Recirc BOV - Why does this work?

[Edub1]

So, I was planning to recirc my BOV to get rid of the rich spot. Then I'm wondering where the recirculated air is going if my throttle plate is closed. Think about it - the injectors inject fuel in back of the closed plate. The fuel is now isolated from the air in the IC pipe, so how is dumping air back behind the MAF going to solve this problem? If anything it seems like the additional air would just back up through the MAF - where else is it going to go?

Now, if we could get it to run through the bypass hose that would make sense. I know lots of people run a recirculated BOV but I just don't see how it can solve the problem. Any ideas?

[KATwo40]

IACV source is before the throttle plate, so if it's closed, the engine still pulls from that IACV port.

[C-Kwik]

Recirculated air simply keeps recirculating. Keep in mind that a turbo still continues to pump air while you shift or let off the gas. In fact, ideally, you want to tune the BOV to flow only enough air to keep from getting into the surge area of the compressor map. But you want to retain pressure in the intercooler pipes so that when you open the throttle again, it will not need to fill the system again.

[Ligouri Rd]

The amount of fuel that the injectors throw into the engine is deterimined by how much air that gets sucked in though the maf...and some other factors like load, tps and sheet computed by the ecu. Once the air has been injested, a specific amount of fuel is injected to match.

If you close the throttle body with an open bov, that air gets vented, and that fuel has no air to burn with and you go rich momentarily. The compressor is also still sucking in air across the maf adding more fuel.

If you close the throttle body with a recirculated bov (diverter valve) and the air gets dumped in front of the compressor. The compressor sucks in the diverted air instead of drawing in more air from the maf so no extra fuel gets added.

I have never had the air flow back out the maf. I imagine that it is possible, if the compressor does not suck all the air in. I dunno if the maf can read backwards and add more fuel that way.

It has also been my experience that at idle the stock ecu does not like to suck air from a slowly spinning compressor. My diverter valve has a very light weight spring so that it opens at idle and pulls air directly from the maf. You caint do that with a open atmosphere bov.

[rotorimp]

The main reason for recirculating is the fact that the ECU has calculated fuel for the the air that has passed by the MAF--if you let some of that air our it will be overly rich because the ECU thinks that air is still there--that is why MAP is a bit better in that respect because it is distributing fuel based on manifold pressure not how much air is flowing past a given point.

[Edub1]

Recirculated air simply keeps recirculating. Keep in mind that a turbo still continues to pump air while you shift or let off the gas. In fact, ideally, you want to tune the BOV to flow only enough air to keep from getting into the surge area of the compressor map. But you want to retain pressure in the intercooler pipes so that when you open the throttle again, it will not need to fill the system again.

This sounds like it makes sense. I didn't consider that the air keeps flowing in a circle.