Dump Valve vs Recirc?
Discussion
Dump valves come in to flavours: recalculating and atmospheric.
Upon lifting off the throttle (after giving it some), the throttle plate is slammed shut so that very little air can get into the cylinders. This means that you have full boost pressure built up, and nowhere for it to go. Air will always try and uncompress itself, so it looks for the easiest escape route.
This escape route is the turbo (where the air has just come from)! The air can now escape this way because there is no longer any exhaust gas driving the turbo (foot is off the throttle remember). Even though the turbo is not being driven at this point, it will still be spinning incredibility quickly, but the compressed air going back through it the wrong way will slow it right down (and can actually damage it in extreme cases).
The atmospheric dump valve works by opening when the throttle is lifted (I’ll explain exactly how it knows when to open if anyone is interested). The easiest escape route for the air is no longer the turbo’s compressor housing, but the dump valve that has just opened. The woosh is the air escaping. The turbo will still be spinning nice and fast, and a lot of the lag between changing gears will be eliminated.
The recalculating valve works just the same, but is much quieter (although some air still escapes back through the air filter). The excess boost is routed back to the piping just before the turbo, so the pressure goes back through the system, which eliminates more lag than the previous method, and still relieves the strain on the turbo between changes.
Laurence
Upon lifting off the throttle (after giving it some), the throttle plate is slammed shut so that very little air can get into the cylinders. This means that you have full boost pressure built up, and nowhere for it to go. Air will always try and uncompress itself, so it looks for the easiest escape route.
This escape route is the turbo (where the air has just come from)! The air can now escape this way because there is no longer any exhaust gas driving the turbo (foot is off the throttle remember). Even though the turbo is not being driven at this point, it will still be spinning incredibility quickly, but the compressed air going back through it the wrong way will slow it right down (and can actually damage it in extreme cases).
The atmospheric dump valve works by opening when the throttle is lifted (I’ll explain exactly how it knows when to open if anyone is interested). The easiest escape route for the air is no longer the turbo’s compressor housing, but the dump valve that has just opened. The woosh is the air escaping. The turbo will still be spinning nice and fast, and a lot of the lag between changing gears will be eliminated.
The recalculating valve works just the same, but is much quieter (although some air still escapes back through the air filter). The excess boost is routed back to the piping just before the turbo, so the pressure goes back through the system, which eliminates more lag than the previous method, and still relieves the strain on the turbo between changes.
Laurence
The dump valve has three openings. One is connected to the inlet manifold before the throttle plate, the second is connected to the inlet manifold after the throttle plate, and the last is just an open hole that is blocked by a valve inside the dump valve (a recalculating dump valve would have a pipe connected to this opening which would route the air back to just before the turbo).
Under gas, the turbo will be generating boost, and the throttle plate is open (throttle pedal down). This means there is no pressure difference between the two sides of the throttle plate as the boost goes straight past the throttle plate and into the engine.
When the pedal is lifted (e.g. to change gear), the throttle plate is slammed shut, and the pressure trapped between the engine and the throttle plate is quickly taken into the engine, so the pressure on this side falls dramatically, but the pressure is still very high on the other side of the throttle plate though (before it).
This means that on one side of the dump valve, there is loads of pressure, and on the other, there is none, so the valve (usually a piston) inside the dump valve gets pushed open by the pressure from one side, and the air escapes casing the "PPPPPSSSSSSHHHHHHTTTT" sound you all know. The valve shuts again when the pressure difference between the two sides is equal. This can happen by either opening the throttle again, or letting all the boost escape through the dump valve.
Under gas, the turbo will be generating boost, and the throttle plate is open (throttle pedal down). This means there is no pressure difference between the two sides of the throttle plate as the boost goes straight past the throttle plate and into the engine.
When the pedal is lifted (e.g. to change gear), the throttle plate is slammed shut, and the pressure trapped between the engine and the throttle plate is quickly taken into the engine, so the pressure on this side falls dramatically, but the pressure is still very high on the other side of the throttle plate though (before it).
This means that on one side of the dump valve, there is loads of pressure, and on the other, there is none, so the valve (usually a piston) inside the dump valve gets pushed open by the pressure from one side, and the air escapes casing the "PPPPPSSSSSSHHHHHHTTTT" sound you all know. The valve shuts again when the pressure difference between the two sides is equal. This can happen by either opening the throttle again, or letting all the boost escape through the dump valve.
DanH said:
So why would you design a car with an atmospheric dump valve over a recirc? The noble is atmospheric judging by the whooshing?
If the engine management on the car utilises an Air Mass Meter then dumping boost to atmosphere causes an overfuel condition as the air dumped has already been metered into the induction system by the ECU, hence why most VAG kit is fitted with recirc as standard.
Matt
I'm still unsure if the 2.5 uses atmospheric dump valves or recirculating ones. I suspect they are recirculating, but so damn loud because of where the filters are positioned.
Perhaps you could confirm this Justin? The dump valves will either be open at the third connection point, or have a load of plumbing attached to them.
Perhaps you could confirm this Justin? The dump valves will either be open at the third connection point, or have a load of plumbing attached to them.
LaurenceFrost said:Sorry - don't know, and I won't be back near the Noble for a few days to have a look.
I'm still unsure if the 2.5 uses atmospheric dump valves or recirculating ones. I suspect they are recirculating, but so damn loud because of where the filters are positioned.
Perhaps you could confirm this Justin? The dump valves will either be open at the third connection point, or have a load of plumbing attached to them.
I know there is an crankcase breather recirc that goes into the filter, but not sure about any other plumbing.
Although I do know it's the same on the 3.0l - Andrew Glover had his air filters moved back to the original position to restore the "woosh" - in that sense it sounds like it's a recirc system no?
J
It appears to be a common misconception, but Lee has never fitted dump valves to his cars. The noise you all love is all that air coming back past the compressors. 
Thankfully, the turbos are small enough that they don't appear to suffer adversely. The racecar now has an atmospheric (for the minute, will be recirculating shortly) dump valve to reduce the shock loadings caused by shutting the throttle (obviously a lot more aggressive and often than a road car) and as an additional benefit, it improves the response in gear changes.
YellowShed
Yellowshed,
Could this be the reason why l have suffered two turbo problems, the inner turbo between the engine and bulkhead has been changed once for leaking oil and l have the same problem again, it's leaking between the two halves.
I do use the car a lot on track so could this be the reason and if so whats involved in fitting the dump valves
Kevin
>> Edited by kj-r on Tuesday 11th May 17:01
Could this be the reason why l have suffered two turbo problems, the inner turbo between the engine and bulkhead has been changed once for leaking oil and l have the same problem again, it's leaking between the two halves.
I do use the car a lot on track so could this be the reason and if so whats involved in fitting the dump valves
Kevin
>> Edited by kj-r on Tuesday 11th May 17:01
YellowShed said:
The racecar now has an atmospheric (for the minute, will be recirculating shortly) dump valve to reduce the shock loadings caused by shutting the throttle (obviously a lot more aggressive and often than a road car) and as an additional benefit, it improves the response in gear changes.
YellowShed
Thank you for finally putting this one to rest. I've been wondering about it for a while.
joust said:
[quote=LaurenceFrost]
Although I do know it's the same on the 3.0l - Andrew Glover had his air filters moved back to the original position to restore the "woosh" - in that sense it sounds like it's a recirc system no?
J
I'm having this on my car too. M.
>> Edited by V6GTO on Tuesday 11th May 18:46
>> Edited by V6GTO on Tuesday 11th May 18:47
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