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Dump valves, great for turbo-charged cars (except diesels-see below), they look great & sound great (atmospheric type only!). They are designed to reduce turbo lag & pressure resistance on the turbine blades- Thus improving the life of your turbo. But Why?

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A ‘Bailey’ Piston Type Dump valve, as fitted to an Impreza.

When you are revving your car at with full throttle & the turbo is spinning at full boost (Over 110,000rpm!).The boost pressure is calculated by the flow efficiency of the turbo, Inlet manifold, Cylinder head etc (pressure does not exist without restriction to flow!). If you are to now suddenly shut the throttle (i.e. for a gear change) this air-flow suddenly has a huge restriction and no where to go. Without a dump valve or some form of pressure release, this sudden rise in pressure will back up to the turbo causing it to slow down dramatically (The rotational energy of the turbine blades will be converted into huge amounts of heat dispelled through the blades themselves & the bearings- typically this will make your turbo not last very long).
Once the gear change has been made the turbo now has to ‘re-spin’ back to over 100,000rpm again to regain the boost pressure. This time taken is known as ‘turbo lag’.

Fitting a dump valve in between the turbo & the throttle butterfly (preferably after the intercooler) reduces this by opening a path for the air flow thus reducing the pressure.

So if they are so important why do manufactures not fit them?
Well they do, but typically they fit ‘rubber diaphragm’ types and these are prone to wear and may leak, due to oil contamination on the rubber.


A Typical ‘Standard’ Re-circulating Dump Valve

Aftermarket ‘up rated’ dump valves are typically a piston with a spring on one side; the spring holds the valve closed when boost is present- When the manifold pressure drops suddenly (Gear change etc) the negative pressure opens the piston by overcoming the springs compression force. The Boost pressure will now escape through the vent holes in the valve body.

Can I have a dump valve on my diesel?
The easy answer would be to say no, But some companies today (Mainly in the hope to increase the market potential for their products) have created dump valve kits for turbo diesel engines. Let me start by saying these do not give any increse in performance, power or even a reduction in lag, but will give a typical ‘dump valve’ whoosh if that’s what you’re after.

So why do dump valves not give a reduction in turbo lag on Turbo Diesels as they do on a Petrol engine?
In a petrol engine the throttle pedal controls a butterfly plate in the throttle body & to shut the throttle causes a restriction to the air flow. Diesel engines do not have this throttle plate & the accelerator pedal on a diesel controls the diesel injection amount & timing. So with no throttle plate to cause a restriction to airflow on the overrun, there is no need for a pressure release.
Secondly, a dump valve on a petrol engine uses the pressure difference on the two sides of the throttle butterfly to control the opening of the valve (when the throttle is open, the pressure on both sides of the throttle plate is pretty equal but once the throttle is snapped shut the pressure on the turbo side can increase to as much as 30psi! whereas on the Inlet side it can drop to as much as -15psi).
As diesel engines do not have this throttle body, the only way to control the opening of the valve is some sort of electronic control & this is what is usually included in the kit.
Once you close the throttle a electric solenoid activates the opening of the valve thus producing the pressure realise 'whoosh'.
There has been talk that putting dump valves on diesel engines actually reduces cylinder ‘scavenging’ & thus reduces the volumetric efficiency & power, but I am not aware of any research being done in this area (Performance engine builders generally don’t mind if there is a whoosh noise or not!).

Re-circulating or Atmospheric?
There has been a lot of questions asked about what is best; re-circulating dump valves (where the pressure is released back into the inlet tract-generally silent) or a atmospheric type (where the pressure is dumped outside of the induction tract-typically noisy).
Well there are good arguments for both, I would say if you like the noise go for atmospheric, they are better- But if you are the more disconcerting driver, go for re-circulating.

Re-circulating:
The arguments for re-circulating stem from points including: *Dumping the pressure back into the inlet tract can actually keep the turbo spinning by ‘blowing’ it- this is not true (blow as hard as you like you could never make a turbo spin- the blades are designed to ‘compress’ air ) and the returned pressure to the inlet tract does not increase the ambient pressure enough to make the pickup quicker (due to huge pressure differences once the throttle is re-opened anyway).
You must also consider where is the valve going to be placed (before or after the intercooler), If it is placed before the intercooler then you are surging warm-pre compressed air back to the turbo- This extra heat will have to be removed or the amount of fuel that can efficiently burn with it will be reduced.
If the valve is placed after the intercooler then intercooler efficiency will decrease, since dumping air that has already been cooled will in turn have to be ‘re-cooled’ again after compression.
These types of valves are usually fitted as standard to production cars mainly for noise reduction & fuel economy, not much effort is made to establish an ‘idea’ location (most are placed before the intercooler).

Atmospheric: These types of dump valves will also have to be placed before the intercooler for best efficiency. Arguments for these are:
If the car had been fitted with an ‘air flow meter’ then the valve is releasing air that has already been measured & accounted for, thus causing your engine to run rich on the overrun. Although this will do nothing for your fuel economy, the un-burnt fuel entering you exhaust stream will rapidly expand (and maybe burn) creating more pressure to keep the turbo spinning longer & give a effect of ‘anti-lag’ as used on top rally cars.

If you raise the boost pressure of your turbo then the engine will get a power increases right?
Wrong, well ok it’s and swings & roundabouts. Here’s why. When you raise the boost pressure- the temperature of the charge increases (universal gas law states that when you compress a gas you raise its temperature) this extra heat lowers the charge potential because it is less dense in oxygen (thermodynamic law states that cooler air is denser in oxygen). This reduction in density lowers the amount of fuel that can be efficiently burned to produce power. Also unless the extra heat is removed (by way of fitting/up rating the intercooler) the engine could be prone to ‘detonation’, (see the combustion article) this is something that you don’t want- although usually you will not have to put up with if for long-
soon after it happens it will destroy your engine dead!.
Most ECU's usually have a boost cut off for safety (usually set around 1-1.1 bar) this limiter is there for a reason, do not remove it- unless you have done suitable work to the engine (Compression ratios-Spark plugs and fuel quality all have effects on the tendency for the charge to detonate) to enable it to run at high boost pressures.

Fitting a dump valve.
This can be a very short section because dump valves are very simple to replace. Basically it is replacing the cars ‘standard’ valve with an ‘up rated one’- generally better quality & longer lasting. Dump valves sit exactly where the standard one did and uses all the same pipes (unless the kit supplies extra pipes) just make sure that if you are fitting a atmospheric type to blank off the inlet hole (blanking plates are usually supplied with the kits).

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