Just curios really?

 

There's an answer to that question, but it is more complicated than a straight yes/no.

The answer is somewhere along the lines that any aftermarket dumpvalve, whether recirculating or atmospheric, if adjusted incorrectly (i.e. set with too high an opening pressure) can lead to air pressure waves travelling backwards down the induction tract, past the airflow meter and out of the air filter when you back off the throttle under boost. The amount of energy in those waves depends on a number of factors, not least the way the DV is set up, how quick you lift off, and the amount of boost you were running immediately before.

There certainly are circumstances where enough of that energy can vibrate the airflow meter (which contains a delicate element and some equally delicate electronics), and if not kill it in one go, start to affect it over a period of time, and, ultimately, yes, if left long enough, affect it to the point where it no longer reads as accurately as you need.

 

8 years and counting...

dunx

 

I am a bit confused with your explanation. If a dump valve is a VTA one then its not really in the induction tract? The backwards energy would have to go back through the Intercooler, through the turbo before it reached the MAF which is at the end (begining really) of its path? and loads more incoming air would push the shockwave away from the MAF?

I could see how the explanation is valid for a recirculation DV, cos that dumps the dirty, hot gases back into the induction tract just after the MAF.
Or have I read it all wrong?

A good reason to fit a proper VTA dump valve .

 

Correct. If your dumpvalve doesn't open when it should (and to all intents and purposes acts like a DV delete), pressurised air that has nowhere else to go will stall the compressor and pulse backwards all the way through the turbo and induction system, just like it would if you were running an outright DV delete. Like I said before, the energy and quantity of that air will vary a lot according to underlying conditions.

No. The pressurised air flowing backwards from the turbo has far more energy than whatever's coming in so in reality you get your "waste" air pushing the fresh inlet out of the induction tract, rather than as you describe. It's that "backwards" air coming out of the induction tract that causes the chatter sound that cars with a dumpvalve delete make.

In practice, it isn't. As you say, with a recirc, the dumped air is sent back into the induction tract, but most if not all of it will be sucked into the turbo and go round again. Don't know whether any of the others can but the 99/00 MAF sensor can tell the difference between forward and reverse flow - so you can actually see it happening on these.

Not when you understand what's really going on.