Dump valves and overfueling
 

The answer why a Blow off valve or VTA makes a car run rich on the over-run is because the Vacuum effect of the Turbo momentarily draws air past the MAF after the throttle is closed and the ECU fuels for it.

On a re-circ BOV the re-cycled air cancels out the vacuum pressure of the Turbo and thus stops new air entering past the MAF.

Jury is still out on a Dump Valve delete and what PSI it would start making the car have a rich fit.



Courtesy of joshnosh!

:banana:

no it fuel for air that should be released back into inlet on recirc, so still fuels for the air released in to atmosphere.

But with the throttle still closed that air isn't going back into the inlet anyway?? why does it matter where the air shoots off to? Into the engine bay or towards the turbo/airbox.

Because the ecu has already accounted for that when it passes the maf, hence the fuel is added. They sound sh1te and make your car run sh1te the end

Gonna jump in here with a question.

My 2001 bugeye wrx has a Forge dump valve but I want to change
back to the original recirc valve. What is the part I need to buy?

Depends if it's running a Forge VTA or a Forge recirculating type.

If it's the latter, just replace it with a standard dealer d/v, or try scrappers.

If it's the former, you may need the pipework.

Yes but when you let off why does it make a difference where the metered air is, the engine won't be using it either way?

The air does not matter at all but the ecu has already calculated how much fuel you need including that air, hence the car overfuels and does not feel smooth

Give it up, Banny, this guy wants one........

Get a better ecu like a syvecs, that lets you remove the maf sensor and you can throw the air about wherever you fancy!

Atmospheric dump valves and maf sensors just don't get on.

MAF = Recirc

Mafless = VTA

Simples :top:

When does the car overfuel, as long as the ecu thinks the pressurised air can be used? split second back on boost?

:facepalm:

People can tell me the MAF has calculated the air all day long, that a VTA causes rich running, that VTA's don't work well with MAF's all day long or a VTA causes non smooth running it won't answer the why or when. I've been reading the same crap for years, it's the same BS that STI owners throw at the capable Newage engine and gearbox.

Your question isnt clear, what exactly is it that dont you understand?

The general consensus is fitting a BOV/DV delete causes rich running for a split second after coming off the throttle. I know the MAF meters the air and the ecu flows fuel to match but why does it make a difference where the unused air dumps to? What causes the rich condition for just that split second considering fueling for 1.5 bar of boost isn't needed when lifting off?

Also with a re-circ does the dumped air rush back to the intercooler or back to the airbox?

It's not rocket science. The air that is vented is already calculated for by the ecu and fuels for it so this causes overfueling. Overfueling means too much fuel in the mix this can cause bore wash and rough running.

Surely your not this daft.

But a re-circ vents air away from use, what is the difference? it must be just me lol :lol1:

Does a pressurised air intake force slightly more air into the engine on closed throttle?

Sorry to jump in but has anyone got a link to the best place to get a syvecs ecu from ?

Can it just be plugged in job done ? Or foes it need some additional works ?

Thank you

Just out of interest what does a dump valve do ? Lol just kidding ;)

It prevents lift off det and cools cylinders before going back on boost. As a bonus it keeps air flowing the right way to the inlet, relieves stress on turbo blades and or bearings and creates a pleasing air-hydraulic pastiche sound.

a recirc "Recirculates"the air that has already been measured by the air flow meter,it simply recirculates within the system before the turbo ,it does not exit the metered system,if it did exit or (vent to atmosphere) the calculated fuel with be too much fir the air that has already been measured

Would you mind confirming that mine is a bov and not a recirculating ?

https://www.scoobynet.com/general-te...ted-wrong.html

I'm concerned that the fact mine isn't recirculating is the reason I'm having issues with the current performance in general

But this air isn't being used off throttle, just like vented air. Only way I could see BOV affecting AFR is if you changed gear fast and on throttle the decrease in air volume can't match the fuel.

The only thing I can think of is that on a re-circ set up the moving pistons on the overrun are drawing in slightly more air from the reserves in the intake and I can't see why a deleted system would run rich using this rhetoric.

no its not.....because its still contained witin the "metered" system,when its vented to atmosphere it gone and yet the fuel is still calculated for it being there

Simon is probably closest to you: http://www.jollygreenmonster.com/subaru.htm

It's not plug in and go, needs mapping and sensors depending on what you want from it.

Yours isn't a recirc. :thumb:

...but when you suddenly come off boost that metered air isn't used either way. I can only see a leaky DV causing rich issues and that would be on moderate to elevated boost.

:thumb: