My son has a 1996 GSX Turbo. He recently got an aftermarket blow-off valve from a 3000 twin turbo his friend is selling and installed it on his car.

I didn't pay too much attention until I drove the car and was disappointed to see that every time I shifted the 5 speed that a loud whoosh took place and the car slowed like you put the brakes on. I was running up to about 4,000 RPM at partial throttle. It seems to me that after each shift this valve is releasing the boost that the turbo worked hard to build up and boost must be rebuilt after each shift. Meantime there is turbo lag in each gear.

I suggested to my son if this valve was such a great idea, the engineers from the factory would have put one on originally. No surprise that he did not follow my advice and go back to the stock valve.

Does anyone know what these valves are supposed to do? As I noted, my impression is that the valve causes boost to be lost and slows the car down at normal driving speeds.

My son is trying to tell me somehow this all makes the car faster. He can't make a good explanation of what the valve really does and how it is a good thing. I think probably because he really does not understand what it does and how it is beneficial. All I see is a silly whoosh sound and a big lag in power between each gear.

Can anyone help me out here?? Also, can he damage his engine with this blow-off valve? It seems that if he is constantly losing boost that he will be making the turbo work that much harder to re-build boost.

Thanks,
The dumb old man

What is it?
The CBV/BOV is supposed to blow open when you slam the throttle plate shut, creating a loop of compressed air running through the turbo and i/c. This loop keeps the turbo spooled up, and prevents the back pressure from slamming into the turbo, stressing and slowing it. The problem is that the CBV on 95+ cars leaks at high boost. Pre-95 vehicles have a CBV which has a stronger spring and won't leak (much) until it is supposed to blow off. One option available to all model years is to remove the loop effect, and just have the CBV blow off into the atmosphere.(whoosh sound) This option in my opinion is not a good idea if you are driving the car on the street. If the pressure is just blown off, that is wasted energy which could be used to keep the turbo spooled up. Drag racers run it sometimes because an open dump can release more pressure than the stock closed loop setup.
Credited to; [email protected] (APalmer)

The reason its not a good idea to vent to the atmosphere, is because the mass air reads how much air being ingested in to the motor threw the turbo. When the bov dumps back into the intake track the metered air remain consistent with what the meter has already read. The ECU determines how much fuel to add dependent on how much air the mass air recognizes. When you dump to the atmosphere the ECU thinks that air that is being ejected from the intake track is actually being ingested into the motor and supplies a given amount of fuel to match the metered air. But that figure is now incorrect because the air is not going into the motor its going into the air, so now you have a bad air/fuel ratio between shifts.
This info was gotten from this forum, Just want to give credit to JDMAWD, good info.....

Excellent and fully explained response. I wish all posts would be answered so fully and completely.

Ok, I did the same thing about 6 months ago. The 3kGT BOV is the same size as the OEM BOV and fits right in. The only difference is the top is metal. The pshhhh sound is normal, its the sound all BOV's make. Its air in the Intercooler pipes being released back into the intake pipe when you shift gears. The reason for this is when the throttle is closed. The air has no where to go and slows down the spoiling up process of the turbo.

Now we need to figure why you are slowing down.

Are you venting the BOV to the atmosphere? Is it rerouted to go back into the intake pipe. Put that wired shaped pipe back on and problem solved.

Its sounds like you have a vacuum leak. There is a black rubber piece that must be inserted with the BOV if he did not put that back on. There is your vacuum leak.

Check to see if the hose clamps are tighten down on both the intake pipe and the IC pipe.

Beisdes that, there is no reason you should have problems.

Thanks for the good info. Maybe the BOV has a valid purpose and the install was wrong and is causing a problem.

The Dumb Old Man

on added note he needs an Aftermarket BOV along his way when he upgrades to bigger turbos ect ect!

Originally posted by EvolvingGST
on added note he needs an Aftermarket BOV along his way when he upgrades to bigger turbos ect ect!

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Or just a stock unit from a 90-94.

The reason blow off valves have to be put on most turboed cars is to stop a pressure wave that results from quickly closing the throttle plate, from hitting the compressor turbine, and causing damage to it. I forget the exact term for it, but ask anybody running high boost, and a BOV that doesn't open well enough what happens.

On some lower boosting cars (older Saabs for example) they didn't have a Blow off valve, because they only ran at a maximum of 6-7 psi of boost.

On a DSM, the way the air incoming to the car is measured happens right behind the filter, any loss of air after the filter results in incorrect air/fuel ratios, because the ECU sees more air than is actually reaching the engine. That is why the BOV on our cars recirculates the air back into the intake, so very little of the already-measured air is lost. Having the BOV vent to atmosphere causes the car to be very very rich when you let off on the gas quickly.

-Jesse

I know what the negitive effects are but would i get any damage from venting to the atmosphere for a day or two? the only problem i could think of would be excess fuel washing oil of the cylinder wall.

Originally posted by Mitsuguy

I suggested to my son if this valve was such a great idea, the engineers from the factory would have put one on originally.

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Oh, no. You told you kid something your dad told you about the '56 Chevy? "Y'know son, if them braght injunears at Chebby thought them big fat tars on the rear end was a good ideah, don'cha think they'da done that at the factory?"

Tsk, tsk, tsk. Congratulations, you're on the verge of getting old.

Remember, it's gonna be a decade before you'll be able to tell the kid anything. Just ride it out.

gvr4 summed it up nicely

basically, the turbo is fully spooled up and throwing tons of air into the engine. suddenly, you take your foot off the gas to shift. the throttle plate slams shut when you do this. between the compressor outlet of the turbo and the throttle plate there is a large amount of compressed air with no place to go, except back through the turbo. this is bad; this is called compressor surge.
a blow off valve, or compressor bypass valve both open right now, and let all this compressed air have a route to leave other than back through the turbo.
the blow off valve, which is what your son has just lets the air shoot out into the engine bay compartment (providing the loud ass sound). a compressor bypass valve does the same exact thing, except the air is ducted back into the intake where it goes back in to the compressor.

a blow off valve typically will not create turbo lag. what it will do, however, since your son's car is designed for a compressor bypass valve, is make the car run rich for a short time during shifts. air that has already been metered through the MAF has left the induction route, but your engine basically thinks its still being detonated, so more fuel is sprayed in.
blow off valves are typically harmless for these cars, minus those two little problems. why didn't the factory put one on? the factory put on a cheap ass little plastic compressor bypass valve that leaks boost quite easily. its also very quiet, which is my guess as to why your son has replaced it.

bottom line: its fine, dont worry about it. if you really dont like it, see if you can route a rubber hose from the outlet of his blow off valve back into the intake tube. this will eliminate those two little problems, while not leaking and being nice and loud for your kid.

my $0.02

Ancient Thread!

wow no shit, who brought this back from the dead?

my reply was a waste of time :/