SW20 Bov Operation

[solitaire]

Hi All,

I'm having an arguement with some yanks over a point... they swear they are right, and beyond all reasonable doubt i think i am right.

Would apreciate it if you could set the record straight for me either way as this is really bugging me now.

My arguement is that air can travel in the stock bov on a gen2 3stge sw20 (in the oposite direction that is vented out of it, i.e. in from outside).

I believe this to be true as when i vented the bov to atmos it caused a rough idle (which i assumed to be un-afm'd air getting into the engine). When i put a one way valve that stopped air entering the bov and only allowed it out, the rought idle stopped.

to illustrate:

Bov open to atmosphere, recirc pipe removed, recirc hole before turbo plugged = rough idle

Exactly the same as above + one way valve stopping air entering the system through the bov = fixed idle.

If i'm wrong that fine, but i would love to know how the above is possible otherwise.

Thanks Peeps

[MR2BOY23]

correct, same thing happend with mine.

Apparently at low revs the BOV bypass the turbo for better low down torque

Kim0663 knows the full story

[fivebob]

Air passes both ways through the valve as it's a bypass valve not just a blow off valve. Off boost air flows through the valve bypassing the turbo, which is why you get a rough idle if you vent the stock valve to atmosphere.

Just one question though, why would you ever bother to argue with septic tanks

[solitaire]

Just one question though, why would you ever bother to argue with septic tanks

Yeah as always you raise a good point Fivebob - i really dont know... the chance of getting into an arguement over such a simple matter seemed remote... but then, yanks were involved i guess

I generally try to avoid posting anywhere as i tend to rise to trolls pretty easily... but a guy was having a problem which i actually knew about and seeing as i seem to spend all my time getting help and not being able to give it i thought id try

[solitaire]

I'm going to stop replying to them because its not helping my sanity, but can you guys answer this?

"look at the idle control valve under the throttle body. there's a large hose coming off that valve that goes to the turbo inlet elbow- THAT is where it is pulling idle air from. that is the only source of intake air at idle. how can it pull air in through the BOV when it's not even connected to that pipe? "

just interested to know...

[fivebob]

"look at the idle control valve under the throttle body. there's a large hose coming off that valve that goes to the turbo inlet elbow- THAT is where it is pulling idle air from. that is the only source of intake air at idle. how can it pull air in through the BOV when it's not even connected to that pipe? "

That might be true if the ISC was the only source of air at idle and if the AFM was the only source for air going to the ISC.

For this to be the case the throttle would have to shut completely allowing no air to pass through. It does not function this way for lots of good reasons which I won't waste my time detailing.

Also last time I looked, the point at which the ISC draws it's air is connected to the same system that the BOV is, and I don't recall the function of the turbo to be that of a one way valve

[solitaire]

Understood, Thanks Fivebob & MR2BOY23

[Akane]

From what I "feel" with my hand-o-meter, the turbo is blowing a fair amount of air, even at idle, if anything the pre-throttle body sections of intake pipes will ALWAYS have a positive pressure, rendering the "bypass valve to suck air in" theory a bit unbelievable.

I believe the turbo can provide more flow than the engine can consume, even while offbost, at all times.

If anything the By pass valve might be venting to prevent air/pressure buildup in the intake pipes causing the turbo compressor to stall, or slow down, while you're in offboost crusing mode.

You can try this on your Gen 3 engine: opening the bypass valve with your thumb (they're quite soft anyway). Rev your engine, hold rev, and push valve with thumb, on mine I can hear a small PSHHH and air is being vented out. Although free revving does not mean normal driving conditions, but I suspect while under normal off boost driving conditions, the turbo still provide more air than the engine can consume.

Just IMO anyway.

[fivebob]

From what I "feel" with my hand-o-meter, the turbo is blowing a fair amount of air, even at idle, if anything the pre-throttle body sections of intake pipes will ALWAYS have a positive pressure, rendering the "bypass valve to suck air in" theory a bit unbelievable.

Actually it's very believable. Given that air will always flow from high pressure to low pressure, if the pressure in the intake pipes ever falls below atmospheric pressure when the valve is open then it will flow through the valve into the intake.

I believe the turbo can provide more flow than the engine can consume, even while offbost, at all times.

Perhaps you don't understand how a cetrifugal compressor works then, I would suggest that you try putting a MAP sensor straight after the turbo and then you might see if your theory holds water... or air in this case.

[solitaire]

Yeah the bit i dont get is, how can air be flowing out when the system is in vacum? the yanks made heaps of arguement with heaps of logical reasons why (although i didn't really understand a lot of what they said)... But i just can't understand how air can be transitioning to an area with higher pressure...

[Akane]

[Perhaps you don't understand how a cetrifugal compressor works then, I would suggest that you try putting a MAP sensor straight after the turbo and then you might see if your theory holds water... or air in this case.

Without actually going through the trouble of obtaining a MAP sensor and the associated hardware, let me take a guess - their flow are exponential to speed until they reach their efficiency range?

[solitaire]

[Perhaps you don't understand how a cetrifugal compressor works then, I would suggest that you try putting a MAP sensor straight after the turbo and then you might see if your theory holds water... or air in this case.

Without actually going through the trouble of obtaining a MAP sensor and the associated hardware, let me take a guess - their flow are exponential to speed until they reach their efficiency range?

You two are already about a million miles above my head