What is a dump valve? PH Explains
Find out how and why a dump valve can improve the life of a turbocharger
What does a dump valve do?
When a driver accelerates in a turbocharged petrol car, the turbocharger will start pressurising the air in the engine's intake system. This high-pressure air then moves past the open throttle blade in the throttle body, into the intake manifold and on into the engine.
If the driver then lifts off, however, this moving high-pressure air will slam into the now-closed throttle blade. Having nowhere else to go, it bounces back and tries to exit through the still-spinning compressor of the turbocharger. This produces a pronounced chattering or fluttering noise as the compressor chops through the air passing over it in the wrong direction.
Besides creating noise, this can generate erratic loads on the turbocharger's bearings and blades. While a turbocharged engine doesn't need a pressure relief system to function properly, these loadings can cause premature turbocharger wear or failure. Consequently, manufacturers fit systems to vent this pressurised air from the intake when necessary.
In factory applications, quiet recirculating valves - sometimes called diverters - are used; these redirect the high-pressure air back into the intake in front of the turbocharger. Aftermarket set-ups sometimes favour valves which vent to atmosphere - these being the type most associated with the term 'dump valve'. These generate a characteristic 'psssch' noise when operating.
How does a basic dump valve work?
These valves typically consist of a spring-loaded piston in a cylinder. The cylinder has an inlet at the top and bottom, and a discharge port in the middle. The bottom port is connected into the pipework between the turbocharger and the throttle body. The other port, at the top, is hooked into the intake system after the throttle body.
At idle, vacuum from behind the throttle tries to lift the valve's piston up - so the spring holds it closed to prevent unwanted leaks. Similarly, at low boost levels, the spring stops the piston shifting. When you're driving along at high boost, the pressure on either side of the throttle body is the same - so the pressure on each side of the valve's piston is the same and it remains closed.
If the throttle closes, however, there will be high boost pressure on the bottom of the piston and strong vacuum from behind the now-closed throttle on the top. The vacuum attempts to pull the piston upwards, while the high boost pressure also tries to push it upwards - and the combined force overpowers the valve's spring, causes the piston to move and the discharge port to be exposed. Excess pressure is then vented, either to the atmosphere or back into the intake system, at which point the valve closes.
A dump valve has no effect - provided it doesn't leak - on the peak boost generated by a turbocharger, however, as that is controlled by the turbocharger's wastegate.
Different types of dump valve
- Atmospheric dump valve: The classic 'blow-off valve' which vents all excess pressure to atmosphere. These generate a 'psssch' noise in operation. Some are offered with a range of fittings and trumpets, which can allow owners to change the volume or style of the noise produced.
- Recirculating or diverter valve: The type favoured by manufacturers. These, as the name suggests, recirculate high-pressure air into the intake system - and, consequently, are far quieter when operating.
- Adjustable bias: Many aftermarket suppliers, such as GFB, offer adjustable set-ups. These can blend between atmospheric and recirculating modes, allowing owners to tailor their volume. They can also work better than purely atmospheric set-ups on certain cars.
Do diesel engines need a dump valve?
As conventional diesel engines have no throttle plate and function differently, a dump valve is not required. In some diesel applications - such as high-performance aftermarket turbocharged set-ups - there may be a benefit to having a pressure relief system of some form, but it will need to be triggered by excessive pressure or an electronic system.
On the local Corsa VXR? No thanks
I think the idea that dump valve = chav is wrong, and I hope this PH guide goes some way to dispelling that, at least amongst us enthusiasts.
On the local Corsa VXR? No thanks
For a long time on my old RB5 I had the decat and full system but never touched the dump valve from the stock recirculating one because I didn't think it would do much and I could take or leave the noise. But when I fitted one for the new owner just before I sold it, I absolutely loved it, you got a massive bang on each gearshift and could usually see the flames in the rear view mirror if the exhaust was good and warm.
I want an Impreza now just to drive around clattering and banging away again. The M3 is positively pedestrian in that respect.
I think the idea that dump valve = chav is wrong, and I hope this PH guide goes some way to dispelling that, at least amongst us enthusiasts.
I'll agree that ones fitted with tweeters are incredibly obnoxious.
Strangely enough, considering it's the era of the turbo, these intentionally whistling valves seem to have gone out of fashion...
Who the f
k is writing these articles?!?If you fit a 'dump' valve to a VAG group 1.8T then it causes it to run rich and causes misfires.
PH articles at there best, written by someone googling.
They dump pre metered air causing the engine to run rich, I can't believe I would ever read such bulls
t on a genuine PH article.
Who the f
k is writing these articles?!?If you fit a 'dump' valve to a VAG group 1.8T then it causes it to run rich and causes misfires.
Most engines will be unaffected, well apart from the lack of blow back into the turbo causing it to spool down... but that is a good thing.
I think the idea that dump valve = chav is wrong, and I hope this PH guide goes some way to dispelling that, at least amongst us enthusiasts.
I think, from my limited experience, that a reputable dump valve is a good mod, easy to fit for the amateur mechanic, has real benefits, has very few drawbacks (on most models of car anyway.. I don't know about the particular VAG mentioned above, but that's hardly a reason to assume they're bad for all cars), they have a reasonable benefit to cost ratio, certianly no worse than an aftermarket exhaust or induction kit, and the noise is personal preference.
It's not true to say that all it does is make a noise. It might be a marginal benefit, but all mods short of a remap or rebuild are marginal.
My Starion had one fitted when I first got it ten years ago. Loved it. Still had it fitted when I sold it a couple of years ago.
To me it went perfectly with that era of turbocharged car- along with the lag, the on/off power delivery, the factory-fitted TURBO stickers, boost-gauge in-between the speedo & rev counter, and the crippling fuel economy (15mpg from a 170bhp 4-cylinder anyone?).
Whilst I suspect modern turbocharged engines are technically superior in just about every way, they have little appeal to me. I want a turbocharged engine to feel & sound like a turbocharged engine- preferably one from the 80's/90's, and the loud dump-valve is an important part of this!
Anyway, some genius in the local area got one of those 'imitation' dump valve kits that, quite bizarrely, dumped the brake vacuum line when triggered by a microswitch on the fuel pump stop
I also remember a company (some 10-12 years ago now) making a speaker that you'd fit under the bonnet of your Saxo/etc that would emit a dump valve noise on gear change
Who the f
k is writing these articles?!?If you fit a 'dump' valve to a VAG group 1.8T then it causes it to run rich and causes misfires.
PH articles at there best, written by someone googling.
They dump pre metered air causing the engine to run rich, I can't believe I would ever read such bulls
t on a genuine PH article.Well, mine doesn't anyway.
I think the idea that dump valve = chav is wrong, and I hope this PH guide goes some way to dispelling that, at least amongst us enthusiasts.
I had a Bailey twin piston one on my S1 RST at 18 years old, i thought i was the bees knees
Gassing Station | General Gassing | Top of Page | What's New | My Stuff