Relief Valve Boost Controllers Explained


Cautionary note

Increasing the power and performance of any car should be considered carefully. Modifications to your car should only be carried out within and in accordance with the manufacturers safe operating tolerances. Standard suspension and braking sysptems can often become compromised if the factory engine power output is exceeded or driving characteristics place constant and heavy load on these areas. In addition modifications to your car should be notified to your insurance company otherwise you may invalidate your insurance policy.

The use of a boost control device such as this places additional stress on the engine, turbocharger and drive-train of your car. Excessive boost levels or incorrect set-up and operation of the relief valve could cause serious damage to any of these components. It is not recommended that the factory boost limit be disabled or that the turbocharger is operated outside of its safe operating range.

The author accepts no responsibility whatsoever for any direct or indirect or consequential loss, injury or damage to any persons, equipment or property arising from the use or fitment of this relief valve boost controller.

Be careful and please read ALL of these instructions.

What is a relief valve ?

A relief valve is a small device which essentially acts as a mechanical air pressure switch. A small ball bearing within the relief valve is held, under pressure, onto a seat by a spring. Air pressure acting on the ball causes it to lift off its seat once a preset pressure has been reached, as determined by the spring pressure on the ball – which can be adjusted of course. Once the ball has lifted off its seat air can flow through the valve. As the air pressure acting on the ball reduces below the spring pressure, the valve closes preventing air flowing through the relief valve. In essence no air passes through the valve until the spring or ‘cracking’ pressure has been reached.

This picture shows a relief valve opened up … Relief valve

How does this work on a turbocharged car ?

If the relief valve is placed in the turbo to waste-gate actuator pipe it acts as a boost controller. The relief valve can be adjusted to give the desired boost pressure by simply adjusting the spring pressure. This is done by lengthening (lower the boost) or shortening (increasing the boost) the relief valve. One of the excellent characteristics of a relief valve is the way the boost is controlled. Due to the way the relief valve ‘cracks’ open once the spring pressure has been reached – no air is acting on the waste-gate actuator until this time. This means that the boost is much more aggressive and rises much more quickly than a turbo car without a relief valve. This is because normally a waste-gate actuator actually starts to open the waste-gate at very low boost levels, gradually opening wider as boost rises until it is fully open – at which point maximum boost has been reached. This has the effect of smoothing the boost all the way until maximum boost is reached. Since a relief valve prevents air getting to the wast-gate until the preset boost level is reached maximum boost is reached more quickly making for a more exciting launch.

Is this better than a bleed valve ?

You bet !

A bleed valve – as the name suggests, bleeds air from the waste-gate actuator fooling it into thinking there is lower boost than there actually is. This doesn’t change the way the waste-gate actuator works so the waste-gate still begins to open early meaning that although the boost level is higher, as defined by the amount bleed dialled into the bleed valve, spool time is not as quick as a relief valve.

Are there any exceptions to this ?

Well yes.

I have found that if the relief valve is set too low the effect can be to reduce the aggressiveness of the boost. This can make the relief valve feel like a bleed valve in that the differences between the two are less detectable.

NOTE: Bear in mind that many cars are actually fitted with a bleed valve type of  boost controller that is activated by the ECU via a solenoid valve. These forms of boost control device may need to be de-activated to gain the full benefit of a relief valve. It is worth noting that factory fitted boost control devices are normally fitted to allow the ECU to control boost by reducing it under adverse conditions i.e. low/high engine temperature etc, rather than for the purpose of increasing performance.

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