What do dump valves REALLY do?
 

Do they actually protect your turbo or are they actually causing lag?

I've always been of the opinion, rightly or wrongly, that a dump valve is used to protect a turbo from stalling or spinning back once the throttle plate is closed by releasing the built up pressure and allowing the turbo to spin freely. The side effect of this being that the turbo can then spool up quicker and lag is reduced.

HOWEVER

Whilst reading up on a few things, I came across an article from an Australian tuning firm that rekon the only thing a dump valve is good for is CAUSING lag. According to them, the only reason manufacturers fit them is to control emissions. They used the example of a rally car - I can't ever recall hearing one with a dump valve but you do hear "flutter dump" / "surge" / "pigeon" whatever you want to call it. Now obviously, a rally turbo only has to last a few hundred miles on each event so it's probably not a great example.

I do prefer the sound of "flutter dump" to that of a dump valve....but not at the expense of my turbo. I don't see how not having some kind of relief valve, even at low boost, could be a good thing? I mean, that pressurised air has to go somewhere, the only place is can go is back through the turbo and I just can't see how that couldn't do damage? That article doesn't exactly do much to allay the common fear of turbo stall and damage so, does anyone know what the real deal is?

The noise that turbos on rally cars is the turbo going into reverse which is not good for a turbo but as rally teams don't have to worry that much about the cost of a turbo it doesn't matter to them.

Sorry, maybe I should have been clearer. At the kind of boost we (most of us on a budget) are running (less than 30psi), can not having a dump valve actually damage the turbo? That article seem to suggest that fitting a dump valve is a common mistake and they actually cause more harm (by reducing performance) than good. It doesn't mention anything about damage. Are they just used to people who can afford a rebuild every five minutes or is the risk of damage so minimal they it wasn't worth mentioning?

The noise you hear from rally cars is anti lag and external wastegates.

Rally cars have antilag which keeps the turbo spinning. Yeah they're getting flutter from the air going backwards through the turbo, but it doesn't matter.

On the case of a normal car a dump valve reduces the strain on the turbo during turbo stall. They can however dump off too much, which means all the charge pipes need recompressing. If they dump off too little then the turbo shaft is still under strain.

Manufacturers opt for recirc setups which make sense from a noise and emissions perspective.

Personally I think dump valves are gay and should only be used on high pressure systems and preferably recirc too.

Most higher performance turbo engines have a dump valve, as Martin says, but rather than vernt to atmosphere they recycle the air back into the inlet tracting. It's silent and efficent.

stop your car from boosting if some douchebag sells you a broken one

If the dump valve leaks in any way then it will shorten the life of your turbo by a considerable amount,and sound gay as well.Recirc ftw.