VTA BOVs

[alhope34]

Well, I have the Forge BPV and I had a sudden urge to close off the recirc tube with electrical tape and try running it VTA. It started fine, worked well. I let the car idle to warm for a bit, then gave it a quick rev to 3000rpm. Sounded really good, had a nice loud whoosh to it. Then, I reved to about 5000 and it sounded amazing! got a HUGE whoosh and a HUGE loud ping from it that echoed in my building's underground. A second after that, the idle gets super low and rough for a second and the car almost stalls.

I'm wondering how people run VTA BOVs on their MS3, how the car acts while shifting at full boost, if they stall or flood at all. Also, is there a chance my engine can hydro-lock from too much gas and bend the rods? After the near stall I put the recirc tube back on before I drove.

[Walrus]

VTA on a MAF car will ruin your cats.

[alhope34]

True, but I'm getting rid of my stock mid-pipe soon. =) Already have a DP and CBE.

I'm just worried about flooding or hurting my engine. I know that unburnt gas will kill the cats, but this car runs freakin rich and backfires all the time due to unburnt gas, anyway, even with the stock tune.

[Aitch]

Your injectors can't flow enough gas to hydrolock the engine. Nowhere near. The reason it stumbles is simply that the air/fuel mixture at idle becomes way too rich and won't ignite, giving the engine no force to keep it rotating.

As you found, this is dependent on the situation right before idle. When you rev really high, the turbo spools up high as well, drawing lots more air and then venting it when you let off the throttle. The engine returns to idle, but the MAF is reading lots more air being drawn by the still spooling turbo; this is why so much extra fuel is dumped in.

So the problem is not 1) flooding your engine, or even 2) killing the cats with unburnt fuel during shifts, but 3) complete inability of the car to idle. Hopefully as I explained it, you can see that even if you were to try and lean out the fueling values at idle (via piggyback etc) it is a variable situation based on what you're doing right before idle, and how fast that caused the turbo to spool.

I can't claim experience with the MS3 setup, but I had a turbo on my Protege5 for a year. Shortly after putting the kit together I converted the MAF sensor to flow-through (i.e. relocated it in the piping to sit between the BOV and throttle body) which meant that it only meters for the air getting to the engine, instead of ALL the air the turbo draws in. People state that this kills the MAF but I didn't have any trouble. I would bet a leaky turbo feeding oily air through the MAF would be a more likely MAF killer.

[alhope34]

Thanks for that detailed explination, you took away all my worries. Yes, I know WHY the engine would bog cause of the AFs and such, just didn't know if it would blow my engine. I'm going to try this now. =)

Also, the CP-e Standback for the MS3 has a fuel cut feature, between shifts or when in neutral, it cuts all fuel till it gets down to 1200-1500rpm or something. So when you shift in boost, there is no fuel going into the engine, no bogging with a BOV.

[Mazda3X2]

I had this problem with my Supra when I vented to atmosphere as well. Shifting wasn't a problem the same as you are back on the throttle and the engine is engaged again to the drive train. But if I clutched while accelerating hard like I was going to shift into say 2nd but just let the RPM fall to idle the car would stall out.

You need to get a BOV that can be adjusted to close quicker (stiffer) so the engine will lose less air from it's loop. The engine will recover from this rich condition very quickly so as you should not even notice it.

[Aitch]

Another option is to use a hybrid BOV which recirculates in most conditions, but vents to the atmosphere under high boost shifts.

[alhope34]

I thought it was under low boost they VTA?

[Aitch]

Under low boost/light throttle applications, they recirculate to keep the system stable. When you really open up the throttle and the boost levels get high, and then you shift, the BOV opens to the atmosphere and dumps a lot of that out - keeping the car from becoming too rich during the shift. Since the spring is setup to recirculate at lower pressures, as the pressure drops during the VTA phase, the BOV switches back into recirc mode and keeps the car from stalling by recirculating the metered air as the engine comes back to idle.