The science to not venting to atmosphere

[daver36]

Hey guys,
I'm asking this question at the moment not owning a dsm, but being a huge fan (and hopeful owner one day) and wanting to know all I can about them. I know you can't vent to atmosphere with the maf is stock location because the maf will meter air that the intake never sees, richening the mixer on deceleration and causing stalling/a bad running engine. I can accept this but it never fully made sense to me. At first I thought "ya, but even with air recirculating while the bov is "engaged" the trottlebody plate is still closed to no air can get in anyways and the MAF can still meter air. As I was thinking about how to write this question I think I can up with the answer, one that makes sense to me anyways. Basically I figured that since the recirculating air circles from turbo to BOV and back to turbo (post MAF) this circulating air could me creating a "block off plate" out of air pressure. This would in essence not allow new, fresh, and more importantly, metered air to come through the filter and into the MAF, thus making sure the MAF doesn't see anymore air coming in, and doesn't tell the ECU to add more fuel. So basically my question is, is my guesstimation on the the topic correct? If not, what is the real science behind it? Like I said before, I love learning all about these things. Come some day in the future when I get my dsm, I don't want and questions and doubts about my own build path. I included this pic to help illistrate what I'm trying to convey in my ramblings.

Cheers,

Daver

 

[the_mork]

It is much simpler than that. First of all the throttle plate may be closed but there is still quite a bit of air going around it (enough to idle). Second, the throttle plate may be closed but when the bov opens there is definitely somewhere for the air to go, you now have a lot of air moving through the maf to go directly out of the bov and cause the computer to inject a lot of fuel. There isn't any pressure wall to stop air from coming in (at least not enough to really matter) and the condition only exists for a little bit.

What kills the idle though is that you have a momentary situation where the car is extremely rich at unpredictable intervals. The computer will get feedback from the o2 sensor and actually attempt to fix this by removing some fuel all of the time and will never manage to satisfy itself what a good amount of fuel is to inject making for a certain amount of fluctuation depending on driving conditions.

Finally the biggest reason is that most BOVs are not closed at idle which allows unmetered (and unfiltered!) air into the engine causing it to be lean at low loads.

 

[daver36]

I thinks I'm starting to under stand, but I have a few more questions then. With the Bov hooked up in recirculating configuration, when you let off the gas the trottlebody plate closes, creating enough pressure to open the bov. I realize some air gets through the plate (the engine still has to run) but not very much. With most of the air recirulating around and around from turbo to bov to turbo etc. how can more air be sucked in though the filter and into the MAF? Wouldn't this create more and more pressure. I realize at idle the bov will be open and the filter and maf will let some air in to let the car idle but i guess the scenario i'm talking about is letting right off the gas from WOT.

 

[MrBoxx]

The time duration of these events is so short that the engine has cycled several times, creating enough vacuum to scavenge the air blown off by the BOV to be used up in a short time.

The way you're thinking of it is that at the time of BOV release, the air entering the intake pipe creates a positive pressure above ambient air pressure (in this case, it would be 0 on a boost gauge), thus preventing ambient air from entering the system. However, I believe the intake pipe would remain in a vacuum state, regardless of the incoming vented air. I don't think the volume of the vented air would be enough to pressurize the intake pipe itself, especially with the turbo compressor wheel still spinning and drawing air into the system.

I suppose if someone were really bored and had a spare boost gauge, some hose, and an open port on their intake pipe, they could test this theory and actually see the vacuum fluctuations on the intake pipe during cruise, boost, and BOV release. That actually might be interesting, if only for archival purposes. I happen to have the spare boost gauge, but no openings on my intake pipe.

 

[crimsondragon]

The instant you close the throttle, the engine goes into vacuum. Once you get a boost gauge you can experiment with it. Develop full boost then close the throttle immediately. It will hit a higher vacuum then at idle. The injectors are shut off and nothing but air is in the combustion chamber. If you get a wideband, you will see the A/F ratio skyrocket.

The time duration of these events is so short that the engine has cycled several times, creating enough vacuum to scavenge the air blown off by the BOV to be used up in a short time.

The way you're thinking of it is that at the time of BOV release, the air entering the intake pipe creates a positive pressure above ambient air pressure (in this case, it would be 0 on a boost gauge), thus preventing ambient air from entering the system. However, I believe the intake pipe would remain in a vacuum state, regardless of the incoming vented air. I don't think the volume of the vented air would be enough to pressurize the intake pipe itself, especially with the turbo compressor wheel still spinning and drawing air into the system.

I suppose if someone were really bored and had a spare boost gauge, some hose, and an open port on their intake pipe, they could test this theory and actually see the vacuum fluctuations on the intake pipe during cruise, boost, and BOV release. That actually might be interesting, if only for archival purposes. I happen to have the spare boost gauge, but no openings on my intake pipe.

I have all the items needed for that. If I have spare time during the week, I'll post my findings.

 

[the_mork]

Much more useful would be a pair of MAP sensors. Even using the stock MDP sensor would give you enough information since you would be at nearly the same pressure above the ability of the stock MDP to read. This would give you clear data and would be much easier to read.

 

[daver36]

I think I got it, thanx guys. I guess I was thinking of the entire event taking more time than it actually does to occur, and the engine needs more are even when the plate is closed than I imagined it would. It's not a pefect recirculating cycle since the engine is still taking in some air, so there is more air is comming in. I would imagine it would be less air comming in the filter than it would if the bov was vented(and therefore no recirculating air). That in return would meter more air than being used and cause the ecu to dump in too much fuel. am i right? Sorry, I find this really interesting and want to know every in and out.

 

[Calan]

With the Bov hooked up in recirculating configuration, when you let off the gas the trottlebody plate closes, creating enough pressure to open the bov. I realize some air gets through the plate (the engine still has to run) but not very much. With most of the air recirulating around and around from turbo to bov to turbo etc. how can more air be sucked in though the filter and into the MAF?

As somebody mentioned, you are over-complicating this

First of all, air isn't "going round and round" between the turbo, UICP, and BOV.

When the TB slams shut, all that excess pressure that the turbo has built up has to go somewhere, since it can't go through the TB and intake now. So the BOV opens to allow that pressure to escape back into the intake track, which is always drawing in air whether you're creating boost or not. This actually lessens the work needed to be done by the turbo, and helps keep it spooled up during shifts (i.e. closed TB).

Closing the TB creates vacuum, which is what opens the path through the BOV...not an excess of pressure. So with the IC pipe under pressure and TB open, the BOV should be closed. When the TB shuts, IM vacuum is created, and the BOV opens to equalize the pressure in the UICP.

The fact that the BOV is at least partially open under vacuum explains why The_Mork said "most BOVs are not closed at idle which allows unmetered (and unfiltered!) air into the engine"

 

[daver36]

got ya. thanks for all the info guys it really helped.

 

[mindset]

That was a good idea though.

 

[the_mork]

I was recently reading some statements from an Evo guy about his past experiences with DSMs and that when he had one he did some testing involving a GM MAF (at the time it was the only way to vent properly), a Tial bov (unrecirculated), and a recirculated bov. He said that in his testing having the bov recirculated kept his charge piping pressurized by an extra 2psi between shifts. My theory is that besides flowing air toward the turbo to reduce the amount of work the turbo has to do it also reduced the amount of air that flowed out of the charge piping which will have a variable velocity based on pressure which increases flow when more flow is needed (to a point). Just a guess though.

 

[Calan]

My theory is that besides flowing air toward the turbo to reduce the amount of work the turbo has to do it also reduced the amount of air that flowed out of the charge piping which will have a variable velocity based on pressure which increases flow when more flow is needed (to a point). Just a guess though.

Sounds reasonable...but I would think that depending on how much of a restriction the BOV poses to the pressure being dissipated back into the intake track, the high speed and suction of the compressor wheel when the TB is shut would equalize things fairly quickly. Besides that, pressure is also being dissipated via vacuum through the TB's BISS/idle path. I just can't see much pressure remaining in the UICP for any appreciable amount of time once the TB closes.

Would be interesting to actually measure it though. Maybe Matt (DSM-onster) will drop in here and edumacate us

 

[kenamond]

As long as the turbo is pumping more air than is making it past the closed throttle plate, the excess is going through the BOV and recirculating back through the compressor. The entire system between the compressor and TB would be higher pressure (LICP, IC, UICP). I'm not sure how much more, though. Air won't move unless there's a pressure difference somewhere (and the air moves from high to low pressure). I'm not sure how much the recirculated air keeps the turbo spooled, but I imagine that a blow-through setup and vented BOV would be more efficient just because there's less piping involved in the "path" of the air being pushed from the compressor. But then you might have less pressure in the intake system when you finally open up the throttle again. So either you maintain a bit more rpm in the turbo or you get a bit more pressure when you floor it again.

And for the OP, don't forget the main purpose of the BOV: keep your compressor from surging.

EDIT: But I think your first post was pretty much on the money as far as the recirculation blocking off additional intake air. It's not going to stop all flow, but if you have no recirculation, A LOT of air will be pulled through the MAS.

 

[Calan]

As long as the turbo is pumping more air than is making it past the closed throttle plate, the excess is going through the BOV and recirculating back through the compressor. The entire system between the compressor and TB would be higher pressure (LICP, IC, UICP).

Right. But I think this would only be for a very short period of time, just until the UICP positive pressure is dissipated and it begins to see vacuum. About the length of a BOV "whoosh"

BTW - hey Mack

 

[kenamond]

Right. But I think this would only be for a very short period of time, just until the UICP positive pressure is dissipated and it begins to see vacuum. About the length of a BOV "whoosh"

BTW - hey Mack

My guess is that in the time to shift gears, the intake tract would still be pressurized. When someone gets intake pressure telemetry data, that'd answer it...but I'd guess someone with that would already be blow-through and vented .

Hiya Craig! Shouldn't you be in the garage putting all those pieces back together? Get the wiring done? Didn't think so

 

[daver36]

wow. i didn't expect this kinda discussion out of it. Thanx alot, getting info from the pros is a big honour. I'm really gonna need to get that dsm soon, but the problem is my truck holds more car seats (got one on the way already)

 

[Calan]

wow. i didn't expect this kinda discussion out of it.

That happens a lot around here

Mack - I'm taking a wrenching break. We happen to be in a short stretch of the clearest nights we've had in months (and with no moon), so I decided to set the scopes up outside and go hunting for faint fuzzies... I'm just haning out between here and out there <<< on the deck under the stars

 

[dsm-onster]

This is a good discussion. It would be neat to get some map sensor logs.

Sounds reasonable...but I would think that depending on how much of a restriction the BOV poses to the pressure being dissipated back into the intake track, the high speed and suction of the compressor wheel when the TB is shut would equalize things fairly quickly. Besides that, pressure is also being dissipated via vacuum through the TB's BISS/idle path. I just can't see much pressure remaining in the UICP for any appreciable amount of time once the TB closes.

Would be interesting to actually measure it though. Maybe Matt (DSM-onster) will drop in here and edumacate us

Only thing I could add is that the location and angle of the the recirculation point could have a slight effect on transient response (respool after a shift). With a 1g, the stock location of the recirculation point is close to the compressor inducer and pointing into the inducer. The 2g has the recirc location at least pointing down the tube to the compressor. Momentum resists pressure drops and encourages pressure rises. Yes, it can do the opposite, because momentum is independent of the energy the pressure change induces. But in this case:

When the air comes rushing from the bypass valve it runs into the compressor fan and helps it spin along. Even with the intake pipe wide open on the other end. You have the compressor blades already moving due to momentum and now you have the air pushing it along due to it's momentum. Less kinetic energy is lost, the wheel stays in motion longer. Less energy is needed to get the rpms back to where it was while in full boost because the wheel is spinning faster already. . . This is one thing you don't get with an open BOV.

Don't know if the difference is noticable. I do know that if a karmen vortex maf gets air coming the other way, from recirculation location to filter, it will throw out so crazy numbers. But you have the turbo wanting to stay in motion to suck air into it, and you have the airflow going to the compressor inducer from the airfilter wanting to stay in motion, and you have air rushing in from the bypass valve. Air moving in one direction tends to create a vacuum (see venturi effect). The bypass valve wont insert more air into the intake pipe than what the compressor has drawn in. So I doubt there will ever be a positive pressure situation in the intake pipe. Just much less air will be drawn into the meter (other end of pipe) when the bypass valve is dumping. Just exactly like a wall of air. Not a high pressure wall, but just that the air is already there from the bypass valve, so little air is drawn into the maf from the compressor inducer.

 

[the_mork]

You can get a Motorola 5bar map sensor for around $30 IIRC. I'm using a GM 3bar that I got for $50. If you're using a good EMS you can use the stock MDP wiring with a bolt-in zeitronix 3,5, or 7 bar unit for about $100.

If someone local to me had a map sensor I would do a log before and after venting (properly with SD) when I have it running next week.

I could also run a test with only one map input from my licp before and after venting improperly if anyone thinks this is worthwhile.

 

[eagle-talon-tsi]

This form is very interesting. I have just began to learn about bov's and want to put one in my 91 eagle talon. Then i learnt that my car has a compressor bypass valve setup witch would cause the engine to run rich (also then i'd have an extra pipe i don't know what to do with). Is it bad for the engine to run rich and is there a way to avoid this?

 

[DSMunknown]

Finally have the thread I was looking for. (Thanks Bri!)


http://www.dsmtuners.com/forums/newbie-forum/157806-what-washing-out-piston-rings-walls.html

 

[Defiant]

Jesus, people.

The MAF measures how much air is coming in, and tells the ECU to adjust the fuel to that quantity of air.

If you run a vented BOV after the MAF, not only are you effing with that measurement, but when the BOV cracks under vacuum, you have a huge intake leak making for a too-lean mixture, which the O2 sensor will read and tell the fuel control to try and compensate for.

No, it's not about the physics of the air. It's about the chemistry of a stoichiometric mixture, and the ECU's desperate attempts to keep it in a power-producing, fuel-saving, minimally-polluting range.

 

[dsm-onster]

Well that's the obvious reason