GM MAF - doesnt the BOV suck in air cruising? debris danger?

[mhuffman]

For those using a blow thru GM MAF setup without a recirculated BOV, doesnt the BOV being open during vacuum conditions present a problem?

1g BOV's open at 15mmHg, and my forge motorsport BOV opens at only 5mmHg... so anytime vacuum is higher 5mmHg the bov is open.. and if i dont recirculate, then cant it suck in dirt and debris from the engine bay?

So, doesnt the BOV need a filter or something to keep stuff from being sucked in during idle and cruise if it isnt recirculated?

 

[polarmoment]

while cruising, the turbo is still spinning trying to push air against a closed throttle plate. since that air can't go through the engine, it goes out the BOV if it's open.

in short, air only ever flows OUT the valve. nothing ever goes IN.

this has been the source of pages upon pages of debate on the mr2 forums, since one of the moderators there (who happened to be a hydraulics engineer) for some reason thought that the BOV opened during cruising to bypass the compressor for better gas mileage. he never actually tested his theory, but everyone just believed him "because he was an engineer."

eventually people started testing it and found that anytime the engine is running, the turbo is spinning, and if the BOV happens to be open, air is ALWAYS coming out, never going in.

my stock BOV on my mr2 is open whenever i'm not in boost and air only comes out.

 

[bastarddsm]

i wouldn't say that, at idle mine definatly sucks in. but its a risk you'll have to take to release the ricer within.

 

[TunaTalon]

I don't know about Mister Twos but the Blow Off Valve on DSM's is also a Compressor Bypass Valve. (Mitsubishi Compressor Bypass Valve Upgrade (1995-1999 turbo)) If the CBV is not re-circulated then it should have a filter.

 

[wret]

I ran with mine open for a while and could find bits of leaf debris on the honeycomb at the MAF. I played around with filters and valves but it was tricky to get it so it didn't interfere with the blow-off function. I believe I had one that worked but it blew off (imagine that). Eventually I stopped worrying about it and left it alone.

 

[99gst_racer]

The BOV is NOT supposed to be opening at idle. That would cause un-metered air to enter the intake tract, causing a lean tune.

It takes two forces to open the BOV; positive pressure at the valve face (boost), and negative pressure (vacuum) above the diaphragm. If only one of these forces is present, the valve should remain closed. This is why it doesn't suck air in during idle - there's no boost pressure in the charge pipes to aid in lifting the valve. If the valve is opening with only vacuum, then it is set too soft.

With that being said; I've had my BOV vented since 2003 and I have no issues with debris entering my crankcase.

 

[wret]

I've bench tested the stock 1g bov/cbv and I can tell you it does open with vacuum only. In a stock set up this would allow it to draw in air that has already passed the MAS, so there's no lean issue. In a blow through set-up it would draw in air prior to the MAF but which would be "counted" before getting to the intake.

 

[99gst_racer]

I wasn't aware of that on the 1G BOV, Ron. Thanks for the input.

 

[turbosax2]

Certain bov's, Tial for example, are held shut by boost. My brother has one with a 7psi spring and it is open at idle. I can't remember 100%, but I believe it sucks air in at idle. He has a blow thru setup so even if it does suck in air the air will be accounted for, but I guess debris could potentially be an issue.

 

[crimsondragon]

I'm ready to test my Forge BOV recirculated for this theory.

 

[mhuffman]

INTERESTING DISCUSSION. SO HERE'S WHAT I THINK AFTER READING ## POSTS:

Just think, if you take the BOV on a bench and apply vacuum, it opens... but it opens because you have ATMOSPHERIC PRESSURE pressing against the bottom of the valve while u have vacuum in the upper cavity. There is a pressure differential between top and bottom.

i know that the pressure differential for my forge BOV is 5 mmHg, because when it is off the car it opens at 5mmHg when the air pressure is ambient. 1g BOV's open at 15mmHg.

Now, when cruising or idling, if the pressure in the UICP is greater than the pressure in the intake manifold, the valve will try to open against the pressure of the spring.

So, the if the BOV is open during idle and cruise, air flows in if atmospheric pressure is higher, air flows out if UICP pressure is higher.

So it all comes down to the following:

1.) pressure differential on the valve between UICP and intake manifold
2.) Ambient air pressure Vs. UICP pressure when the BOV is open

For 1, the valve's open when: UICP pressure - IM pressure > spring pressure
For 2, air flows in the UICP when: Ambient Pressure > UICP pressure

The only unknown here is UICP pressure during cruise and idle. I.M. pressure is measured with boost gauge, spring pressures of BOV's are known, and ambient pressure is known.

Follow?

UICP pressure is probably a function of engine rpm, since at higher rpm the turbine will spin faster even at low throttle.

ANYWAYS, all someone really has to do is start their car in the garage and put their hand over the BOV inlet and see if it sucks or blows! That will tell us if at idle the B.O.valve is open or closed, and if UICP pressure or ambient pressure is higher.

(im betting ambient pressure is higher than UICP pressure at idle, meaning the valve would be sucking air in if it is open)

I think this might be an important subject to really experiment on so that people not recirculating know if there is possibility of ingesting debris thru the BOV.

I think My forge BOV will be more likely to open under cruise and idle because it opens at only 5mmHg.

 

[RipperXX]

I have done that, in the past a few times, because I would sware I could HEAR air being sucked in. I think it was just an acoustic thing, hearing the air passing though the UICP. bnecause I even lit a cigarette and put it next to it, and it didn't draw in the smoke.

I also wet my fing and stuck it just inside the lip of the 1g bov and did not feel any air coming in. So I stopped worrying about it.

If there is any coming in at idle I would think it is an extremely small amount, and nothing to get to worked up over.


ALTHOUGH on the new Tial BOV's coming out I did notice there new vented valve looks to have a screen around it's ports. I duno about it though thats a totally different valve which I have never seen in person etc.

 

[architechnik]

I find this an interesting discussion. As mhuffman stated, while the BOV would open under only vacuum, the pressure differential between the intake mani and the IC piping required to create a vacuum greater than that of the valve spring would not occur under normal cruising conditions.

The intake mani pressure would need to somehow be greater than required air to operate the engine at that particular rpm. Unless the throttle plate were rapidly opening and closing, artificially creating greater air needs than what the engine would be cruising at, then there would never be a point that vacuum line would exceed the negative pressure inside the intercooler piping unless there was a malfunctioning BOV.

Now, this would be an interesting modification for anyone looking to create an N/A idle position for a turbo car. It would just simply use a solenoid, similar to that on the boost control (wastegate), only it activates during idle to open the bov and pull air in.

 

[RipperXX]

I think the only time I could see it opening is right when you go WOT, the throttle body opens but theres only X amount of air in the IC pipes, and untill the turbo starts to spool it could create a vacume. However! It would be like what a half sec or so? Nothing to worry about if it were doing it.

 

[joe]

I thought I would just throw this out there since it would make things more difficult but..couldn't the blow off valve be closed at idle and open on deceleration while in gear when the vacuum is more than at idle.

oh and a side note my greddy type rs is closed at idle but I guess that could also depend on where you have it set at.

 

[Mr Peepers]

I find this an interesting discussion. As mhuffman stated, while the BOV would open under only vacuum, the pressure differential between the intake mani and the IC piping required to create a vacuum greater than that of the valve spring would not occur under normal cruising conditions.

The intake mani pressure would need to somehow be greater than required air to operate the engine at that particular rpm. Unless the throttle plate were rapidly opening and closing, artificially creating greater air needs than what the engine would be cruising at, then there would never be a point that vacuum line would exceed the negative pressure inside the intercooler piping unless there was a malfunctioning BOV.

Now, this would be an interesting modification for anyone looking to create an N/A idle position for a turbo car. It would just simply use a solenoid, similar to that on the boost control (wastegate), only it activates during idle to open the bov and pull air in.

You have to remember, the throttle plate is the largest restriction in the entire intake tract at idle and cruise. The engine wants to suck in a liter of air every rotation but the throttle plate keeps it from doing so, generating ~-20 inHg in the intake manifold. The intercooler piping only has to work against the air filter, IC itself, MAS, and compressor; which is spinning anyway, although most likely not creating more airflow than the engine is consuming. This would be only about negative 2-3 inHg I'm guessing during idle and cruise.

I think the only time I could see it opening is right when you go WOT, the throttle body opens but theres only X amount of air in the IC pipes, and untill the turbo starts to spool it could create a vacume. However! It would be like what a half sec or so? Nothing to worry about if it were doing it.

Once the throttle plate opens, there is a drastic reduction in manifold vacuum; the intake tract and manifold will be about equal in pressure(slight vacuum in this case). The CBV spring will likely keep it closed.

I thought I would just throw this out there since it would make things more difficult but..couldn't the blow off valve be closed at idle and open on deceleration while in gear when the vacuum is more than at idle.

oh and a side note my greddy type rs is closed at idle but I guess that could also depend on where you have it set at.

This is very likely since manifold vacuum often nears -30 inHg during decel.

 

[99gst_racer]

ANYWAYS, all someone really has to do is start their car in the garage and put their hand over the BOV inlet and see if it sucks or blows!

My Type-S only open when my intake plenum goes from a positive pressure to a negative pressure. I have mine set tight enough so that the differential opens it - not just vacuum.

Have any of you ever shot compressed air into your charge pipes (with the engine off) to see if your BOV opens? I'm not sure if this is even possible unless the vacuum line has a leak....

 

[Simpleman1]

My Synapse BOV has 2 air fittings on rear and 1 on side. When we hooked up (following the instruction) the 2 rear in "T" configuration attached to intake vacuum line and MBC, it was blowing air out and the car will not idle properly. So a friend suggested to just hook only 1, it worked after that and did not notice if it was sucking air.