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Dump Valves


Sniper
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Dont know if this is the right place to post, anyway one stupid question...

Can anyone give me aproper definition for `Dump Vavles`

Why they are fitted to turbo diesals

And how do they work

Many thanks :blink:

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Guest diesel_jim

They're not fitted to diesels, only petrols.

When you lift your foot off the accelerator to change gear, the engine obviously drops its revs, and usually comes off of turbo "boost", so when you put your foot back on again, you have to wait (seconds, sometimes more) for the turbo to start producing enough boost again to give a power increase, so the dump valve "vents" the turbo outlet to atmosphere, and at the same time keeps it spinning at a fast speed, so when you've changed gear and you lift your foot off the clutch, the turbo is ready at the correct pressure to give some oomph.

wikipedia knows best

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Jim / Wikipedia is 95% there - it drops all the pressure in the manifold, turbo inertia etc so that your petrol engine doesn't go very lean (too much air) when you open the throttle in the next gear. Not an issue with a diesel (and to be quite honest, not really an issue with most road engines/gearboxes but if rally cars make a 'pish' noise, Kevin's Corsa must make a pish noise too.

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I thought that they were used on petrol engined cars because closing the throttle butterfly (ie when changing gear) causes the compressor to stall as without the dumpvalve the turbo is pressurising a closed tube. They are not generally used on diesels as most ;) don't have a throttle butterfly so don't need it.

Pete.

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raceface has the most accurate definition here.

On a petrol when u close the throttle the turbo, still spinning at 100000rpm, is trying to force air against a closed plate. This causes the compressor wheel to slow down rapidly which on some turbos can cause damage to the shaft and or bearings. The dump valve is fitted to the boost pipe between the turbo and the inlet manifold and has a vacuum feed, when the throttle is closed the engine generates a vacuum and opens the valve which creates an exit for the compressed air as the turbo slows down. pretty much ALL turbo petrols have them, although from the factory they are fitted with a recirculating valve which doesnt create any audiable noise, this works by dumping the compressed air into the inlet side of the turbo. The dump valves you hear making a TISHHH sound are the same device doing the same job as the standard valve, but are venting to atmosphere instead of the turbo inlet.

On a derv the "throttle" simply controls the fuel input and there is no actual throttle plate on the inlet to stop the airflow, the dump valve is therefore not required, u can however fit at atmospheric valve in order to attain the TISHH noise, u will require some way of creating a vacuum pull on the valve tho, this is usually done using a EGR solenoid, microswitch on the throttle pedal and a feed from the brake servo vacuum pump.

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There is a diesel tuning co who advertise in the comics who *claim* they are the only ones who can make a dump valve work on a diesel.

I know it's been tried and rarely works / makes any difference on most diesels, although what the difference is I have no idea. A guy at work fitted an old one from his cosworth to his transit and it did nothing.

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Guest diesel_jim
erm i thought landcrusers had them fitted on the diesel? either that ir something else makes a big airy noise !!

Is that not an exhaust brake? blocks the exhaust flow off to nearly "stall" the engine, and thus give you mega engine braking. a lot of trucks have it.

my 90 makes the "Pshhhht" noise..... whenever i release the ARB's! :lol:

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Is that not an exhaust brake? blocks the exhaust flow off to nearly "stall" the engine, and thus give you mega engine braking. a lot of trucks have it.

my 90 makes the "Pshhhht" noise..... whenever i release the ARB's! :lol:

ive got a recording of the Pshhht which a play out really load... everyone thinks ive got ARBs then :P

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a dump valve will not provide a power increase on any engine, petrol or diesel. Its sole purpose is to stop compressor stall damaging the shaft and compressor

Exactly.

P.S. My brother in law has one on his Seat Cupra Diesel, sporty diesel thing, factory fitted...

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It's funny how so many different interpretations of the same thing there are. Here's mine.

For me, I've always though of the dump valves (either venting or recirculating type) as just being inlet manifold pressure release valves. At whatever boost the engine or turbo design gets scared that's what he sets the limiting valve too. :rolleyes:

The reason you generally don't see them on CI engines is that turbos are heat scavenging devices and have a far harder life in SI engines where exhaust gases are considerably hotter and so they run at higher rpms. With CIs the total boost just isn't going to get to a damaging level although really modern diesels are starting to rev so freely that it's probably becoming a problem.

There is also the issue of noxious NOx which is produced in abundance with very high boost pressures leading manufacturers no choice but to limit boost or fail type approval on emissions.

To confuse us even more there are also waste-gates that do a similar job of limiting boost but by redirecting exhaust gases away from the turbo when boost gets too high. These are usually just spring held valves in the exhaust tract operated by the positive pressure found in the turbo'd inlet manifolds. Inlet manifold pressure up, spring pulled, valve opens, exhaust gas bypasses turbo, turbo winds down, inlet manifold pressure falls, spring pulls back, valve closes, etc..

They also have the effect of reducing turbo lag under certain throttle on/throttle off circumstances but you can get round that in a race by not bothering to come off the gas when you dip the clutch - no lag at all then :P

So, although you can shape your power curve through the gears on a dyno by reducing lag, you won't get any more power out of an engine just by dumping inlet pressure.

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I've got 3 perfectly good vacuum-operated dump valves on the garage wall. TD5 EGR units. We replace them with a fabricated plate and a pipe with no restriction. I thought they might come in handy. Now I need a turbo petrol car (fat chance of that).

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