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Discussion Starter · #1 ·
This may be a daft question (not unknown for me), but why do people fit aftermarket dump-valves on their cars. I've an evo 1 and had a squint under the bonnet and it is fitted with a re-circulating dump valve. It appears to dump the excess into the intake just after the airflow meter.
Why would you want to fit a valve that dumps to atmosphere (forget the noise for now) ? I know the rally teams do, is this becuase the valve operates quicker, or for others reasons.
Is the recirulating type a bit useless, or a good idea ?
Everyone seems to want to fit them for the noise, but is their a good reason for dumping to atmosphere ? I notice a degree of spin-up time going up through the gears etc, would it help reduce this compared to the one fitted as standard ?
I've raced on normal engines for several years, but this turbo lark is a bit of a puzzle, any guys in the know that can answer me ?

Discussion Starter · #2 ·
I confess that I know little more than you Tony, but until someone more knowledgeable comes along (RobP, Alan, etc etc), I think you've answered the question yourself.

Aftermarket (atmosphere) dump valves make a 'nice' noise, and the very best might just make a tiny difference to spin-up time, but that's it. I'm not sure that all the rally cars do use them - although it's difficult to hear them through anti-lag !

Are you based up North somewhere ?

Discussion Starter · #3 ·
I am up from up North (Skipton, North Yorks) does my accent show even through the keyboard border|EQU| 0 align|EQU| middle >

Most of the Rally cars I've seen running up close was before anti-lag became big, and they all seem to chitter and chirup. My friends Scooby WRX does the same.
And when I've been hillclimbing, all the turbo cars there vent them to atmosphere (You should hear the group B Audi -pikes peak car- with 700BHP.. Incredible)
perhaps it only applies to really modified cars (and my mate posing in his WRX he he)
Think I'll go dig a book about turbos out and see what all that plumbing does [img] border|EQU| 0 align|EQU| middle >

Discussion Starter · #4 ·
Thanks for the compliment Kurt, but I'm not sure I have any more knowledge than the two of you.

What I do know is that a lot of cars can't run with a dump valve that vents to atmosphere. Well, they do run, but they won't idle (something to do with drawing in air that hasn't been through the airflow meter and therefore is not there as far as the ECU is concerned). That's why all these fancy HKS ones operate on a piston as opposed to a diaphragm. I know Cossies etc can run them vented to atmosphere no problem - it's all to do with how the air going into the engine is metered.

And it's probably easier to get them running properly if it recirculates.

Whatever anyone says, the main reason for fitting a dump valve by 99% of people is for the tsscccccchhhhhhh noise you get when you back off the throttle (mostly Max Power readers who fall into this category).

The real reason for fitting one is that when running plenty of boost, when you close the throttle, all that boost has nowhere to go. So it backs up through the pipework and intercooler, back into the turbine housing. In extreme cases, this can bring the compressor wheel to a sudden halt and thereby dramatically shorten the life of the turbo.

As for the other turbo noises you can get........the Audi noise is actually the wastegate chattering . This noise is present on all turbo cars, it's just that cos of the tortuous route the air takes before it gets to the turbo, the noise can't be heard.

If you dumped all the pipework and just had an air filter on the front of your turbo, you too could sound like that....................although your car would run pretty crap (unless you put an aftermarket engine management system on it).

I've never been able to understand the relationship between anti-lag and dump valves. They always seem to be a contradiction in terms to me: one keeps the boost up when you back off and one dumps it when you back off.......any suggestions about how that works?

I'm waffling now, so I'll let someone else have a say........

Discussion Starter · #5 ·
I didn't write the following, It was posted by John Felstead on Scoobynet. It should answer some peoples questions.Also, a valve that dumps to the atmosphere has NO advantage to one which recirculates. Also, the chattering you hear on the rally cars is compressor surge:

I run a Group A Cosworth rally car with Pectel P8 mountune mapped engine management, using ALS and water injection.

This is how it works.Firstly, boost is controlled very acurately by using 2 fuel injectors as air valves, these feed the boost/atmospheric pressure to the waiste gate actuator, allowing minute and very fast acting boost control.

Secondly, the normal method of controlling engine iddle via an idle control valve, that
bypasses the throtle butterfly is dispensed with, the static idle is set in the old
fashioned way by a locked off bypass screw.

Thirdly, where the iddle control valve used to be, a new valve is fitted that has a
solenoid controlled valve (looks like an engine inlet valve internally) that can
open/close the throttle buterfly bypass orifice very qickly.

Forthly, a miram turbo shaft is used to withstand the massive increase in turbo
temperature generated by ALS.

Fifth, there is no dump valve used on an ALS equipped engine, the whole point of ALS
is to keep boosting the engine, even on closed throttle.The way it works is that when you come off the throttle, for example braking hard into a corner, the throttle butterfly closes as per normal, once the boost drops to a certain level, the ECU starts to pulse the Throttle butterfly bypass valve, this has the same effect as you constantly stabbing the throttle on and off very quickly, this means thatthe engine is being driven by the ECU as though it wants to accelerate again, however this occilation of the throttle bypass valve is happening so fast, it has the effect of allowing the turbo to stay spinning yet produces no additional torque to drive the car forward.
The effect of all this is that massive heat is generated in the turbo exhaust side, which
ignites all the excess fuel, there is no deliberate overfuelling going on to explode the turbo impellar area.

The most modern of ALS systems on the current world rally cars are far more efficient
than a couple of years ago, which is why the ALS banging was less aparent this year
on the RAC rally.

The turbo used in ALS equipped rally cars is smaller than you would see on a race car
for two reasons. the first is that the FIA has a mandatory 34mm turbo inlet restrictor
that limits air flow into the turbo, this means that it is pointless useing a large inletturbine unit as the inlet could not flow any more air, secondly the lag would increase
due to the inertia of the larger impeller.

A conventional dump valve uses a flexible membrane to vent to atmosphere, this
membrane is held shut by two things, firstly there is a spring that pushes the
membrane valve shut, secondly the back of the membrane is presurised by the boost in
the inlet plenum chamber.

What happens when you come off the throttle is that the throttle butterfly closes, this
causes a vacumb in the inlet plenum chamber, this causes a negative presure on the
dump valve membrane forcing it open to vent the boost to atmosphere.

The problem with these standard valves is that the construction is cheap, the first
thing to go is usually the spring looses its effectiveness, this has the effect of not
closing the dump valve fully untill the boost in the inlet plenum starts to build, the
presure on the membrane daiphram then forces the valve shut.

The second thing that goes wrong with these dump valves is that the membrane splits
due to fatigue, this is more common on uprated boost or older cars. This has the effect of leaking away to atmosphere some of the boost, resulting in a loss of top end power in particular.

Discussion Starter · #6 ·
Thats great ! I knew the anti-lag worked by chucking fuel into the system but never how it all came together.
So basically, no point in fitting one that dumps to atmosphere, and as the evo has a recirculating one as standard (evo 1 anyway) no need to change it.
Only advantage I can see now is to get a decent piston type re-circulating one so you don't have problems with worn diaphragms/springs etc) If you've had a look at the standard one it is a the cheap plastic looking thing near the airmeter. Looks to cost about 20p to make (and 50 quid to buy no doubt) so it might not last very well.
Thanks all, you are obviously Car and amp; car conversion readers and not Max power or Banzai(urk) border|EQU| 0 align|EQU| middle >
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