Dump valves and overfueling
01 January 2014, 08:02 PM
#31
01 January 2014, 08:14 PM
#32
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So if it is down to the fact that the recycled air is still being used it must mean the engine still draws in a tiny bit of pressure on the beginning of overrun on just a re-circ system? Why does a delete set-up run rich on overrun then?
Last edited by RS_Matt; 01 January 2014 at 08:16 PM.
01 January 2014, 08:16 PM
#33
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but it is used,its recirculated back to a point before the turbo and after the maf,then pulled back through the turbo again when needed,the volume of air is calculated and must be used,if its not used there will be too much fuel
01 January 2014, 08:26 PM
#36
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Also how long does the pressurised unneeded air stay in the system if you are going down the gears? It can't possibly stay in the inlet tract for long?? Surely this would mean a re-circ system would run rich until the throttle was opened up again? On the contrary I believe the AFR tries to get back to 14.7
01 January 2014, 08:28 PM
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01 January 2014, 08:29 PM
#38
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The answer is like 'the storm destroyed my fence' and not it was the wind/flying debris/lightening that destroyed it.
So if it is down to the fact that the recycled air is still being used it must mean the engine still draws in a tiny bit of pressure on the beginning of overrun on just a re-circ system? Why does a delete set-up run rich on overrun then?
So if it is down to the fact that the recycled air is still being used it must mean the engine still draws in a tiny bit of pressure on the beginning of overrun on just a re-circ system? Why does a delete set-up run rich on overrun then?
The throttle is closed on over-run.....
01 January 2014, 08:31 PM
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01 January 2014, 08:33 PM
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01 January 2014, 08:47 PM
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Taken from Enginemappers site
Dump Valves and Blow off valves.
These are fitted purely to redirect the turbo pressure which builds up in the intake system, post
turbo, pre-throttle plate when the throttle is closed quickly, i.e when you lift off. In all of the
production based vehicles with MAF sensors reading ECU load it is highly likely that any dump valve
will be of the recirculating type. Meaning the air pressure built post turbo, pre-throttle will be
dumped back into the turbo inlet side, post MAF. This is a key point to note regarding what dump
valve you choose and why. On a MAF system, all the air that enters the engine will travel through
the MAF to be measured for the engine management to work out how much fuel is needed for the
combustion process. When you lift off and the pressurised air back up against the back of the
throttle plate, the dump valve, (on a standard recirculating system), will dump it back into the intake
system, so the same total volume of air that the MAF has measured remains within the system and
will still be consumed by the engine. Fitting an externally venting dump valve, or a vent to
atmosphere, (VTA), valve will cause a number of problems when you lift off and general engine
running, as that dumped air has been measured by the MAF, but is then not used by the engine,
although it expects it.
The other main issue regarding externally venting dump-valves and MAF sensors, is that the
dumpvalve will always open under vacuum, i.e when not is boost, (positive pressure). This means
that on idle and cruise, an external venting dump valve will always be letting in extra air post MAF,
(essentially an air leak). This means that initially it will run leaner than it should until the ECU learns some corrections, (standard ECUs only). This problem is not applicable when running a mafless ECU,
like a Simtek, Syvecs, Link etc.
My advice; on a standard ECU stick with a recirculating dump valve for drive-ability and general
integrity of the engine management control systems. As said before, if you are running mafless, then
running an external dump valve won’t matter or not.
These are fitted purely to redirect the turbo pressure which builds up in the intake system, post
turbo, pre-throttle plate when the throttle is closed quickly, i.e when you lift off. In all of the
production based vehicles with MAF sensors reading ECU load it is highly likely that any dump valve
will be of the recirculating type. Meaning the air pressure built post turbo, pre-throttle will be
dumped back into the turbo inlet side, post MAF. This is a key point to note regarding what dump
valve you choose and why. On a MAF system, all the air that enters the engine will travel through
the MAF to be measured for the engine management to work out how much fuel is needed for the
combustion process. When you lift off and the pressurised air back up against the back of the
throttle plate, the dump valve, (on a standard recirculating system), will dump it back into the intake
system, so the same total volume of air that the MAF has measured remains within the system and
will still be consumed by the engine. Fitting an externally venting dump valve, or a vent to
atmosphere, (VTA), valve will cause a number of problems when you lift off and general engine
running, as that dumped air has been measured by the MAF, but is then not used by the engine,
although it expects it.
The other main issue regarding externally venting dump-valves and MAF sensors, is that the
dumpvalve will always open under vacuum, i.e when not is boost, (positive pressure). This means
that on idle and cruise, an external venting dump valve will always be letting in extra air post MAF,
(essentially an air leak). This means that initially it will run leaner than it should until the ECU learns some corrections, (standard ECUs only). This problem is not applicable when running a mafless ECU,
like a Simtek, Syvecs, Link etc.
My advice; on a standard ECU stick with a recirculating dump valve for drive-ability and general
integrity of the engine management control systems. As said before, if you are running mafless, then
running an external dump valve won’t matter or not.
01 January 2014, 09:01 PM
#43
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I'm all the less wiser for reading that and I wasn't aware my VTA lets in air off boost. 
Going on that EM site I can't see why a car with a re-circ wouldn't run rich lifting off and then slowing down. You won't be using that pressurised air the MAF accounted for.
Going on that EM site I can't see why a car with a re-circ wouldn't run rich lifting off and then slowing down. You won't be using that pressurised air the MAF accounted for.
01 January 2014, 09:41 PM
#44
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this is literally all new to me but as i understand it surely its just a case of - For Example
1 litre of air enters maf
maf then dictates how much fuel is need for that 1 litre
but that 1 litre doesn't go through as its expelled into the air
but the maf doesn't know that so still fuels for that original 1 litre
im not here taking the mickey but is that pretty much a basic simpletons way of the process ?
1 litre of air enters maf
maf then dictates how much fuel is need for that 1 litre
but that 1 litre doesn't go through as its expelled into the air
but the maf doesn't know that so still fuels for that original 1 litre
im not here taking the mickey but is that pretty much a basic simpletons way of the process ?
01 January 2014, 09:53 PM
#45
this is literally all new to me but as i understand it surely its just a case of - For Example
1 litre of air enters maf
maf then dictates how much fuel is need for that 1 litre
but that 1 litre doesn't go through as its expelled into the air
but the maf doesn't know that so still fuels for that original 1 litre
im not here taking the mickey but is that pretty much a basic simpletons way of the process ?
1 litre of air enters maf
maf then dictates how much fuel is need for that 1 litre
but that 1 litre doesn't go through as its expelled into the air
but the maf doesn't know that so still fuels for that original 1 litre
im not here taking the mickey but is that pretty much a basic simpletons way of the process ?
01 January 2014, 10:14 PM
#48
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ive never been a fan of the noise really but as little as i know ive always been aware of the amount of pressure involved and how its better to reduce it to decrease wear on the turbo but never knew more than that in a nutshell, how naive i was 
ive ordered a recirculating one now and will remove my current vta, lets just hope it comes with instructions !
just out of interest though would the vta dump valve really effect performance or not in the slightest ? is it just a case of the damage that can be done over time ?
ive ordered a recirculating one now and will remove my current vta, lets just hope it comes with instructions !
just out of interest though would the vta dump valve really effect performance or not in the slightest ? is it just a case of the damage that can be done over time ?
Last edited by ScoobyFanatic33; 01 January 2014 at 10:16 PM.
01 January 2014, 10:26 PM
#49
A vta on a mafless setup wouldn't really matter and some people will argue that a vta will prolong the life of your turbo. As long as the turbo is supplied with good clean oil it's happy and doesnt really matter what you do with em. A turbo will go usually around the 100k mark depanding how much boost they've run in the past and how regular oil services are.
01 January 2014, 10:37 PM
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oh dear thats just opened up another question then if you dont mind 
is a turbo timer something that is best to go with ? ive heard it said that just stopping the engine after a good run causes the turbo to spin almost dry as no oil is pumping hence keeping the tick over going for 30 sec's or so ? or is this a whole new debate lol
is a turbo timer something that is best to go with ? ive heard it said that just stopping the engine after a good run causes the turbo to spin almost dry as no oil is pumping hence keeping the tick over going for 30 sec's or so ? or is this a whole new debate lol
02 January 2014, 12:30 AM
#51
oh dear thats just opened up another question then if you dont mind
is a turbo timer something that is best to go with ? ive heard it said that just stopping the engine after a good run causes the turbo to spin almost dry as no oil is pumping hence keeping the tick over going for 30 sec's or so ? or is this a whole new debate lol
is a turbo timer something that is best to go with ? ive heard it said that just stopping the engine after a good run causes the turbo to spin almost dry as no oil is pumping hence keeping the tick over going for 30 sec's or so ? or is this a whole new debate lol
The best way of combatting against this, is simply to drive off boost for the last mile or two of your journey, before switching off and parking up. This is because nice, chilling air is passing over the turbo housings, thus efficiently cooling them down. And of course, the oil is still circulating, which helps guard against localised hotspots from establishing themselves.
The above course of action is better than just letting oil circulate on its own, by using a turbo timer for a minute or two. (Although, this is still better than shutting engine off immediately after a lengthy hoon, obviously).
Despite all of the above though, modern day water-cooled turbos are pretty reliable, in terms of taking abuse.
Last edited by joz8968; 02 January 2014 at 12:38 AM.
02 January 2014, 08:20 AM
#52
If you dump it to atmosphere, the car has already fuelled for it and the air is gone.
TBH, bud, you are arguing against knowledge gained from YEARS of ownership, mapping etc.
02 January 2014, 10:03 AM
#53
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You must understand that even when you close the throttle, that a certain amount of air/fuel is still being put through the engine. Why you ask? How would the car idle without this? 
So, you shut the throttle, the ecu reverts to idle mixture. But, there is a higher % of fuel due to some of the air being vented to atmosphere when you came off the throttle.
Hence why until the extra fuel is burnt, the mixture is rich.
It isn't hard to work out.
Whether it is bad for your engine, probably not in the short term. But running rich on any car can cause issue long term, just as running lean.
So, you shut the throttle, the ecu reverts to idle mixture. But, there is a higher % of fuel due to some of the air being vented to atmosphere when you came off the throttle.
Hence why until the extra fuel is burnt, the mixture is rich.
It isn't hard to work out.
Whether it is bad for your engine, probably not in the short term. But running rich on any car can cause issue long term, just as running lean.
02 January 2014, 10:05 AM
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oh dear thats just opened up another question then if you dont mind
is a turbo timer something that is best to go with ? ive heard it said that just stopping the engine after a good run causes the turbo to spin almost dry as no oil is pumping hence keeping the tick over going for 30 sec's or so ? or is this a whole new debate lol
is a turbo timer something that is best to go with ? ive heard it said that just stopping the engine after a good run causes the turbo to spin almost dry as no oil is pumping hence keeping the tick over going for 30 sec's or so ? or is this a whole new debate lol
02 January 2014, 11:08 AM
#55
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You must understand that even when you close the throttle, that a certain amount of air/fuel is still being put through the engine. Why you ask? How would the car idle without this?
So, you shut the throttle, the ecu reverts to idle mixture. But, there is a higher % of fuel due to some of the air being vented to atmosphere when you came off the throttle.
Hence why until the extra fuel is burnt, the mixture is rich.
It isn't hard to work out.
Whether it is bad for your engine, probably not in the short term. But running rich on any car can cause issue long term, just as running lean.
So, you shut the throttle, the ecu reverts to idle mixture. But, there is a higher % of fuel due to some of the air being vented to atmosphere when you came off the throttle.
Hence why until the extra fuel is burnt, the mixture is rich.
It isn't hard to work out.
Whether it is bad for your engine, probably not in the short term. But running rich on any car can cause issue long term, just as running lean.
So basically running a VTA causes a split second of richness on the overrun because the the engine is still trying to pull a tiny bit of non-existent pressurised air in on closed throttle/idle?
I've run a AFR gauge and it's only straight after a gear change on the overrun that I notice a slight rich dip, not any other time.
I have a stiff spring on my VTA that only opens at 4k, so I'm guessing the rich fueling only really comes into play when I ease off at high revs as changing gears lightening fast only sees a subtle pressure loss? Or would it still be bad at 4k and changing fast as the air is heading out of the intercooler and causes the air to vacuum away from the inlet?
02 January 2014, 11:15 AM
#56
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That air is put back in the system, AFTER the meter, but before the turbo, so it isn't metered again and does get used.
If you dump it to atmosphere, the car has already fuelled for it and the air is gone.
TBH, bud, you are arguing against knowledge gained from YEARS of ownership, mapping etc.
If you dump it to atmosphere, the car has already fuelled for it and the air is gone.
TBH, bud, you are arguing against knowledge gained from YEARS of ownership, mapping etc.
I can imagine after you vent/re-circ at high revs there is maybe a window of 800ms?
Last edited by RS_Matt; 02 January 2014 at 11:17 AM.
02 January 2014, 11:45 AM
#59
Consider non turbo cars, when you shut the throttle it stops pulling in air..... simples.
With a spooled turbo in that inlet tract, the momentum of the spooled impeller doesnt stop pumping air. If there was no pressure releif valve the air would either over pressurise the inlet or force its way back through the turbo, hence the chattering noise associated with a dv delete.
A recirculating dv simply cycles that pumped air around the turbo so no metered air is lost and intake pressures dont get excessive.
Modern high performance cars (150bhp + per litre) have to govern the amount of air and hence amount of fuel very accurately and in the mid 90's the best way to do this was to pass the inlet air through a known size tube containing a heated wire. Its resistance was then used by the ecu to measure the mass of air going in. Used along side the lambda signal you then have very accurate control of the air fuel mix.
By wasting air out of the system rather than just recirculating it around the turbo you are inevitably screwing with the complicated calculations going on in the ecu.
You talk about very fast gearchanges as if your left hand and left foot can move quicker than the air going into the engine at redline ........ trust me on thisun, you cant! Theres some fundamental thermodynamics you need to get your head round to understand this properly.
With a spooled turbo in that inlet tract, the momentum of the spooled impeller doesnt stop pumping air. If there was no pressure releif valve the air would either over pressurise the inlet or force its way back through the turbo, hence the chattering noise associated with a dv delete.
A recirculating dv simply cycles that pumped air around the turbo so no metered air is lost and intake pressures dont get excessive.
Modern high performance cars (150bhp + per litre) have to govern the amount of air and hence amount of fuel very accurately and in the mid 90's the best way to do this was to pass the inlet air through a known size tube containing a heated wire. Its resistance was then used by the ecu to measure the mass of air going in. Used along side the lambda signal you then have very accurate control of the air fuel mix.
By wasting air out of the system rather than just recirculating it around the turbo you are inevitably screwing with the complicated calculations going on in the ecu.
You talk about very fast gearchanges as if your left hand and left foot can move quicker than the air going into the engine at redline ........ trust me on thisun, you cant! Theres some fundamental thermodynamics you need to get your head round to understand this properly.