Relief Valve

What's this "Relief Valve" I keep hearing about?

It's a little device that is placed between the turbo housing and wastegate actuator. This is similar to a Dawes Device.

An assembled RV. Note the threaded section which adjusts the tension applied to an internal spring Install the RV between the nipple on the turbo housing and the wastegate actuator as indicated RV installed between the turbo housing and wastegate actuator, as shown

What do I want one of those for?

It is a cheap method to increase boost pressure and therefore power output costing ~25.

Can it be used on all the GT4 variants?


How does it work?

Boost pressure is fed out from the turbo housing to the wastegate actuator. As the boost pressure rises then this forces the wastegate actuator to move. As this moves it pushes a rod and in doing so pushes open the wastegate. See my what is a wastegate article. If you put something between the turbo and the wastegate then you can alter the pressure at which the wastegate will open, this is what the RV does.

The RV is very simply in design and construction. A ball bearing is kept on its seat via a spring - this stops any air passing through the RV. As the boost pressure rises then this is sufficient to dislodge the ball from its seat and let air past and on to the wastegate, thus opening the wastegate. The pressure on the spring can be altered by screwing in or out the body of the RV, thus the pressure at which the ball bearing lifts from its seat can be altered and so the pressure at which the wastegate opens can be altered.

RV unscrewed. Note the ball bearing and spring which is used to set the desired boost pressure The ball bearing sat on it's seat. Note that the spring provides tension in order to keep it sat here until the boost pressure overcomes the spring tension
RV screwed in for high boost pressure RV unscrewed for low boost pressure

Why are you waffling on about the wastegate? What's that got to do with it?

See my wastegate article. In short at what pressure the wastegate opens will determine the boost pressure. The wastegate is a mechanical device that regulates the turbo boost pressure.

Any drawbacks?

Yes. Whilst this is a good cheap solution to increase the boost pressure I think this method is a little "rough". My reasons for this comment are:

What are the good points?

It is cheap, reliable, easily removed, no wiring involved and easily adjustable.

Where can I get one from?

Contact Martin Farrell via e-mail or visit his website

Are there any others ways to control the boost pressure?


Probably best of all is a Solenoid Boost Controller, but they are expensive ~300 depending upon what specification and maker.

Another way is to install a bleed valve, which as the name suggests, bleeds away a proportion of your boost pressure so that the wastegate never sees the true boost pressure. This unfortunately does not seem to produce an aggressive rise in boost and the pressure seems to rise at a slower rate.

Another alternative is to fit an uprated wastegate actuator. This has a couple of effects, first you can run a larger boost pressure reliably and also a stiffer spring in the wastegate will help to prevent wastegate creep - in plain speak this will prevent boost from rising as quickly as it could do. A supplier I would recommend would be Forge Motorsport. The can supply a wide range of spring rates, an adjustable actuator rod and the assembly can be overhauled at a later date.

How do I set this RV thing up?

You need a boost gauge. Don't do this mod without one! Why not have a look at my first 10 mods article.

Firstly don't go mad with screwing this thing up tight, things will go mad and you won't know what's going on! Okay, you've got the RV stuck between the turbo housing nipple and the wastegate as shown above. Make sure it's round the right way or it will not operate correctly! i.e. screwy end closest to the wastegate actuator.

Screw it in a couple of turns. Take it for a drive and watch your boost gauge. May read a little bit higher?...stop turn it up half a turn drive it, watch pressure.

Keep repeating this until you get about 10psi - a nice safe level on 165 and 185 GT4s. I don't recommend going above this unless you have done more modifications to ensure the longevity of your engine.

If you foul up and the boost goes beyond the 10psi level then you may well hit fuel cut. Fuel cut is when the boost pressure reaches such a level that Toyota deemed it unsafe for the engine. The level varies ~12-14psi depending upon the car. If you hit fuel cut the engine check light will come on and you'll have to peel your face off the windscreen as it feels quite severe. Don't worry you won't break anything, you'll get used to it! See the article on checking for error codes for more information. Fuel cut on the 205 also varies and seems to lie anywhere between 1.2 - 1.4 bar (17-20psi).

Adjust the RV in small steps.

Anything else I should know?

You may well hit fuel cut. If you want to raise your boost above the safety levels then you'll need a Fuel Cut Defencer (FCD). Have a look at my:


Why wouldn't I want to fit an FCD to a 205?

The ECU relies on the MAP sensor information to provide the correct fuelling and ignition timing for the engine. Messing about with this signal in a random manner may well prove VERY expensive i.e. new engine. Leave it alone!


Altering the boost levels and/or design of your car away from the manufacturer's design specification is not recommended and may seriously damage your engine/car. If in doubt or not qualified to do it then don't do it! This article was written as a helpful guide as to how I installed and tweaked the RV. Running too much boost WILL damage your car - I'm not kidding!

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