Does anybody know anything about these? I wondered roughly how much it would cost to have them fitted to a car , whether they alter performance / economy, and what they actually do?
I just like the noise they make!
Thanks

They don't do anything apart from making your car sound like a bus with air brakes.

Save your cash :)

First of all has your car got a turbo-charger? This is the primary requirement. If you do have a turbo'ed car it will probably already have one.
Most production turbocharged engines are fitted with a dump valve. This is to vent away excessive turbocharger boost pressure when the throttle is closed. The dump valve has basically two operating conditions; throttle open, and throttle closed. When the throttle is open, the boost pressure coming into the valve from below the diaphragm is equalled by the pressure onto the top of the diaphragm that comes via a small hose from the engine plenum inlet. As the pressures are equal, a spring holds the diaphragm onto its seat, and prevents the pressure venting away. When the throttle is closed, the pressure coming into the top of the valve is significantly reduced, (often to a vacuum), and the pressure below the diaphragm lifts it away from its seat, allowing the excess pressure to vent away. As the throttle is reopened, the pressures again equalise, the spring pushes the diaphragm onto its seat, and the boost pressure cannot vent away.

Why Fit a Dump Valve?

It is for a number of reasons. Without a dump valve, when the throttle was closed, the build up of boost pressure would be immense, very similar to closing a quickly flowing water tap. This excess of pressure, (which could be over five times the running boost pressure), would put the components of the system under a great strain. It would try to burst the intercooler and pipe work. More significantly, it would put a large strain on the turbocharger itself, firstly stalling the compressor shaft, then trying to force the compressor wheel out of the turbo, against its bearings. So, the dump valve prevents mechanical damage to the intercooler & turbo pipe work, prolongs turbocharger life, and prevents excessive turbo - lag that would be caused by the compressor shaft stalling.

They only impress numpties...

You know the whoosing noises lorries sometimes make when stopped at traffic lights due to their air brakes? Well some boy racer's fit devices that make similar noises onto their cars, little aware that it makes them the laughing stock of the general population. It is quite amusing seeing a whacked out car zoom by making a little whooshing sound as the driver changes gear - a bit like seeing a cat roar like a lion I guess :) Many of these noise-producing devices are proper dump valves for cars with turbos, designed to reduce pressure on the engine. However some are just basically speakers with a prerecorded whoosh noise. We kid you not.
taken from a boy races site

wow- quite a technical lowdown! thanks for this SHarper.

That's a good rundown SHarper - I was trying to be more consice :D

I don't remember the last time I heard a real performance car with an audible dump valve, even 911 Turbo's, and as SHarper says, a non-turbo 911 doesn't have one because they serve no useful purpose - apart from amusing bus queues that is ;)

Most turbo production cars don't have the kind of dump valve we're discussing here. The turbo installation features a wastegate, which does the job of getting rid of pressure when not needed. Noise regulations and refinement expectations mean these don't vent audibly. The most you'll hear is a discreet 'chatter' when you lift off the throttle.

On occasion a dump valve can actually help, particularly on cars modified with a bigger turbo than standard. Bigger turbos have more inertia, and tend towards 'lag', I've seen one very good situation where a chap used two aftermarket dump valves to keep the turbo unpressurised when off throttle. It worked well, making the thing very responsive. But it still sounded like a proper dick's motor. There are several dump valves out there you can actually tune the sound on, I'm afraid.

Tragically there is actually a dump valve for non-turbo cars. It's essentially a throttle-actuated tape recording of the rising swish of a turbo, followed by the phishhh! of the dump valve. It plays through a speaker under the bonnet. Essentially, your 1-litre Saxo becomes turbocharged, but without the inconvenience of improved performance. You would have to be a pretty hardcore muppet to install one.

With apologies to Steeve - just read the tail end of his post.