Dump valve question

"Kevin Scott" wrote in news:JCMJe.137668$ snipped-for-privacy@fe2.news.blueyonder.co.uk:

Usually fitted by boy racers with small TD engines (eg 1.5 corsa). Makes a nice noise but not sure if there's any other significant gain.

Yes, I know what you mean about the burberry-baseball-cap-corsa brigade but they are also huge on the modified import scene and often on big power units like Supras', Scoobys', Evos', RX7s', Skylines etc My understanding (and I couild be wrong about this!) is the valve opens and allows the turbo spin freely with the result of reduced turbo lag but I can't think of a instance where a manufacturer fits one as standard and I suspect this is because they reduce the lifespan of the turbo.

Am I right or am I talking bollocks?

No, I'm fairly certain you're talking bollocks. I think they're there to release pressure or something.

On a non-throttled turbo diesel no gain at all.

To make a nice noise it'd need to "dump" something, there's nothing to dump on a Diesel.

"I think they're there to release pressure or something" Good answer - and pretty much what I meant when I said - "the valve opens and allows the turbo spin freely with the result of reduced turbo lag"

Yet somehow I'm talking bollocks. Pots and kettles mate. lol. ;-)

No, but seriously I'm interested in starting a flame war or trolling - just an answer to a legitimate question.

No gain whatsoever on pretty much any Turbo Diesel. In fact I think you have to be quite clever to get a dump valve to make a noise at all on a TD.

Douglas

Heh! Hang around a bit more in here and you'll find that me talking bollocks isn't exactly a break from the norm!

But seriously, I don't really know the ins and outs of them, but I do know that they generally dump whatever they dump when the turbo isn't boosting as much, hence why you hear the "pssst" sound when the clutch is depressed just prior to changing gear. So I doubt it reduces turbo lag. If it was reducing turbo lag it'd happen when the throttle's on full. I could be wrong though - it could, as you say, allow the turbo to spin freely, and not slow down quite so much, doing exactly what you're saying - reducing turbo lag.

I think you'll find that pretty much every modern car manufacturer making turbo'd petrol engines fits dump valves as standard. This probably means that they are a good idea and I very much doubt that they affect the life of the turbo detrimentally.

Car manufacturers tend not to fit the sneezy/squeaky noisy Dump Valves. Dump valves that dump into the atmosphere can, depending on the method of air/fuel management, be bad. Car manufacturers generally fit ones that dump the air back into the induction system, this doesn't knacker air/fuel metering and it doesn't make a noise like something is wrong.

Douglas

I could also be spouting bollocks but here goes:

When the turbo is on full boost under hard acceleration and you close the throttle (to change gear or whatever) the high pressure air being thrown out of the turbo has nowhere to go. This air gets all rowdy, tries to burst things and ends up stalling the turbo. When you go back onto the gas in the next gear, the turbo has to spin up to boost speed again (this is lag).

In a Dump Valve equipped car, the throttle butterfly closes but the dump valve opens uatomagically and gives all the rowdy high pressure air somewhere to go (be it into the atmosphere with a sneeze noise or back into the inlet tract). The turbo can keep spinning and be closer to boost speeds during the gear change, and is ready for some hardcore acceleration sooner after you let up the clutch as it doesn't have to spend so much time spinning up.

HTH

Douglas

Sounds about right.

The first example certainly catches me out in the Passat far too often. I'm getting used to driving around it now, but it's a bit of a surprise after years of n/a cars delivering the same sort of power output.

The purpose off a dump valve is to reduce lag when changing gear.

On a normal turbo'd engine, when you change gear and lift off the accelerator, the throttle valve closes. This effectively creates a blockage for the air flow from the turbo, which leads to a pressure build up before the throttle valve. This pressure build up acts as a brake on the turbo, and combined with the reduced exhaust gas flow to power the turbo, the turbo slows down considerably. So when you put your foot down again, you've got to wait for the turbo to pick up speed (turbo lag)

Now fit a dump valve, and when you close the throttle, the dump valve opens, venting the air before the throttle (to atmosphere with pleny sound for the ultimate boy racer effect), preventing the pressure build up which would otherwise slow the turbo. Result being, that when you put your foot back down, the turbo is still spinning at a fair speed reducing turbo lag.

Correct. You get compressor stall and you end up going over the surge line on the compressor map. This wrecks the turbo bearings very quickly.

Guess why turbodiesels don't have dump valves....:-)

Actually, you can buy a little speaker arrangement to go under the bonnet that makes all the noise without having to even buy a turbo.

I think you're thinking of the Waste Gate. That also reduces lag, but also limits maximum pressure too, as it will dump boost pressure once it hits maximum pressure. That is there to help engine life. On Ford Escorts (and probably a lot of other cars), the ECU will actually cut the fuel if it reads an over-boost, so fiddling with the Waste Gate adjustment has no benefit.

Douglas Payne wrote in news:42f7a477$0$18643$ snipped-for-privacy@news.sunsite.dk:

Oh, you can, believe me. My old apprentice had one fitted to his 306TD (besides chav alloys & suspension etc). Made a lovely hissing noise ever gearchange, right up till the point where he ran a red light and got himself wiped out. Dickhead.

I'm pretty sure I'm thinking of a dump valve. I think you're thinking of a waste gate. (c:

Douglas

That's not really a dump valve though is it?

Douglas

I'm aware that you can get them to make a noise, indeed, I know that at least for the 306 you can buy them off the shelf. I don't think they work quite the same way as they do on turbo petrols however. Diesels not having a throttle butterfly to close being one of the main reasons.

Douglas