If you read lots of popular magazines you could be forgiven for thinking that there's no way of increasing turbo boost unless you buy a mega-dollar electronic boost control. Or, if you use any other technique, the results will be terrible. But nothing could be further from the truth.
Let's make it really clear: if you have a turbo car where the factory ECU controls the boost level (most turbo cars of the last 15 years), you can make boost adjustable for less than AUD$50.
This boost control gives factory driveability and the boost delivery remains very progressive. (Note: 'progressive' means that boost doesn't come on hard - it builds at more or less the factory rate.)
Factory ECU Boost
Pretty well all factory boost control systems use the same approach. The wastegate (the valve that controls how much exhaust gas bypasses the turbine) is operated by a rod. The other end of the rod is connected to a diaphragm that is deflected by boost pressure acting on it. If more boost pressure gets to the diaphragm, the rod moves further and so more exhaust gas is bypassed around the turbo. This limits boost.
How far the rod moves depends on how much boost pressure is allowed to push against the diaphragm, and also how strong the internal spring pressure is that's pushing back against the other side. But really all you need to know is this: the lower the boost pressure acting on the diaphragm, the higher the turbo boost on the engine.
Turbo cars with electronic factory boost control use a 3-port solenoid valve which is pulsed by the ECU. This allows air to be bled out of the wastegate control line, decreasing the amount of boost seen by the diaphragm. If the ECU wants to increase boost, it pulses the valve so that it's open longer than it is shut, giving a great bleed. If the ECU wants to lower boost, it lets less airflow through the bleed valve.
Changing the Boost
The boost pressure to move the diaphragm of the wastegate actuator comes via a hose that connects the turbo outlet (or plenum chamber) to the factory boost control valve. If you put an adjustable control valve in this hose, you can regulate how much air gets to the factory boost control solenoid. This then changes how high the boost will rise. You can't decrease boost from the factory setting, but you can increase it. Increasing boost is as easy as reducing the flow of air that can pass along this hose.
The ECU will still pulse the valve, giving much the same shaped boost curve as standard, but boost will reach a higher level before holding at the new value.
The diagram shows the standard and new boost curves achieved with the control fitted to an R32 Skyline GTR. Engine-wise the car was standard but for a cool air duct to the airbox. The boost measurements were taken at full throttle in second gear. As you can see, the boost delivery up to about 3600 rpm is the same with or without the control fitted. But from there, boost rises smoothly to the new maximum setting, which is set to 14.5 psi (1 Bar). Note that these are actual measured values - not estimates!
The boost control system will give the same peak boost in different gears (a common criticism of pneumatic-based systems) and if the valve is mounted under the bonnet with short hose runs, good throttle control is retained.
Probably any variable orifice valve could be used, but I used a Festo Pneumatics valve which has two advantages. Firstly, it features a lock-nut that prevents accidental boost changes. Secondly, the valve allows adjustable flow in one direction but free-flow in the other direction. This characteristic can have advantages in some cars.
The actual valve used is the Festo 6509 GRA -¼-B flow control valve, which costs about AUD$33. You will also need ¼ inch barbed hose fittings and some ¼ inch ID hose, some hose clamps and cable ties. Festo valves are available from Festo and other industrial pneumatics suppliers in each state and many countries (check your telephone book).
The first step is to identify which hose is the one that supplies boost pressure to the factory boost control valve. It will come from either the turbo outlet or plenum chamber in most cars. Insert the new valve in this hose, making sure that the valve is installed with the arrow pointing in the direction of the normal flow to the factory boost control solenoid. Try to keep the hose runs short, otherwise you'll build some wastegate delay into the system. This will give quicker boosting but reduced fine throttle control. (Click on the diagram to enlarge it.)
After that, follow these steps to set the boost level:
- Set the valve to the fully open position (wound anticlockwise). Drive the car, and check the boost. It should be dead-standard or at the most up by just a very small amount.
- Undo the lock-nut and close the valve a little. Drive the car again and check the boost. It should be up a little in peak boost but the car should otherwise behave normally.
- Close the valve a little more until you get the boost level which you want. Tighten the lock-nut.
- That's it!
If you lift the boost without performing appropriate changes to the engine management and intercooler, you may cause major engine damage. The air/fuel ratio should always remain suitably rich, and detonation cannot be allowed to occur. However, in general a small boost increase (2-3 psi) will cause no problems but give a noticeable performance gain.