V2Rocket

My basic understanding of the 951 bypass valve is that when you close the throttle, the air basically bounces back down the pipe the direction it came, and the valve sees this pressure being higher than it should be, opens the valve and routes that pressurized air to in front of the turbo, after the AFM, to prevent that air from slowing down the compressor wheel. It is a blow-off valve that "blows" back into the intake.

Is this correct? If so, what kind of pressure is needed to make a stock valve open? And does this valve work when off-throttle, routing air around the turbo?

Darwantae951

For the most part yes, I don't know if you can really describe it as the air bouncing back down the pipe in the direction it came. It's more like the air hits the closed throttle plate and backs up on itself, eventually making its way to the compressor wheel and causing compressor surge (a fluttering sound, I'm sure you've heard it before). The bypass valve does just what you said, by re-routing the air in front of the compressor housing but after the AFM so as not to turn into a rich situation. The valve itself is controlled via vacuum.

theedge

The valve is opened via manifold vacuum and closed via pressure basically. It has a vacuum line going back to the manifold (post throttle body). Once the throttle is shut, the vacuum pulls/the boost pressure pushes it open.

It is open during non boost driving, yes.

V2Rocket

so on light throttle the air going into the intake is not being sent through the turbo/IC?

lee101315

No, its just being drawn through via vacuum, it still has the same path.
This website has a diagram to help you understand a bit better
http://en.wikipedia.org/wiki/Blow_off_valve

fortysixandtwo

No. All of the air to run the engine still goes through the IC, regardless of manifold pressure.
The valve will open when if intake vacuum is sufficient to overcome the spring pressure, but the bypass side of the valve is not connected to the plenum. Either way, this should not happen if the valve is functioning properly.
The actuator side of the valve, which is connected to the plenum, is just a dead end as far as the plenum is concerened.

schip43

Can you vent to atmosphere if you run a MAF setup? I see hardpipe kits that have a blow off valve option.

Andy1212

You could, depending on where on the intake tract the maf is located. But venting to the atmosphere is kinda pointless.

fortysixandtwo

Yeah, it can be done. The air fuel ratio will go super rich for a second, because you'll be venting metered air. If you went to a speed-density system, this would not be an issue.

V2Rocket

does the valve only close under positive pressure? or a particular amount of vacuum?

95ONE

Blow off valves have a spring in there.. some higher, some lower, but like to remain closed.. When you shut the throttle butterfly, it creates vacuum.. that vacuum pulls against the spring ,opening the bypass valve.. it will not want to open until there is enough vacuum to overcome the spring pressure. (usually light) When you boost.. the boost pressure pushes at least the same boost lbs your runinnng against the valve plus the spring pressure... therefore shouldnt' open no matter how much boost you run....... because the same boost pressure is being used to push it closed.. always equal.

V2Rocket

assuming you have a bone stock 951, about what point does the valve close to allow full airflow to the turbo? quarter throttle? half throttle? or only above half/WOT?

Darwantae951

Depends on the spring pressure of the bypass valve as to when pressure is great enough for the valve to close. If it's a stock unit, I can't imagine it's very much.

V2Rocket

anybody got a good used one they dont need?

drift a 944

I have a couple of stock Bosch bypass valves laying around I think..