting the engine off in cases of overboost. Probably not as safe, but it's there.
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Jon Furst

924 turbo · Posted: Thu Apr 10, 2003 1:47 am Post subject:

This picture might help. Upon closing the throttle, the buildup of vacuum in the intake manifold allows air to escape from the pressurized outlet of the turbo compressor to the inlet via the compressor bypass. The vacuum line attaches at the bottom of the intake manifold.
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Jon Furst

Smoothie · Posted: Thu Apr 10, 2003 2:09 am Post subject:

Nice paperweight you got there Jon.

The end-plate that's facing up in your photo is where I was thinking it would be good to route the air released from a blowoff valve if one were used. It could either be drilled and tapped for a fitting or an end plate like the one on the other side with the fitting built in could be used. You'd leave the vacuum hookup on the other side to function as it was designed and use the fitting on this side to hook up the pressure output from the blowoff valve. This way the bypass would be activated by either vacuum or pressure as required and the output from the blowoff valve would be put back into the intake instead of being released which would alter the air-fuel mix. Just theory though - haven't tried it or seen it done before.
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox