Boost Control on the Toyota Celica GT4


This page contains details specific to the Toyota Celica GT4 however much of it may be relevant to other turbo-charged cars so it may be worth a read.


What about the VSV on the GT4 ?

On the Celica GT4 the factory fitted boost controller is known as the Turbo VSV and allows the ECU to limit boost in 1st and 2nd gears and also in unfavourable conditions i.e. low/high engine temperature etc. Boost control is achieved by the use of a low pressure wate-gate actuator which opens the waste-gate at approximately 5psi. An electronic solenoid, under the control of the ECU, is energised when the ECU decides to provide full boost. When the solenoid is energised it opens a bleed port through which approximately 4 psi of boost is bled from the waste-gate actuator causing the turbo boost to increase by an additional 4 psi. This means that the turbo is generating 9 psi of boost but the waste-gate actuator is only seeing 5 psi, the amount it needs to open the waste-gate fully, the remaining 4 psi is being bled away.

NOTE: For the relief valve to provide maximum effect it is HIGHLY recommended that the VSV be de-activated. This will provide the advantages of full boost in all gears and when YOU want it not when the ECU decides to let you have it.

How can I disconnect my VSV ?

This can be done in a number of ways. The following is a list of suggestions on what to do NOT instructions on how to do it ! (Check out my article at

Disconnect the VSV connector the VSV and its connector are under the inlet manifold
Block off the VSV outlet pipe as it rises from the VSV behind the inlet manifold and attaches back to the induction system this is my preferred method as it can easily be reversed.
Block off the VSV inlet pipe after the waste-gate actuator outlet pipe this can be done while fitting the relief valve but is extremely fiddly and difficult to reverse.

What about a FCD (fuel cut defensor)

A fuel cut defensor will only be necessary if you wish to exceed the factory boost limits programmed into the ECU. The following table provides the approximate factory limits in the ECU. The factory boost limit normally provides a realistic limit for the standard engine, turbo and head gasket. The use of a fuel cut defendor may push a standard engine beyond its limits.

ST165 12/14 psi
ST185 12/14 psi
ST185/RC & CS 17 psi
ST205 17psi

NOTE: Whilst the use of a fuel cut defendor is not recommended for initial tuning it is a very effective modification. Check out my fuel cut defendor pages for more information.

How do I know if I have hit the factory bost limit ?

Quite simply you WILL know !

The engine will cut out when the boost limit is reached. The harshness of this action is dependent on how hard the car is being driven at the time. If you are driving hard on full boost ALL power will be lost causing severe deceleration and possible jolting of the engine. The ECU will allow the car to be driven off boost but as soon as boost is registered acceleration will be prevented.

Also bear in mind that boost will tend to be higher for a given adjustment if the ambient temperature is low outside so you may experience fuel cut in cold  weather if you set the relief valve in hot weather.

NOTE: During the adjustment procedure fuel cut is to be expected and to experience is a valuable lesson. Fuel cut during overtaking can be an unnerving experience and it is important to set the relief valve so as not to trigger fuel cut at maximum power and engine load.

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