RussB

On a turbo MC1 motor, I wonder if adding a bypass valve (to reduce intake pressure spikes between shifts) momentarily pressurizes the crankcase (via the crankcase and valve cover breather tubes)?

If so, I can't believe that this could be a good thing (from the standpoints of output power and longevity of various crankcase seals).

Black-Wolf

no it does not..

the pressure is redirected to in front of the turbo not into the crankcase. the pressure helps keep the turbo spooled up between shifts. the crankcase is under vacuum when the bpv is active. if a small amount of boost makes it into the crankcase the vacuum is greater so the boost is nullified. another point to consider is if a bpv is installed correctly the boost stays in the intake. if a bit of boost there was a problem whats happening when your driving with full boost?

RussB

I'm just examining how a hypothetical BPV would work . . .

I examined my stock MC1 engine today and didn't see any sort of check valve or PCV valve that would keep the pressure pulse from the BPV out of the crankcase.

RussB

I wonder what (if anything) they do to keep the pressure pulse from a stock BPV out of the crankcase?

I never thought about it before, so never bothered to look.

boxerfan

and considering I've had 12 too many beerz tonight...

The BPV routs pressurized air from the area infront of the throttle body back in front of the turbo and is usually actuated by vacuum from the IM.

How is that going to pressurize the crank case?

The TB slams shut under shift. The turbo is still spooled and is trying to cram air into a closed door. The BPV opens, and true to it's name bypasses the intake and dumps the air into the intake stream. At the same time the slammed shut TB creates a vacuum as the motor purges all the air from the motor side of the TB and r3esults in opening the BPV and giving theb air another path to foolow. The pressure spike never hits the crank case as far as I can tell.

Perhaps you ahve thought of another scenario or an alternate way of thinking about this (I'm WAY outside the boix right now). What's the thought?

Twisted5000

I dont know how the crankcase would become involved at all? What a BPV does is takes already compressed (ex-turbo air from before the TB, and reverts it back to before the turbo. In effect, the air never reaches the TB, and thus the engine at all... never mind the crankcase.

In a strange way of thinking, the BPV is no different than a turbocharger... puts air into the Throttle Body.

....Remember, a BPV is only in effect when you're off throttle. That is, basically, when you shift. At that point the pressure on the Intake is released... the TB closes (mostly) The BPV is just an afterthought in an effort to save the turbocharger life.

RussB

I notice that the crankcase breather system on the 20 valve engine includes a one way check valve; I don't think that our 10 valve engines have such a valve. The 20V engine also has a turbo bypass valve. Could that be why it has a one way check valve in its crankcase breather and ours doesn't?

Did Audi find that on the 20V engine the pressure pulse, or a portion of it, from the BPV would momentarily pressurize (or at least reduce the vacuum of) the crankcase?

I'm sure that the flow and instantaneous pressure inside the intake tract is wild . . . but when the BPV opens it must become even wilder for that instant, maybe creating standing waves . . . I don't know.

boxerfan

The crank vent hose on my 10v goes to the rear of the intake manifold and that is behind the TB valve.

If anything, the crank is exposed to sudden changes as the TB closes when shifting under heavy load. This might suck the seals into failing... just musing now...

The only things that I can think of that might be affected are the ISV and the Decel valves.

RussB

Mine's a 1988 5000CSTQ. What model year is yours?

boxerfan

There is a tripple splitter hose off the rear of the IM.