what is it for?

When you lift off the throttle, almost all air is stopped from going into the engine (in a petrol car), so there is a build up of unused air between the turbo and throttle butterfly, which can 'stall' the turbo. A dump valve gets rid of this air, so keeps the turbo spinning ready for when you put your foot back down, and so reduces lag.

funny you should ask as that's one question i asked the MR2 people at imoc.co.uk yeah just stops stall on the turbo, not factory fitted as they aint quiet :)

i'm starting to save for an MR2 MKII Turbo next year Rev 3 and will probably have one on that as the REV1-2 couldn't have one because of the AFM (Air Flow Meter)

here's one of the useful and short replies i got :)

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The BOV debate. The short version;

Rev 1/2 - has air flow meter. This records how much air is to be burnt. If you vent to atmosphere the car doesnt realise and cant compensate. = Shit running

Rev 3 - uses MAP sensors etc, can compensate for it so its ok.

Good Points of a BOV,

Prolongs turbo life by avoiding whats called turbo stall. Reduces intake temperature by not venting hot air back into the intake, less likely to knock and gives more power. Cool noise.

Reason not standard on turbo cars which dont have the AFM issue - might annoy nice drivers.

Hope this covers it

and makes a good sound :)

Atmospheric ones do, but you'd find it more difficult to hear a recirc one!

Should be able to put a nice quiet recirc one on with the AMM shouldn't you? The air to be burnt is still in the system so shouldn't screw the fuelling up. Or am I (probably) missing something?

Mike

I think most manufacturers fit recirculating to their turbo'd petrol models anyway. The ones that make noise are atmospheric dump valves and folk fit them to let everyone know they have a turbo.

There's been muchos discussion about this on here before, I'm not sure what the advantages are of atomospheric dump valves, I do remember reading that in some cases they are are worse for performance/your engine than recirculating ones, I think it depends how and where the air flow or air mass is measured for engine management as the ECU expects air to be recirculated and isnt with and atmo valve.

Douglas

Douglas

snip

Removing the airbox and replacing it with a BFO stainless-mesh type air filter helps ;-)

You can on mine, with the cone filter. All you have to do is floor the throttle in a gear that is touch high the speed you are doing (not labouring, but too low speed to really get best acceleration, but ideal for lowish speed cruising along such as 20 in 3rd just leaving traffic). Then just as it spools and boost is about the happen back off, and it gives a very satisfying "Ksssch" sound, not loud, but enough to be heard over the engine on over-run. Works best in

3rd when overrun sounds so absolutley bubbley. Strangley, when it is nicely spooled up, and making it's max boost it doesn't make hardly a noise at all, or maybe the engine is too loud or I'm not completley off the throttle as I shift at higher revs, and get back on the gas too fast).

Same problem as Saabs have. But a double spring one like Bailey do should still be made to work, without giving bad idling, or black smoke.

Also they probably already have a dump valve fitted, but it will be a plastic recirc type like the Bosch that Saab/VW etc Euro companies fit, or a japanese copy.

You could fit an uprated Bailey or Forge recirc and then if you add a cone filter on a very short induction tract before the turbo, you would hear the recirc dumping back.

BTW, have you heard of Hybrid dump valves with double springs. They have two vents. One side dumps back to the inlet, the other is outlet through a trumpet to make the "Tssccch" sound. You can adjust the balance so you can get the engine running right then. SO with small shifts like changing gear at town speeds, it recircs only, if you shift at higher speeds it splits the dump between the recirc and trumpet, so you get a proper woosh sound.

Something like this

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In news:K1QYb.2248$ snipped-for-privacy@news-text.cableinet.net, Theo decided to enlighten our sheltered souls with a rant as follows

it's for people who're not running enough boost to have a proper "Wuurrrrrrrrrrrrble"

...as do all Turbo-charged cars !

You can't run a Turbo without a dumpvalve, otherwise there's nowhere for the pressure to go when you lift off the throttle. This does bad things to the turbo !

Some old cars don't (R5 GTT etc).

I should have rephrased, atmospheric dump valves I was reffering to.

No they don't. R5GTT, Saab 900T8 and more didn't have them as standard.. Modern ones probably do though..

Mike

on my dad's volvo S60 D5 you can hear the turbo spool at low revs when cruising slowly. if you don't let it come in you can hear like a high pitch sound, hard to describe...

As i drove round with a large chest in the back of my car today, so that the boot was open i discovered that my 205, like the old one, whistles like a banshee when the turbo spools up under load. Most amusing :)

In article , snipped-for-privacy@Somewhere.Somewhere spouted forth into uk.rec.cars.modifications...

Saab 99 and the early T8 900's didn't have them. A lot of them survived OK.

You can run a turbo without a dump valve, its not an essential item. Most modern cars will have recirculating ones, but some older turboes wont have one at all. It will just make them more 'laggy' and probably wont help the turbo life.

Well some will say chucking warm air back into the intake isnt really the best of ideas. But often they're just fitted for the ptttttttttttttttsssssssssssch noise they make. Top tuners (as in show cars) have them to mostly sound good to boy racer types. I dont think WRC cars have dump valves because that depressurizes the intake manifold, which although will help the turbo last longer and not stall, also creates lag as the system has to repressurize. To counteract this the cars use anitlag. Basically when the driver lifts off, more fuel is chucked in, and ignition is retarded so that it all ignites as the exhuast valves are opening, causing the turbo to keep spinning and not stalling from the intake manifold back pressure, whilst still keeping everything pressurized. Now that would be cool on Nom's ti! In fact i'm surprised the FM900/2 hasnt got it!