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Bumped Up Boost

Bumping up the boost curve of your late-model turbo car while maintaining the factory ECU control logic and hardware...

By Michael Knowling

Click on pics to view larger images

This article was first published in 2003.

When it comes time to squeeze more boost into a late-model turbo car, many people assume they need to get their ECU remapped or fit a stand-alone aftermarket boost control. But guess again!

The following approach - which we first introduced using a R32 GT-R Skyline way back in 1998 (see "Brilliant Boost") - retains all of the factory boost control system and maintains the original boost curve characteristics and safety strategies. The only difference is you'll enjoy an elevated boost ceiling - the factory boost curve is simply 'bumped up' to the level you want. Neat, eh?

Oh, and you might also like to know it can be done for a total cost less than AUD$50!

But before we dive head-first under the bonnet, we should cover some background...

Factory Electronic Boost Control Systems

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Most late-model production turbo cars are equipped with an ECU-controlled boost system. In these systems, a bleed solenoid is fitted to the hose that connects the turbo compressor outlet to the wastegate actuator. When electric pulses are sent from the ECU, the solenoid bleeds some air pressure from the hose, pressure that would otherwise be acting on the wastegate actuator. By reducing the amount of pressure felt by the actuator, the wastegate bypass opening is smaller, forcing more exhaust gas to drive the turbine wheel. This creates more boost.

In a standard late-model turbo car, the amount of pressure that's bled is determined by the ECU, which takes into account rpm, load and various other parameters. The more parameters that are taken into account to determine the appropriate boost pressure, the safer and more efficiently an engine will run.

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For the home tuner wanting a mild boost increase, the only way to retain the factory boost control logic is to tweak what's already there - there's no need to start from scratch and effectively throw the baby out with the bath water...

Lifting Boost

In order to maintain factory boost control logic you must leave the factory bleed solenoid connected to the ECU and the wastegate actuator. All that you need do to incease boost is to insert a variable flow control valve in the hose connecting the bleed solenoid to the compressor nozzle. (Note that, in some cars, the hose from the bleed solenoid may not necessarily be connected to the compressor outlet - it may connect to the plenum chamber, for example.)

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Inserting a variable flow control valve in this position reduces the amount of air that reaches the bleed solenoid. The solenoid is then able to bleed a greater proportion of the air pressure left in the hose downstream of the flow control valve. This reduces the pressure that acts on the wastegate actuator, enabling more exhaust gas to drive the turbine wheel and creating more boost.

The Festo 6509 GRA -¼-B flow control valve is ideal for a turbo boost application as it's rated to a maximum pressure of 10 Bar, it's simple, rugged, easily adjustable and comes with a locking nut. The central adjustment knob has 6 full turns from fully open to fully closed, giving decent tuning resolution.

Fitting the Valve

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Note that installation of the Festo valve also requires a pair of ¼-inch screw-in barbed fittings, some hose clamps and, in some cases, a length of ¼-inch ID hose. The flow control valve can be purchased for AUD$39 through Festo Pty. Ltd. (branches worldwide)and the screw-in fittings can be bought from pretty well any pneumatics supplier for a couple of dollars each.

Closed Loop or Open Loop Boost Control?

Most factory turbo cars with electronic boost control use an open-loop boost control system. In these vehicles, the ECU is relatively 'dumb' and does not compensate for manifold pressure feedback from a MAP sensor. Sure, the ECU comes pre-programmed with the appropriate bleed solenoid pulse-width data to achieve the factory rated boost pressure, but these values do not vary in accordance with, say, changes to engine breathing or turbo control system dynamics. In other words, we're free to do anything we like and the ECU will obliviously continue reading off its pre-programmed boost control maps!

However, some of the latest turbo cars incorporate a more sophisticated closed-loop boost control system. In a closed-loop system, there is a feedback loop between the boost control circuitry of the ECU and MAP sensor. In these systems the ECU is constantly varying its output to the bleed solenoid to achieve a target boost value.

Note - the boost control system described in this article will work only in an open-loop boost control arrangement.

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As you can see, the Festo valve should be installed in the hose connecting the bleed solenoid to the compressor outlet. Note that an arrow on the side of the valve body indicates the intended flow direction of the valve.

You may find that your vehicle is factory-fitted with small restrictor 'pill' or a step-down hose connector in the hose leading to the bleed solenoid. It doesn't matter if you leave either in place - all that will change is the amount of adjustment of the Festo valve that's needed to achieve your desired boost lift.

With the Festo valve fitted as described, the next step is to hit the road to set your new boost ceiling...

Adjusting the Variable Flow Control Valve

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Extreme care must be taken when setting up a boost control system - it's all too easy to blow up an engine.

When starting off, make sure that the Festo valve is fully unwound - that is, turned fully anticlockwise. This will deliver a boost curve identical to standard, except in cases where you've removed the factory restrictor pill or step-down hose. If either of these has been removed, the fully unwound flow control valve will give slightly less boost than standard.

The boost pressure graphs we've provided were obtained in second gear driving our demo car - a lightly-modified 1994 Subaru WRX. This first graph illustrates the standard boost curve versus the curve with the factory step-down connector removed and the fully unwound Festo valve fitted. These are the blue and pink plots, respectively. As you can see, removal of our WRX's factory step-down connector caused boost to drop about 5 kPA below standard all the way from around 4000 rpm to the redline. Note that the standard boost pressure taper toward the redline is still retained - the curve has been knocked down by 5 kPa.

In search of more boost we then wound the Festo valve in 1 full turn (out of a maximum of 6 turns).

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As you can see, this adjustment gave a considerable boost lift. The new boost curve (plotted in yellow) was increased about 5 kPa above factory from 4000 rpm to redline. Again, the boost taper at the top-end is retained and the rate of boost rise at the foot of the curve is pretty well factory.

The second adjustment of the Festo valve gave the boost curve we were after.

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This was achieved with the Festo valve wound in 4 full turns out of 6 (in other words, it was two-thirds closed). Peak boost in the mid-range now touched 90 kPa and again tapered back by about 5 kPa to the redline. The rate of boost increase at lower revs was also pretty well line-ball with standard.

These curves indicate that we've simply bumped the factory boost curve about 15 kPa above factory - and that's how it's supposed to be with this approach!

The Verdict

For just under $50 we've increased the factory boost curve without throwing away any of the factory boost control logic or fail-safes.

Note, though, this approach does not give the rapid rush of boost associated with control systems incorporating 'anti wastegate creep'. This means the car is comparatively slower than a car fitted with such a system, but ultimately more controllable and progressive and less brutal on the driveline. If you own a constant AWD vehicle - such as a Subaru WRX - this is a benefit worth thinking about.

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During our test period, ambient temperatures were relatively cold (about 15 degrees Celsius) and varied little across the day. But we expect that boost will settle to a slightly lower level in warmer ambient conditions. Again this is a characteristic of the factory boost control logic that's retained. While boost pressure - and performance - may drop off slightly in warm weather, the upside is a reduced chance of detonating your engine to death.

We know which we'd rather have!


As always, lifting the boost pressure of a turbo engine is an easy way to end up with a collection of melted pistons. At minimum, make sure air-fuel ratios remain at a safe (rich) level and keep an ear out for detonation. Be careful!


Festo Pty. Ltd. (South Australia)
1300 889 696

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