Dump valves
 

I apologise as this is a newbie question, but could anyone help me with the following on dump valves.

1) What exactly is a dump valve and what does it do?

2) Is it better for the engine and turbo?

3) My car backfires a bit with it on, why does it do that?

4) Will the car use more petrol with it on? or will it be marginal

Thanks in advance!!!

1) What exactly is a dump valve and what does it do?
Releases Air in the Intake produced by the turbo to prevent turbine stall thus cutting down on lag (The Time It takes to get the turbo spinning).

2) Is it better for the engine and turbo?
Supposidly (sp?) better for the Turbo as it prevents the air in the intake when throttle is released stopping or at least slowing the turbine down

3) My car backfires a bit with it on, why does it do that?
So does mine, guess this goes back to the spinning turbine and unburnt fuel freely entering the exhaust system

4) Will the car use more petrol with it on? or will it be marginal
May do - Not Sure though, I doubt you would notice much difference apart from resisting the temptation of reving harder to produce more pressure in the turbo to make a bigger WOOOSH!!!.. :) thus using more fuel ??

Though I may be heading in the totally wrong direction with these answers... :) someone will correct if I am wrong (Which no doubt I am..LOL)

HTH

Regards

Chris

Thanks for the help with all that Chris. Im new to scobbies and to be honest i dont know that much about how engines operate, etc.


Ta

Your car already has a dump valve which is perfectly suited to the car. The reason it doesn't give the bus doors opening sound is that it is a recirculating valve which redirects the unused air back into the system instead of blowing it out under the bonnet with a silly noise. Just after the DV is a sensor called the AFR (air fuel ratio) which expects a certain amount of airflow at any given time. When a VTA or BOV DV dumps the air out of the system, the ECU reacts to the missing air by dumping a bucket load of fuel in to prevent a stall. This causes the pops and bangs, but also a few other symptoms. Some of the overfuelling is forced under compression past the pistons into the sump thinning the oil (borewash) and removing the microfilm of oil which lubricates the cylinders. Also the excess unburt fuel being forced through your exhaust will be doing your cats no good. The detrimental effects of an aftermarket DV may take a considerable time to become evident (maybe longer than you will own the car), but they do happen. Another point to note is that there are many people on here who have fitted BOV's and thought they were the bees knees, but were astounded that the cars power actually increased when refitting the original recirc DV, leading many to presume that the perceived benefits are purely placebo justifying the financial outlay. Save the cash and if you need more power, try a decat and remap. Better still, spend it on either enhancing the braking or handling of the car, or the competence of the driver.

Cheque for £0.02 in the post :D

I think it would also be a valid point that some standard DV's limit the amount of boost that can be run meaning most after market DV's will not leak at higher boost.

Regards

Chris

Mr Carradoboy, some good, valid points there mate.. ;)

Correct.

Sorry, but this is patently wrong. The MAF sensor is immediately after the airfilter. This meters all the air entering the engine and this information is passed to the ECU which adds fuel accordingly in the correct quantity.
When a VTA dumpvalve dumps the air to atmosphere, that air has already been accounted for by the MAF, so the ECU fuels as if that air is still there as it has no way of knowing that some of the registered air had been dumped. It doesn't add any EXTRA fuel at all. There is simply a reduction in the amount of air that the ECU has provided fuel for, hence a MOMENTARY overly rich state after the car has dumped off.

In theory I'm inclined to agree, certainly the pops and bangs DO do your cats no good at all and I wouldn't recommend fitting a VTA valve to a car with cats in, in fact i wouldn't recommend a VTA at ALL unless you specifically want the noise (which some people do). Recirc is a far better option.
BUT I think the borewash argument is a bit OTT and people should bear it in mind but not be unduly worried by it. The cars overfuel considerably anyway most of the time and the further effects of a VTA will be mimimal.

Power will not and cannot be altered by changing a dumpvalve unless one of them is leaking boost. The change from VTA to recirc or the other way will make no difference to power whatsoever. I agree though with the fact that money is far better off spent elsewhere.

Not having a go here (although it might look like it), so please don't think that. :)

Thanks both for your feedback on the issue.

I now need to decide whether to remove the bailey dump valve i have added, or go back to the factory fitted one! http://bbs.scoobynet.co.uk/images/icons/icon9.gif

Many thanks.

If you like it, keep it. If you dont like it, then remove it.

Easy :)

Bob