A poser for the turbo experts...

[EvilScotsman]

Ok,

if the factory DV lets boost pressure bypass back to the intake side of the turbo, and deleting the DV or adding an aftermarket VTA one will cause overfuelling on lift off, that doesnt make sense.

If the throttle slams shut, any air, whether it backflushes thru the maf or re-circulates round into the turbo inlet, is not going into the cylinders either way, so why would the ecu add fuel for air that has not been used yet.....?

My point being that adding a VTA DV will let metered air out, I can see that but surely that cant cause overfuelling as the air wasnt going to be used anyway as the throttle is closed and only idle air is admitted, hence idle fuelling only on closed throttle?

I think this seems like one of those urban myths, unless someone can give an engineering explanation of why its actually true? (if it is) - and I think, it's just taking the turbulence loading off the turbo blades till the throttle opens again...recirc'd air needs no fuel till its admitted into the engine when the throttle is open!

cheers!

[clive]

take it from someone that has a AFR gauge with a wideband sensor and VTA BOV... it over fuels like a bitch on lift off and you get pops, bangs and flames!!!!

[chapman-s13]

the maf or air flow meter is programmed to allow for that re-circulation of air back into the system!

if you rid the stock DV its allowing for a % thats not there, therefore causes the over fueling

[EvilScotsman]

hmmm, just dont see why it needs to add fuel while the air hasnt been sucked into the cylinders yet? Even if it remembers the air it took in past the maf, it shouldn't add fuel for it until its about to be burned, adding fuel while it's actually still circulating round the bypass is just daft. Dont you think?

[itrhondaboy]

fq 360s have the same problem!

[sideways14a]

It most certainly is not a myth as its very noticeable with a wideband hooked up, also the amount of popping and banging as that excess fuel goes down the exhaust also proves it.
The air is metered at the afm, when the throttle plate closes the air does not go to the cylinders but the afm has already metered the air and the ecu has asked the injectors to pump the fuel in, hence more fuel than air in the combustion chambers, not a lot of bang as its too rich and it tends to piss out the exhaust side into a hot mani-dp and explode
Thats about it.

[Drifter]

It's already measured the air and fuels for it, when you shut the throttle it doesn't get that air and over fuels. It will also overfuel as the air washes back of the AFM as it will read more air (regardless of direction) and dump more fuel. AFAIK

[T2-lEE-G]

like "sideways" and "Drifter" said! the MAF measures the amount of air thats going into the engine, once past the MAF the ecu calculates how much fuel is going to be needed and when! theres is no more measuring of air past the MAF. when the throttle is shut the re-circ will place the air behind the MAF by which time the throttle will be open again so the air will already be there for the amount of fuel injected, where as when you have a VTA there will be not be the correct amount of air ready for the fuel it thinks it needs, hence over fueling! the air is not there but the car thinks it is so adds the fuel.

[EvilScotsman]

ok, not wanting to labour the point....but if you dont open the throttle, you just lift off, the ecu doesnt know if you will open it again or not, or when, and when the air rushes in as you do, the fuel will increase as fresh air comes in the intake anyway. The circulating air cannot be fuelled for if you dont know when its going to enter the engine, you see where im going with this?

So when does the extra fuel get added, when the throttle closes? (not needed then) - when the throttle opens next? MAF will measure incoming air as new volume - recirc air is a small fixed volume of the total and is gone instantaneously - lads in physics this makes no sense!

Now im not saying you are wrong in that the car actually does this but why the fook should it have to?

I am not talking out me arse, I designed chemical rocket ignition systems for Raytheon Defence Systems amongst many other things over the years, and I know other cars do not do this. It is a stupid fuelling strategy to measure air, then remember where a small amount of it is stored until a random time when it is admitted again and then massively overfuel if that small amount of air isnt there?



The air in the whole inlet tract moves as one liquid mass. The ECU can only ever reliably measure the live reading from the anemometer in the maf so it should fuel for the moving air mass only.

[itrhondaboy]

i presumed there is pressure in the intake that the vta lets out, but a recirc sends it back for the next induction
meaning with vta there is a small delay in building pressure that makes the car stutter
the maf/ecu expects the recirced air there so the fueling is set for that as the maf meters the incoming air and the recirced is after the maf so it cant meter that! If that makes sense

[sideways14a]

Eh??

The air moves past the afm
Ecu reads... Need x amount of fuel
Air moves down tract.
Air hits Throttle that closed in the time the air had been through the maf/turbo/IC/intake and thus is metered for x amount
Ecu still injects x amount of fuel as when the air passed through the afm (its only reference point) it needed x ..... not a little bit for idle.
Fuel fires out the exhaust making noises
Throttle re opened or whatever
afm reads more air
ecu adds
bla bla bla

Its down to the fact that the ecu has already started fuelling for the cycle by the time the air is flying towards the throttle plate, when its quickly closed the fuel is already being readied so thats where the extra comes from.
At least thats how i see it.

[PR200SX]

makes perfect sense, there is less air than the system thinks there is, so it is overfuelled.

simples.

[EvilScotsman]

ok, when you shut the throttle plate with the air already flying towards it, and the stock dv recircs the air round and round, past the closed throttle, it would be very stupid to put fuel in the cylinders for air thats still running round the recirc path....surely you see that the air has not entered the engine whether it vents or recircs, so when is this extra fuel getting added?

You are saying the extra fuel goes in when the throttle shuts then?, and if the air escaped it would have overfuelled, but when its in recirc on a factory dv its not gone onto the engine either way. It did pass the maf, yes, but if it circulates in recirc it cant be fuelled for cos its not in the engine yet! And if you let it go to atmosphere its such a small piddly volume that how much fuel can it add for that?

Those with WB Lambdas and AFR's - what does the AFR go to on VTA dump? 12's for a split second?

What you are saying makes sense in terms of a boost leak as air already ENTERING the engine is lost as the fuel for it is added...

Not as justification for "massive" overfuelling on lift off

[clive]

ok, when you shut the throttle plate with the air already flying towards it, and the stock dv recircs the air round and round, past the closed throttle, it would be very stupid to put fuel in the cylinders for air thats still running round the recirc path....surely you see that the air has not entered the engine whether it vents or recircs, so when is this extra fuel getting added?

You are saying the extra fuel goes in when the throttle shuts then?, and if the air escaped it would have overfuelled, but when its in recirc on a factory dv its not gone onto the engine either way. It did pass the maf, yes, but if it circulates in recirc it cant be fuelled for cos its not in the engine yet! And if you let it go to atmosphere its such a small piddly volume that how much fuel can it add for that?

Those with WB Lambdas and AFR's - what does the AFR go to on VTA dump? 12's for a split second?

What you are saying makes sense in terms of a boost leak as air already ENTERING the engine is lost as the fuel for it is added...

Not as justification for "massive" overfuelling on lift off

MINE GOES TO 10.... WHICH IS THE MAX

[EvilScotsman]

ok so 10 parts air to 1 part fuel. So thats 29% richer than the perfect 14.7:1 stoichiometric ratio for a second as a ratio (in volumetric terms). Standard Stoich is 6.8% fuel by air mass, so thats 8.7% fuel by air-mass at an AFR of 10:1.

At what AFR does borewash occur? Only if the airmass is saturated! 10:1 is only 1.9% more fuel by mass than perfect!

8.7% FBM is waaaaaaaaay below saturated levels required for borewash to strip oil off the cylinder wall.

BUT I agree if you were running 10 to 12 constant AFR then that would spell trouble. Hitting 10's for a split second each gearchange though, is not likely to cause any damage even in a fast road car.

btw this is good discussion material, as it affects us all in some measure, and isnt meant in an argumentative way! I want to dig out the real facts

[dan_t]

subscribed to this one

[EvilScotsman]

Good one mate! Everyone is just saying "it is so, because it sounds plausible" - but no one has put up any physics to prove the myth that it is harmful to set the air free, or pinpoint exactly when it overfuels and why this is necessary in the map strategy.

Now, obviously letting the pressurised air back-wash through the turbine blades is not good for the turbo's performance, as it causes resistance and slowing down of the turbine speed, but apart from straining the blades slightly more than normal it is just a resistance for the turbo to huff against. However you should consider the air-motor in a vacuum cleaner.....why does it accelerate to fever pitch when you clog the suction pipe? Because it is UNLOADED - and doing no work. A turbine in a closed space can move no air, so its speed rises due to the fact its not shifting air, just cutting thru it, and the short burst of turbulent reverse pressure against the turbo on lift off, if the DV wasnt there, just makes a weird noise! Thats primarily why car manufacturers fit them. There may be a little tiny bit extra lag on a pidgeon mod motor, but its probably immeasureably small.

[nitrodave]

when running no dump valve at all, closing the throttle when on boost causes the pressure to fire back at the turbo giving the pigeon sound, some of that air gets through the turbo and is re-read by the MAF and dumps fuel also causing bangs and flames.

[clive]

ok so 10 parts air to 1 part fuel. So thats 29% richer than the perfect 14.7:1 stoichiometric ratio for a second as a ratio (in volumetric terms). Standard Stoich is 6.8% fuel by air mass, so thats 8.7% fuel by air-mass at an AFR of 10:1.

At what AFR does borewash occur? Only if the airmass is saturated! 10:1 is only 1.9% more fuel by mass than perfect!

8.7% FBM is waaaaaaaaay below saturated levels required for borewash to strip oil off the cylinder wall.

BUT I agree if you were running 10 to 12 constant AFR then that would spell trouble. Hitting 10's for a split second each gearchange though, is not likely to cause any damage even in a fast road car.

btw this is good discussion material, as it affects us all in some measure, and isnt meant in an argumentative way! I want to dig out the real facts

who know just how rich it does go when you take your foot off the throttle.... my gauge reads a max of 10.... so it could be less for all i know... less as in 5, 8 who knows...

if i change gear quickly it will not pop or bang at all... but if i take my foot off the throttle, for say approaching a junction it will... takes a few seconds before i get a pop/bang/flame etc...

[Drifter]

No one said it's harmful, just not ideal. I'm sure the tuners on here would post as they have seen it, but you've already been told by someone with a wideband lambda what it does. Feck it, I'm too pissed to argue, I'll leave it to sideways