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· Registered
10,051 Posts
Re: (MASH)

Quote, originally posted by MASH »
A blowoff valve is made to take the extra pressure and dispense it. (thus the chirping) We do not have one. Our cars are made to "hold" that pressure so the next gear has enough boost to have little to no "lagg".With a blow off valve, all of that pressure has to build up again. Thus the "lagg" that is noticed with a blow off. I would not trade "lagg" for a nice chirping sound. I hope this helps, Rick

We have one...

A Blow off Valve is exactlt doing what you are telling here only with the term BOV a free to air type of pressure release is often meant instead of what we call a CBV, Compressor Bypass Valve.
The difference is that instead of releasing free to air, outside the intake system, the pressure will be released back in to the intake system before turbo compressor.
The reason for this setup is because the R uses an air metering system so it needs to know how much air is in the system.

· Registered
10,051 Posts
Re: (Stoobey)

Quote, originally posted by Stoobey »
wow I have no clue what to believe now

Do blow off valves benifit th car or not

Eveytime i shift my car makes a small chirp which i would assume is my blow off valve

Somebody who knows cars like the R should explain how this crap works all i see is a bunch of stupid bickering

It isn't that hard to understand.

The R uses an airmetering system which expects a closed loop intake setup. Closed loop means that any air going around inside the engine and plumbing need to go in and out through the air meter (MAF)

when a turbo is delivering boost and the throttle plate closes the turbo still keeps producing boost because of the swing effect but if the boost has nowhere to go it is basically pumping up the intake system. The bad thing about it is that it will cause a backpressure on the compressor wheel which will slow it down and also cause stress on it. The other thing is that it might cause harm to the intercooler which is made out of plastic end tanks which are clamped on the cores. To much boost can tear the clamp area open.

For this reason nearly every single turbo engine nowadays uses a so called recirculation valve or compressor bypass valve (CBV) close to or like with the R inside the compressor.

These CBV's have been present on Volvo's from day one. 200,700 800 and P2 series engine's.

Here you can see the CBV as used on the R's KKK-K24,

This is what it looks like inside,

What it does is to route the air between turbo and throttle plate back to the intake between MAF and compressor, the side where the turbo sucks up the air. Basically the airwill be circulated around the compressor wheel so that it can't build up boost. When the throttle plate closed vacuum will operate this valve.

From a performance point of view the only gain you can expect when changing to a larger CBV or a BOV is that the compressor wheel might keep spinning longer because the stock setup could have to much resistance.

What a free to air BOV basically does is to create a leak in the closed loop intake system and some cars/ECU's simply do not like that. Some ECU's go limp mode, other dump fuel and change timing to fix the leak, other revert back to a replacement value for MAF reading because it is thinking the MAF is out of scale.

In any case, more negatives than positives come with installing a BOV on a Volvo.
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