How to alter boost pressure

One of the first mods people tend to do is to up the boost on the turbo. What does this mean? Well it means altering the control mechanism somehow to alter the air pressure produced by the turbo to make it higher.

There are various methods of altering the boost pressure, and I’ll describe two of them here:
Altering the spring tension on the actuator
Using a boost controller to alter the air pressure that the actuator “sees”

The first is the most common method as it is free, requires no extra parts and works well for most people.

The second is a bit more complicated (not very though), but requires a boost control valve of some sort and some extra pipework to be installed.


How to adjust the actuator

First things first, what is the actuator. Well it is a device that alters the length of a rod attached to it depending on the boost pressure it “sees”. This is done by a spring forcing the rod in one direction, and air pressure on a diaphragm forcing the rod in the other direction. If you alter the spring pressure or preload then you alter the pressure at which the rod moves. The rod is connected to a valve that diverts exhaust gases around the turbo (the wastegate), rather than through it, i.e. it bleeds exhaust gases away from the turbo in order to slow it down and hence control the air pressure that the turbo is generating in the compressor half of it. It’s a very simple control method that works very well in general.

Ok so now you see how it works, how do you adjust it. Well if we shorten the rod that is attached to the wastegate we increase the spring pressure on the actuator that is trying to keep the wastegate closed. This means that more air pressure is required to open the wastegate and so the boost level produced by the turbo is also increased.

So in summary, to increase the boost pressure we shorten the actuator arm length.

Fortunatly doing this is not too hard as the actuator arm is threaded and the connection to the wastegate is also threaded meaning that if you turn one in relation to the other you alter the length. I guess most people reading this will have an L series although this applies to many many turbos out there. Some of the L series models have a knurled adjuster which means that the boost can be adjusted without removing the actuator arm from the wastegate. Others are more tricky as you have to remove the arm from the actuator, turn the end and then replace it back onto the arm. Both types have a 10mm locknut to prevent rotation that must be loosened before any adjustment can be made. Although removing the actuator arm from the wastegate sounds easy, and indeed is easy when the turbo is cold, (as it is only retained by a small circlip), when it is hot after a test drive adjustment becomes a lot trickier! And to get the correct boost level often requires several test drives adjusting the boost level in small steps, you can see the potential for burnt fingers.

To make it easier you need not replace the circlip on the wastegate arm until you have got the boost level correct, as the arm rarely falls off in use even over extended periods (my old maestro never had a circlip fitted!).
I’d also recommend stout gloves (or lots of patience for the engine to go cold before adjusting it again).
If you struggle to stretch the actuator arm back over the wastegate you can attach a hose from the wastegate to a foot/hand pump and use this to create air pressure to move the rod to make it slip over the wastegate arm more simply.


How to fit a boost control valve

There are a few variations of boost control valve, but all work in the same way. They reduce the amount of boost pressure the actuator "sees" and so effectively tricks it into running a higher boost level than it would normally.

The schematics below should hopefully give you an idea how to fit them to your car. Always refer to the instructions that come with the valve as different designs exist but this will give you a basic run down of how they are usually laid out.

Bleed valve

These valves bleed off a fraction of the air that is passing to the actuator and so reduce the air pressure that the actuator experiences. They can be tricky to set up as they tend to be rather sensitive, and are generally frowned upon in the petrol tuning arena, however for the diesel tuning arena they work just fine once they are set up properly.




Mechanical Ball-on-Spring type

These are generally more popular as they are easier to set up and tend to reduce wastegate creep (the wastegate starting to open slightly before the set boost pressure is reached).






These are the most sophisticated types often with various special features such as reduced boost levels in the lower gears to reduce wheelspin and consequently the most expensive and most difficult to set up by far, and really are for extreme modifiers only at the moment.


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