Blow Off Valves

[Jana]

Tell me how they work? (continuing from previous off topic thread)

[A//// Guy]

Move that stuff that pertains to the mazda and its setup in here then if it doesnt belong the FMIC topic. Which I think is fine as it is not that off topic because the BOV and its operation are directly related to the setup you guys installed.

[Matt D.]

Originally posted by DSMChick@May 24 2004, 12:14 PM
Tell me how they work? (continuing from previous off topic thread)
Blow off valves are simply valves that release pressure between the turbo and the throttle body. They are activated by vacuum, negative pressure in the intake manifold. This vacuum happens when the throttle is closed, and the engine is sucking air in faster than the throttle body is providing it. When vacuum is created the blow off valve (BOV) opens. Under normal circumstances the BOV should never open at idle. If it does certain things must be done so that the car operates properly.

[LightningGSX]

Its actually a good thing if they open at idle or anytime you aren't building boost(recirculating setup that is), it will give you much better throttle response and quicker spool.Most factory BOV recirculating setups are designed to do just that, thats why OEMs usually call them Compressor Bypass Valves.Ideally you'd want the BOV open anytime the boost is less than 0 psig, and open when the throttle plate closes of course.

[A//// Guy]

So since no one will try an answer my previous question about how my stock BOV would blow air out at idle, I did some research. It seems as though I was right an a few of you were wrong.

At idle the BOV will spill air out of the BOV because there is positive pressure in the piping before the TB, and the vacuum in the intake manifold will slightly open the BOV.

I verified that with this page http://www.racetep.com/bypass.html

So in this case the Mazda was running like crap (rich not lean) because some air that was being metered was being released out of the BOV and therefore creating a rich idle.

A strong BOV like a tial shouldnt open at idle like a stock one does so if it does there is either really strong vacuum or the spring is faulty.

[LightningGSX]

You are wrong, I'm not even gonna bother to look at that page because I'm guessing they are wrong also.At idle(when MAP is vacuum), the air inside the I/C piping is below atmospheric pressure(or at max in equillibrium with atmospheric), they is no way(without breaking the 2nd law of thermodynamics) air can flow out of the BOV.Its possible your turbo(spinning at idle) brought the air inside the pipes above atmospheric(not likely), in which case it could flow out.In the case of the Mazda, it was in fact running LEAN, causing shitty idle.

[LightningGSX]

BTW IIRC the Tial BOV comes with a .7 Bar spring, which means it will open at 9.24 in Hg of vacuum, most vehicles will pull twice that at idle.

[A//// Guy]

So according to you my car was not pushing air out of the BOV at idle.. I guess I was lying and I guess a few pages I visit were also wrong.

I am going to have to do a few test and find out the results becuase I guess no one can totally clarify.. even though I tested it already.

Mike or Josh you want to chime in on why my car is an exception then?? At first I use to think the same way as you guys did but I just dont believe it... It does make total sense but not in my case for some reason.

[Jacek]

blowing air out? you running anti-lag or something? ;)

blow off valve is just a pressure relief valve. works in the same manner as a boost controller but different use..

[JET]

Originally posted by EclipseTurbo@May 24 2004, 09:26 PM

I verified that with this page http://www.racetep.com/bypass.html

So in this case the Mazda was running like crap (rich not lean) because some air that was being metered was being released out of the BOV and therefore creating a rich idle.

Anyone else find a Starion page at that link? I don't see anything about a Mazda and nothing on there said they were blowing air out.

Just think about it though. If you are at idle and floor your car, how high does the boost gauge go? Usually to -2 in hg or so. This is under atmosperic, so it can't be pressurized.

I am a little intrigued by the fact that it is in the IC piping though. That is a smaller area and could possibly have a higher pressure, but I still don't see any way that the compressor is putting out more air than the engine is using at idle.