M_D_B

Being rather thick when it comes to things of a mechanical nature, I have a couple of questions regarding dump valves.
How do they know when to dump?
As you can get flames out of a de-catted exhaust using an atmospheric dump valve (or can you do this with both?), where does a recirculating type dump the air to that doesn't confuse the airflow sensor?
Or am I getting completely the wrong end of the stick (again)?

Thanks,

Martin.

brickboy

They're a simple, mechanical pressure relief valve which only operates when there's enough pressure. When the throttle plate shuts (e.g when you lift off to change gear), you get a pressure wave back down the induction tract which opens the valve. The factory (recirculating) valve routes the dumped air into the turbo inlet, so it doesn't mess with the air metering.

I *think* (not sure) that a non-recirculating valve may give flames 'cos you get a temporary rich mixture when they operate (as the motor gets less air than it thinks it's going to) but I'm sure others have more nous about this.

RichieC

Thats pretty much the size of it.

With a dump to atmosphere valve, youre fooling the ECU into thinking its lean and so more fuel is added resulting in that temporary rich mix, when you come back on the throttle this raw fuel goes down the exhaust and is ignited.

Looks good but MPG suffers

Richie

BTW Mine does it without a VTA valve if you change up just as full boost is reached!!

pat

It's worth pointing out that a dump valve is operated by vacuum not pressure, or more precisely the vacuum generated in the manifold when the throttle is closed.

This can cause a problem, if the spring rate of the dump valve is insufficient, since on cruise the manifold experiences a vacuum and therefore opens the dump valve (unless it has a strong spring).

On cruise there is sufficient exhaust gas to generate slight positive pressure infront of the butterfly. If using an "inadequate" dump valve, this slight pressure will be bled off through the dump valve, which will reduce throttle response slightly (but not enormously).

When running a vent to atmosphere dump valve, a non-return valve is fitted into the body of the dump valve which prevents air being drawn into the intake via the dump valve (it shouldn't anyway, but just to be sure....). As a result of this non return valve, a dump to atmosphere valve will not vent all of the pressure in the intake (some pressure is required to open the non return valve...), this again can improve throttle response over a recirculating valve (which is essentially "open" and air can flow both ways).

The flames are probably a result of overfuelling during lift off when the ECU "sees" more air than there really is, coupled with the fact that when you press the LOUD pedal again there is positive pressure before the butterfly causing a lot of fresh air to be blown through... when the fuel and the air mix, it goes bang :-)

Cheers,

Pat.

M_D_B

Thanks Gents,
I think I need to find myself a nice simple diagram showing me how everything is plumbed and where the sensors are mounted. I still can't quite get my head around the recirculating type, not confusing the air/fuel ratio, or does the air still end up in the engine somehow?

Even more confused than before,

Martin.