Jolly Green Monster

iTrader: (4)

Everything I read on here in the past said that a recirculate dump valve puts the dumped air back into the engine and more fuel is injected to compensate.. fine.. but how the flip does that dumped air which is put into the air intake just before the turbo get into the engine through a closed throttle butterfly??

After building my own intercooler pipework and now my own air filter intake pipework I cannot see how the air can possibly get into the engine?

Also looking into ecu maps I cannot see the extra fuel injected on over run previously spoken about? they only way I can figure out that extra fuel is injected is due to the MAF reading the air going passed it the wrong way from the recirculate dump valve.

Was this some myth started to try and get people not to buy noisy dump to atmosphere dump valves or am I missing something? (often happens, as anyone that knows me can confirm)

Help..

JGM

Tone Loc

iTrader: (1)

From various logs i've taken with recirc and VTA both give an increase in AFR when you lift off the throttle (standard ecu). The VTA was usually a bit higher but the recirc also did this. I see it as this, the ecu measures the air going past the MAF and fuels accordingly. However when you lift off the air that is expected to go into the engine (as previously measured) doesn't and is either recirced or VTAd. So the AFR goes up as there is less air than expected.

How's the Link coming along now Simon?

Tony.

dowser

When you lift off, DV opens and recirculates the air to before the turbo, but after the MAF.

The point is that the air flowing through the opened DV (and back into the inlet pre-turbo/post-MAF) has already been metered by the MAF, so the ECU has already made the fuelling decision.

Just because you've closed the throttle, doesn't immediately stop the turbo from spinning. It's still sucking air through the compressor. Recirc DV ensures this air is the already metered stuff, VTA variant means you'll be pulling new air past the MAF.

I've never tried a VTA (can't stand the noise), but with a strongly springed recirc DV a sudden lift off between peak torque and power will see a split-second shudder.....this is the shockwave of air passing the MAF in the wrong direction I assume....confusing the hell out of it.

I also don't know how the accel enrichment algorithms work on the classic JECS ecu, but logic suggests it should be using the information on airflow past MAF with 0% throttle to calculate the amount of air in system to make the right decision when re-opening the throttle? Is accel enrichment adjustable on the Link? Do you ever play with it?

Richard

Jolly Green Monster

iTrader: (4)

Okay that is making a bit more sense.

So the extra fuel with a VTA (which I too hate the noise of) is because the compressor sucks more air in through the maf rather than using the already meter air. So the recirculated air doesn't actually go back into the engine and the recirculate dump valve doesn't cause the extra fuel to be injected.

The AFR being richer slightly on lift off anyway makes sense.

My new air intake has no allocation for the recirculate dump valve input.. so I am hoping to run a pipe off the dump valve and some how silence it or quieten it down at least less than a set of bus air brakes anyway. So was thinking about the effect on the engine as I would essentially have a bit of extra fuel injected due to the maf seeing more air passing it.. although I won't as I will be running the link

Tony,

Link is not on the car yet, I have plugged it in to check it worked and was curious to see if my car would run on it.. it started but wouldn't tickover.. I only had the tuning module but not have a serial Link too (thanks Trout) so can change the map easier but I don't trust my mapping to do the final map so intend getting the car mapped, so might as well remain on standard ecu + scoobyecu until I can get it mapped.. Which won't be until I have changed the Uppipe on Friday and received the GM 3bar sensor from the US..

Cheers

Simon

RB5_245

Once the link's on that will be totally MAP based will it not? Therfor eliminating the problem of overfueling with a VTA, Have you considered no DV at all? (not good for the turbo i know). I have an HKS SSQV, with no insert in it's quite quiet.

Jolly Green Monster

iTrader: (4)

RB5,

Yes will be map based when the Link is on so no problem

Just how I believed it worked I suddenly realised it didn't.

But in reading the replies it nearly does.. lol

I ran the car for a week with no dv when I was doing the pipe work on my fmic.. I worry too much about doing the blades on the turbine as I do 300miles a week.. so would rather run a dv.

JGM

PS: Sensor arrived in the post whilst I was at work today Tony.
See 22b

JamesS

The `shudder` some people have reported is simply the dump valve not flowing enough, quickly enough IMHO.

Had probs on mine till I changed the dump valve for a high flow version, both recirc. Problem solved.

WRX_Rich

iTrader: (5)

has any one ever had problems where there revs seem to rise ( say 500 rpm) when you lift off with a VTA D/V Im sure this only seems to happen when you lift off when you just have come on boost????