A question that pops up near enough weekly on our Facebook page: 18T 20v Tuning is which dump valve is best for noise and which is the best for performance. To begin with we need to understand the two terms that will be used when people answer the question.
Dump Valve or Recirculating Valve?
You might tell yourself, ‘These are both the same thing!’ right? However, there are big differences on your engines performance depending on whether you vent your boost pressure remaining inside any pipework when the throttle plate closes, to atmosphere (dump valve) or recirculate it back into the intake pipework (recirculating valve). So, new we know where the excess air is going to end up when we lift off, lets have a look at the advantages and disadvantages of both and we’ll give you our opinion on what’s recommended and what tuners like to see when you take your car to them for mapping.
Dump Valve
This is what most people will call them when someone asks. It is also a very ‘popular’ modification for those who want a ‘Whooshhh’ sound that lets everyone know that you have a turbo under the bonnet. However, there is a huge issue with running a valve that vents to atmosphere and that’s the fact that any air that has past the MAF sensor before it goes through the turbo and intercooler to engine has been measured and fuelling inside the ECU has been accounted for. This means, that releasing the excess air left in the pipework when we lift off, is lost so the ECU, still thinks that this air is going to re-enter the turbo via the intake pipe and won’t be accounted for in the fuelling tables.
Ultimately, over time, the ECU will adapt to this as it will see that the air/fuel mixture read by the Lambda sensor will be too rich, so it’ll adjust the fuel trims inside the ECU, to ‘pull’ fuelling to prevent the richer air/fuel mix seen by the lambda sensors. These fuel trims can be seen in block 032 in VCDS and are short term adaption and long term. The closer to zero these are, the less adaptions the ECU has to make to the fuelling tables to ensure everything is running properly.
Secondly, and what a lot of people don’t consider, is the boost recovery when changing gear during a full acceleration run. What happens, is the turbo looses an amount of shaft speed during gear change as there is no exhaust gases powering the exhaust turbine when the clutch is pressed in the engine as the combustion cycles will have stopped for the period. So this means, that the air lost to the atmosphere, wont be entering the turbo, helping to keep the turbines running at higher shaft speeds, so you may feel a short period when you begin accelerating where it feels a bit ‘numb’ before the turbo is fully spooled back up.
Recirculating Valve
A recirculating valve will redirect any excess air back into the turbo intake and back though the engine. As we are recirculating the air that has been accounted for by the MAF sensor and ECU, the fuelling will be much better and it’ll also mean that the fuel trims are a lot better.
As we discussed above in the dump valve section, the boost recovery when changing gear will also be a lot better, this is due to the air helping to maintain higher turbine shaft speeds when changing gear.
It is still entirely possible to get a ‘Whooshh’ sound whilst still using a recirculating valve, it just means that you may need to run a open cone air filter so the noise of the air re-entering the turbo can be heard, rather than being muffled by OEM paper filters inside a standard air box.
Recommendations
Here at 18T 20v Tuning, we will always recommend a recirculating valve, purely because it keeps your fuel trims in check and it’ll give you better performance overall.
So now I can hear you asking, which brand is best for a aftermarket valve.
There are a wide range of aftermarket valves out there that all do the same thing, some valves, such as the Forge Split-r will even claim to do both functions of recirculate the air and vent it off to give the ‘Whooshh’ sound. However, we have found that these Split-r valves don’t really work that well for performance.
One of the two recommended valves we suggest are the Forge 008p and, believe it or not, a OEM factory replacement; commonly referred to as the 710N, due to the part number: 06A 145 710 N. The OEM replacement can be had for around £30.
A key point that lets most aftermarket valves down, is the lack of seal on the piston inside the valve. If the piston to body is allowed to leak, then there is a scope for boost pressure to be lost through the dump valve, which means that the turbo will have to work harder to meet the required boost demands set by the ECU. A great ability of the Forge 008P valve, is that it can be serviced with a fresh rubber o-ring seal on the piston should the old one deteriorate over time.
Conclusion
Hopefully, we have outlined the key points regarding the difference between both types of valves that are on the market and the importance of running a recirculating valve due to the ECU maintaining good fuel trims and the boost recovery when tipping-in on the throttle when beginning to accelerate or changing gear.
Any good tuner who is set to remapping your car will typically not remap it with a atmospheric valve on due to the reasons we have outlined above.
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