rooky1

this may seem a silly question to some of you, but what is the purpose of a dumpvalve? do they cause any damage or problems? any advice would be helpful.

Nick200

They simply let excess air pressure out of the system, never heard of any car having any trouble because of a dump valve.

john coffey

They are only really worth it if:

a) your existing re-circulating dump valve is broken, or you are running serious levels of boost causing the standard one to leak.
b) you specifically want the "pssht" noise when you lift off.

If you fit a vent to atmosphere one, then there is a chance that your car may actually perform worse, use more fuel and idle poorly. The reason for this is that the air coming into the engine is measured by the ECU, and if you vent some of this to the atmosphere, then the ECU gets the calculations wrong (more air coming in than expected), so it adds loads more fuel to compensate. This can cause the car to run rich for a second or so on each gearchange, and also cuase poor idling when coming to a stop.

If you have a replacement ECU which doesn't use the measured air mass as an input, then you probably won't have these problems.

ScrappyDoo2

Hi,

Been asking Q's about dump valves in the main forum, I thought that the excess fueling isn't a case of the ECU seeing more air and therefore increasing the fuel. The way I see it is that the MAF sees the air coming into the system and is placed before the point where the DV would push back the expelled air into the system (air which was already accounted for when it entered past the MAF). The ECU has this measurement and is fueling accordingly, if however you vent some of that air with a VTA DV then that pre-accounted for air is lost and the 'original' amount of fuel is still used by the ECU as it expects that air to have been returned, but it hasn't and this in proportion to the fuel delivered means for that cycle the mixture is rich?? All IMHO of course. You also mentioned poor idling which I've also read about, I thought this usually only affected diaphragm type valves and not the twin piston designs used in Scoob applications. It surely wouldn't be due vented air of a VTA as scoob DVs are closed on idle and therefore no air is being vented which would lead to incorrect AF ratios.

This is what I have picked up from trudging through loads of threads as I had put a forge VTA on my 02 WRX but have decided to put the factory unit back on due to the momentary rich running on gear change, I also didn't like the way it would vent boost if I backed of to part throttle which would jerk the car. Hope some of this helps - IMHO don't bother changing the factory DV unless you really want the Pssshhh noise. I noticed that Merv at PE and Graham at TSL both running above the 260bhp mark with raised boost both use the factory unit...