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Dump Valves – Are They Worthwhile

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PremiumNo Difficulty Mod ID250 CreditEvilution Cost££50 - £200 For450 Fortwo451 Fortwo452 Roadster453 Fortwo/Forfour454 Forfour Link Copy to Clipboard

The fact of the matter is that almost all of the respected smart tuners will not fit a dump valve, that goes for Brabus as well and they should know what they are talking about.The reason is because they were using stock DVs and didn’t consider a smart specific DV.

How Does A Dump Valve Work?

As the throttle plate closes, air pressure builds up between the engine and the turbo. Because the turbo is still spinning it continues to pump air into the pipes even though it is not needed.

A vacuum builds up on the other side of the throttle plate (inside the engine air intake).

A dump valve is connected between the turbo and the engine, being a valve, it can open and close to allow excess air to be vented from pipe. The top of the DV is connected to the engine intake manifold via a 3mm rubber or silicone hose. The differential pressure between either side of the DV opens and closes the valve to vent the gasses.

What Does The Dump Valve Do?

The DV opens and releases trapped boost that builds up between the turbo and the throttle body when it closes. Without the DV, the boost air builds up and then forces its way back passed the turbo the wrong way causing it to stall (which is the fluttering noise you can sometimes hear).


The turbo can only pressurise a certain amount of air into a closed area. When it can’t push any more air into the intercooler the air stops moving and can make its way back down the pipe towards the turbo.

If high pressure air is pushed the wrong way across a turbo compressor it can cause it to slow down suddenly or even stop, this is called stalling. Over time, this stalling can wear the bearings and seals and cause juddering of the main turbo shaft.

The DV opens to vent enough air so that the turbo can continue spin yet maintain a slight back pressure in the pipes ready for when the DV closes and the throttle plate opens.

So Why Is It So Bad For The Smart?

The problem is all down to the size of the turbo and the back pressure in the pipes.

On a larger turbo car, the DV outlet is smaller than the turbo outlet. This means that the turbo can put more air in than the DV can let out which maintains the back pressure needed later on.

On the smart car, the turbo outlet is SMALLER than the DV outlet, this means the DV can vent faster than the turbo can put in resulting in no back pressure at all.

A good comparison is push starting a car. Everything is fine when you have a car to push against, when the car suddenly drives away you speed up in attempt to stay upright but end up falling flat on your face.

The smart turbo is quite happy to pressurise air into the relatively small engine but as soon as the DV opens there is nothing for the turbo to push against. With the restriction lifted, the turbo suddenly spins up to a much faster RPM.

As soon as the turbo races it begins to slow as the wastegate opens and diverts the exhaust gasses away from the turbine side of the turbo. In the 1.5 seconds it takes for the car to change gear the turbo is now spinning at about half of its normal operational speed.

With the correct gear selected, the throttle plate opens again to feed air into the engine. As all of the boost has been vented there is no pressure available for the engine. To compensate for the lack of boost, the ECU closes the wastegate to quickly spin up the turbo. For a brief second, the turbo spins up over its usual operational speed to quickly supply the engine with air. Once the pressure is corrected, the wastegate is regulated and the turbo assumes its correct speed.

In the space of 4 seconds the turbo has been made to race twice, once from half speed.
The constant change of speed damages the bearings and the oil seals.

People have been saying that they have been running DVs pretty much from day one on modified smart cars with no problems but that just isn’t true. The damage isn’t a quick thing, it slowly damages the turbo which can cause problems elsewhere in the engine. To be honest, most smart engines don’t last long enough to see a fault with the turbo but is it worth the risk?

If you still don’t believe it, there is another reason to adapt your DV.
With the 12mm adapter fitted, the car will be quicker from 0-60 and that is something you can test yourself. The ECU spends less time trying to equalise and rebuild boost pressures so the car spends more time accelerating and not sitting in ‘gear change limbo’.


Fitting a DV will save your turbo.
A DV causes oil to appear in the air pipes.
All dump valves are suitable for the smart.
Dump valves increase performance.


Fitting a DV will eventually damage your oil seals and bearings and cause a judder.
The Oil spat out by a DV comes from the crank case breather and isn’t caused by the DV.
Only the restricted Forge DV is designed especially for the smart.
Dump valves give you quicker access to the performance you already have.

But I Want A DV And No Damage

In that case, don’t just blindly fit “any old dump valve”, what you want is a restricted DV.
A restricted DV will give you all the benefits of the DV on a smart without the downsides.
The only difference you may notice is that the restricted DV is quieter.

A Bit Of Maths For You

The golden rule for working out the correct outlet size for a DV is 1/3rd the area of the turbo outlet. The smart outlet is 21mm in diameter so we need to work out the area.

21mm / 2 = 10.5mm this gives the radius
Using that we can use Pi x Radius squared
3.14 x (10.5 x 10.5) = 346.185 which is the area in mm.
1/3rd of 346.185 is 115.395 meaning 115.395 is the correct area needed.
115.395 / 3.14 = 36.75
Square root of 36.75 = 6.06mm
6.06mm is the radius so double it to get the diameter.
12.12mm is the required diameter to create a hole 1/3rd the size of the turbo outlet.

12mm Outlet Dump Valve!

Yep, that is tiny compared to the 25mm found on most dump valves.
In fact, normal DV’s are 4 times too big for the smart!

When the calculations were made there were no ‘off the shelf’ 12mm dump valves.
C_H_A_L_K_Y from Ebay

can machine an adaptor for your existing DV to make them fit for service working on my calculations seen above.On the 21st of December 2006, the first ever smart specific DV was made for testing.
Made by one of the ‘big guns’ of the DV world, Forge Motorsport, it was sent to me for initial trials and tests. All info about the smart specific Forge dump valve is here.

To test my own theory I bought a sequential DV and sent it to Chalky for machining.
Read about the ‘Smart specific dump valve’ here.