What did you do to your Polo today?

**sambb** · 14-09-2018, 09:58 PM

hi mate, in case you didn't already know the BOV will act basically as an air leak in the induction system. The MAF will meter exactly how much air goes in and what goes in should basically be combusted with fuel and that's it. Even if the DV opens, the air that is exhausted goes back into the inlet for another lap of the induction system until it is eventually burnt. Whats going to happen is that the MAF will see a certain amount of air and tell the fuel injectors exactly what to add to it to get the mixture right. But then you are going to let some of that air escape which will cause a rich condition. It may also suck some air in when under full vacuum depending on how strong the BOV spring is causing a lean condition at those times. End result is that your fuel trims will be all over the shop as the O2 sensor will be seeing mixtures that don't square up with what it expects and so the ECU will be putting in fuel offsets/trims to compensate for the average of what is going wrong. If you are on a mafless tune, different story, but with a MAF a BOV aint a good idea if you want it to be running tip top. You probably can block the vent with a GFB plug and turn it into a DV again. Just didn't want you to do it without eyes wide open cos then you might chase other things if the car feels a bit wrong with it is all.

**MattSA9N3** · 14-09-2018, 10:28 PM

Originally Posted by sambb

hi mate, in case you didn't already know the BOV will act basically as an air leak in the induction system. The MAF will meter exactly how much air goes in and what goes in should basically be combusted with fuel and that's it. Even if the DV opens, the air that is exhausted goes back into the inlet for another lap of the induction system until it is eventually burnt. Whats going to happen is that the MAF will see a certain amount of air and tell the fuel injectors exactly what to add to it to get the mixture right. But then you are going to let some of that air escape which will cause a rich condition. It may also suck some air in when under full vacuum depending on how strong the BOV spring is causing a lean condition at those times. End result is that your fuel trims will be all over the shop as the O2 sensor will be seeing mixtures that don't square up with what it expects and so the ECU will be putting in fuel offsets/trims to compensate for the average of what is going wrong. If you are on a mafless tune, different story, but with a MAF a BOV aint a good idea if you want it to be running tip top. You probably can block the vent with a GFB plug and turn it into a DV again. Just didn't want you to do it without eyes wide open cos then you might chase other things if the car feels a bit wrong with it is all.

Yeah this one's hybrid, can be run in full recirc, 50/50 or fully open. According to GFB, you can run it in atmo mode on VW's with an MAF and you won't get stalls/backfiring etc.

HYBRID : T9225 HYBRID DUAL OUTLET VALVE

Will do a bit of trial and error and suck it and see.

**h100vw** · 15-09-2018, 10:17 PM

Unless the car is tuned for a 4BAR FPR, all that will happen is the ECU will trim out the extra fuel and you end up back at the start.

**MattSA9N3** · 16-09-2018, 08:11 AM

Originally Posted by h100vw

Unless the car is tuned for a 4BAR FPR, all that will happen is the ECU will trim out the extra fuel and you end up back at the start.

Yeah, aware of that. I got it for a good price and it's just another 11 year old part replaced. Preventative maintenance is cheaper than fixing it when it breaks.

Plus it gives me options for a tune down the track. I'm looking at stage 2 Revo among others.

**rgvlee** · 17-09-2018, 08:09 AM

Vent to air is aids for polos

**h100vw** · 17-09-2018, 09:19 AM

Originally Posted by MattSA9N3

Yeah, aware of that. I got it for a good price and it's just another 11 year old part replaced. Preventative maintenance is cheaper than fixing it when it breaks.

Plus it gives me options for a tune down the track. I'm looking at stage 2 Revo among others.

They put some strees on the fuel pump as it has to cope with the higher constant fuel pressure.

It's your choice but unless you have run out of fuel at the top end. There's no need

**Sydneykid** · 17-09-2018, 10:20 AM

Provided the BOV doesn't leak, the running rich (due to venting to atmosphere) is much overrated. Since the BOV should only vent air when the throttle is closed (closing), when the ECU is reducing the fuel flow, not much over fueling (if any) will occur. I have watched the lambda trace many times, as well as it being logged on the race cars, and it's barely noticeable if at all.

Recirculating BOV's are for emissions standards, which always seemed strange to me as the vented air had passed through the air cleaner so would be "cleaner" than the ambient air that it was venting into.

Cheers
Gary

**Grant** · 17-09-2018, 11:00 AM

Noise emissions maybe.

Originally Posted by Sydneykid

Provided the BOV doesn't leak, the running rich (due to venting to atmosphere) is much overrated. Since the BOV should only vent air when the throttle is closed (closing), when the ECU is reducing the fuel flow, not much over fueling (if any) will occur. I have watched the lambda trace many times, as well as it being logged on the race cars, and it's barely noticeable if at all.

Recirculating BOV's are for emissions standards, which always seemed strange to me as the vented air had passed through the air cleaner so would be "cleaner" than the ambient air that it was venting into.

Cheers
Gary

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