miragezero

Hey guys, I'm switching to an aftermarket blowoff valve (hey, it's shiny). I will be running a more or less stock app for the moment. I am assuming that I shouldn't vent it to atmosphere even though it allows for this. This got me thinking though- what situation would you actually WANT do do this and what would the tell be.

Thanks!

Richgreenster

If you want a good laugh, tape a duck call whistle to the vented end of the BOV.
Other than that it will make your car run rich every time it opens up.

schip43

What he said but... if you have a map (Standalone usually in 944 world ) or MAF you can vent to atomsphere w/o issue as the motor only measures the air that the engine actually uses. With the AFM venting to atomsphere cause issues because it jacks up the AFR ie run rich air has been accounted for but now it's gone?
As to why one would want to do that... it sounds cool!

951Saga

With a blow through MAF positioned between the BOV and throttle body/ intake there will be no un-metered air issues.

Tom M'Guinn

Typical MAF's on the 951 are before the BOV and therefore the BOV air needs to recirculated, and not vented to atmosphere.

Miragezero -- once the MAF/AFM measures the amount of air going to the motor, the computer delivers enough fuel for that amount of air. If the air is vented, you still get that amount of fuel and end up rich.

schip43

Aww point taken...so on the 951 with MAF to vent to atomsphere in a typical MAF install could you mount the BOV in the itercooler pipe to vent to atomsphere?

On a side note I plan on using a Map sensor on mine and vent to atomsphere.
That is as soon as I can get my 951 off it's lazy ***!

Tom M'Guinn

Not sure I follow your question, but with a normal MAF arrangement you need to reciculate the air.

schip43

You answered it while I was writting but I have seen BOV installed in the intercooler hard pipes on the intake side I think?

Tom M'Guinn

Not sure I follow. The stock bypass valve installs between the i/c pipe and the turbo inlet (j-boot) etc.

Willard Bridgham 3

The blow off valve dumps air when you are shifting. Under hard acceleration, the turbo is putting out a lot of air and has high rotational energy and is making high pressure.....when you shift, the turbo is still making high pressure but the engine has no use for it and there will be an immediate pressure spike that could damage the turbo or other parts.

Laust Pedersen

Until now this is the typical BOV vs. recirculation valve (RV) discussion, where the conclusion is that “the engine will run rich” if vented to atmosphere.

The next logical question is: “Is the BOV/RV ever open when the engine needs metered air?”

When shifting gear (and during engine deceleration) the injectors are off and metered air is not needed, but in all other cases shouldn’t the WG make sure that the pressure difference before and after the throttle plate is small enough to keep the BOV/RV closed?

From an efficiency standpoint it makes no sense to have the turbo push on the throttle plate, but it does have an advantage with respect to throttle response.

I have another case where I can hear the BOV opening under mild acceleration (MAP set-up), but maybe that is just a case of “mismatched” turbo, WG and BOV.

Laust

Tom M'Guinn

Well the question was a BOV 101 question so I think the responses were geared in that direction. However, to your point, it's always a little more complicated. Either way, in practice, venting to atmosphere does seem to cause real driveability issues. If you vent to atmosphere and floor it until you reach max boost (say 3500 -4000 rpms) then step on the clutch as if to shift and let the rpms return to idle, it will fall right through the idle and die from the excess fuel -- at least of the 4 or 5 different MAF's I've experienced. Heavy part throttle is another situation as you mention....

shortyboy

i can confirm the running rich issue on 2 951s ive owned. 1st 951 was a stock 2.5L with HKS SSQ BOV, worked fine but backfired between shifts, but didnt die after letting off the throttle. My current 951 with a 2.8L knife-edge crank, with the same BOV would die after a shift and letting of the gas. Im guessing the less weight of my crank,the dme cant recover from a rpm dip.

odb812

schip43, if you want to configure your BOV to vent to the atmosphere, you need to move your air metering mechanism in relation to the BOV, not vice versa. So instead of moving the BOV before the MAF(still not sure how that is accomplished), you move the MAF after the BOV, most typically between the IC and TB. Since the air in this section is pressurized and most MAF systems are engineered to measure air that is not pressurized, you need to ensure your sensor is up to the task.

I'm no expert here, but there have been tons of 951 owners who know more than me and have put more miles on cars making more power than mine with BOV recirculating. Because of that I have had no reason to consider changing the BOV to vent to atmosphere rendering my car's configuration different from the majority and making it harder to diagnose issues. Unless there's power or reliability to be had, there's no reason to stray from the herd here. As far as sounding cool, if you have a MAF kit with cone filter and aluminum hard pipes, the BOV still sounds pretty damn cool when recirculating.

schip43

Wow well this managed to get complex fast! Sorry I was under impression that with a MAF venting to At was a none issue. It looks like it's a bit more complicated than I thought. Not to worry if I go MAF I'll recirclate. But if I replace the AFM with a Map Sensor I'll vent to atomsphere.