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.
