Dump Valves

How does a dump valve work ?




There are many other reasons car manufacturers fit blow-off valves to their cars, mainly to do with emissions, fuel economy and drivability. In aftermarket applications though, the main reasons for fitting a Dump Valve are to hold higher-than-standard boost levels, to give better throttle response (than a factory Dump Valve) by staying closed whenever it's not venting, and of course to make noise!


How a blow off valve works


At idle there is engine vacuum on the top of the Dump Valve piston trying to suck it open, and no vacuum or pressure on the bottom of the piston. Since a vent-to-atmosphere Dump Valve needs to be shut at idle to avoid air being drawn in through it, there is a spring inside a Dump Valve with the job of holding the piston closed. The spring preload adjustment is to allow for differences in engine vacuum from car to car, and variations in atmospheric pressure at different elevations.

On airflow metered cars the air drawn in through an open vent-to-atmosphere Dump Valve at idle would confuse the ECU and cause over-fuelling and stalling and in any case, the air drawn in is unfiltered.

Under cruise conditions (off boost) the Dump Valve is experiencing similar conditions to when the car is at idle, but there is less vacuum present on top of the piston because the throttle is partly open. If the Dump Valve spring has been adjusted to keep the piston closed at idle, it will also be closed at cruise.

On boost there is boost pressure on both top and bottom of the Dump Valve, the forces from which counteract each other, so the Dump Valve remains closed.

Immediately after the throttle is closed under boost there is vacuum on the top of the piston and boost pressure on the bottom of the piston, which together, quickly open the Dump Valve to release the pressure. When the pressure has been released, the Dump Valve closes.

How to fit a blow off valve


If there isn't a bolt-on kit available, don't worry, you can still fit a GFB Dump Valve to almost any turbo car. Using the standard adaptors supplied with a Go Fast Bits blow-off valve you can…

1. Hose mount



2. Pipe mount


Select a suitable location on the factory inlet plumbing (somewhere between the turbo and the throttle), and weld the suitable pipe into position. The GFB Dump Valve then pushes onto this pipe and is sealed by the supplied o-ring that sits in a groove inside the base. The Dump Valve secures on the pipe with grub screws and locking nuts (also supplied).

3. Vehicle specific adaptor







Car manufacturers fit recirculating (plumb back) Dump Valves to give the pressurised air an alternate path when the throttle is closed: back into the turbo compressor inlet. This eliminates the ‘undesirable in a brand-new car' fluttering noise.

Aftermarket Dump Valves typically vent the pressurised air into the atmosphere for the purpose of making noise, and are characterised by the 'standard trumpet' sounds. Some other brands do different things with the air to make different noises, but this is not to be confused with the fluttering noise.

In some cases, aftermarket Dump Valves do not flow enough air either as a result of their design, or the way that they are adjusted. In this case, fitting an aftermarket blow-off valve will result in the fluttering noise being emitted from the turbo. While this is extremely popular, it is worth noting that if this is your objective, then simply removing the factory Dump Valve and replacing it with a pair of hose plugs would have been more cost-effective!

Incidentally, fitting a pod air filter can make any fluttering noise that was already present more audible. Also, large front-mounted intercoolers can increase the likelihood of ‘flutter' for any given Dump Valve, due to the larger volume of air present in the intake system. If the Dump Valve is any good, some adjustment of the spring preload would be all that is necessary to once again eliminate the flutter.

Finally, it is possible to set up your GFB blow-off valve to cause some ‘pigeon' noise by increasing the spring preload slightly (turning the spring preload adjustment clockwise). The aim is to have the flutter occur at low rpm and boost, while allowing the Dump Valve to vent freely at higher rpm and boost levels. Experiment with it; you can't do any harm!




So whether it is the Stealth FX hand-adjustable blow-off valve, or the Deceptor Pro in-car electronically adjustable blow-off valve, you're getting the best and most adjustable Dump Valve on the market!








I want my Dump Valve to be noisy, but I've been told that I can't vent a blow-off valve to the atmosphere. What's the deal?

There are many people who will say that if your car has a MAF (Mass Air Flow) sensor (which is most modern turbo cars), you can't vent a Dump Valve to atmosphere. This is not entirely true. In most cases you can, but it pays to be aware of the possible side effects. Quite often you may find the side effects are so minimal that they are not really a concern. This section describes in detail what happens when you vent to atmosphere.



This is where the idea for the GFB Hybrid valve came from. It is made to eliminate such backfiring by its unique design that evacuates the boost pressure to the atmosphere, while sending most of the turbo overrun air to the inlet, still giving the distinct Dump Valve sound but reducing the effects of over-fuelling.



Are GFB blow off valves adjustable?

Many of them are. The spring supplied is used to keep the piston shut at idle (it has no effect on the boost) and will work fine with the majority of engines. A stiffer spring is supplied with most of the valves in the case of Idle vacuum opening the valve.

Is there a possibility that dirt could enter the engine through the

The turbo piping before the throttle body will only ever be at atmospheric
or positive pressure, except for a very brief period when you rapidly open
the throttle from off boost conditions.
way unfiltered air can ever enter the engine through the valve.


Do I need to buy extra parts to configure my Respons Valve as a plumb back or vent-to-atmopshere only Dump Valve?

The beauty of the Repons valve is the rotary adjustment to vary the boost vented to atmosphere vs recirculation. It can be configured in three ways; either as a hybrid, a plumb-back or full vent-to-atmosphere, for maximum noise.







GFB DUMPVALVES - Probably the finest Dump Valves in the world. Click to buy

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