The Audi's DIY Boost Control - Part 1

This article was first published in 2000.

Using cheap off-the-shelf pneumatic engineering products, this new DIY boost control system allows you to set how quickly boost comes on as well as how high it goes. If you want, you can reduce wastegate creep to zero, giving extraordinarily quick boosting. Or you can adjust the system to give a more linear throttle response - the choice is yours!

This week we present the basic system, while next week we add some refinements that make it even better.

So why a new boost control?

Part way through the development of a new pre-turbo intake system for my Audi S4, I realised that the factory electronic boost control was causing boost variations that were making a greater difference to acceleration than the intake mods. For example, no matter how much I improved the intake, the power gain wasn't consistently there - the ECU simply kept dropping boost pressure further and further at high revs. While I didn't want to increase the max boost developed, what I did want was to have boost that was not varying on the whim of the ECU.

The Audi

The '93 Audi S4 uses as standard a KKK turbo and an external wastegate. Also factory is the nominal 1 Bar (14.5 psi) of boost and electronic wastegate control. While the external wastegate looks quite different to normal internal wastegates, it works in much the same way. Boost pressure pushes against a diaphragm that is backed by a spring holding the wastegate shut. As boost rises, the diaphragm moves against the spring, gradually compressing it. When this occurs, the wastegate is opened, bypassing exhaust gases around the turbo and so controlling turbo speed and thus boost pressure. An ECU-controlled solenoid valve regulates the boost pressure that is fed to the wastegate.

There are several interesting characteristics of the factory system:

• On hot days it increases boost (yes, I said 'increases'), while on cold days it reduces boost. This is presumably so that the car goes pretty well as hard on hot as cold days - its performance is very consistent, irrespective of the weather.

• Full boost (it varies from 0.8 to 1.1 Bar depending on the above) is only achieved if the accelerator is absolutely floored. When this is done an 'overboost' function kicks in which permits this high level of boost for a certain length of time and in a certain rev range only. This gives a two-step performance feel - normal boost and overboost.

• The performance changes that occurred as the intake system was being modified indicate that the boost control logic takes a signal from the mass airflow meter, dropping boost if mass airflow increases, so as to retain the same performance level. This is probably also the mechanism by which the boost varies with the temperature of the day.

• The factory electronic boost control system gives very quick boosting, with maximum full-throttle boost available from as low as 2000 rpm in some gears.

What was wanted was a new system that provided:

• Consistent boost, irrespective of the weather.

• Full boost without having to totally floor the accelerator (ie to have overboost available all of the time).

• Low rpm boosting that occurs at least as fast as the factory system.

• Under-bonnet boost adjustment.

• Low cost.

The System

The new boost control system uses two off-the-shelf pneumatic components - a pressure regulator and an in-line pressure relief valve. While this system was used on the external wastegate Audi, any internal wastegate turbo should be able to be controlled with the same approach.

Let's look at each of the two components in turn.

The pressure regulator that is used is very similar to the regulators used on compressed air systems - it reduces the inlet pressure to a constant outlet pressure. But instead of feeding (say) an air tool, the pressure is being fed to the dead-end of the wastegate diaphragm, meaning that a 'self relieving' pressure reg is required. This allows the air that would otherwise be trapped between the reg and the wastegate to escape when the boost pressure being fed to the reg drops. The pressure regulator imposes a ceiling on the pressure that the wastegate sees - if you set the reg to give a 12 psi outlet pressure, that's as high as the wastegate pressure will go, irrespective of the inlet pressure coming from the turbo.

If the pressure in the wastegate line is decreased over normal, the wastegate will stay shut to a greater degree and so boost will be higher. The pressure regulator will drop the pressure in the wastegate line if it is unscrewed (the opposite to what I first thought!), so turning the pressure regulator anticlockwise will increase boost. Sounds good; so what's the purpose of the other valve?

It's there to control creep. So what's that then? Let's look an example. Say that the wastegate is fully open when the wastegate pressure hose has 10 psi in it. That means that at half of this pressure (ie 5 psi) it will already have started to open, slowing the rate of boost increase. This nasty is called wastegate creep. However in the system so far described, the pressure regulator only starts to limit pressure when the boost level reaches the set pressure setting. Below this pressure, boost can flow through to the wastegate, allowing it to creep open. To prevent this, the second valve - an inline pressure relief valve - is used.

The pressure relief valve prevents any airflow passing through it until the input pressure reaches a certain, adjustable, level. For example, this valve can be set to 10 psi. Below this pressure the valve will be shut; above it, the valve will open and allow flow to occur through it. If this valve is placed in front of the pressure regulator, no boost will be fed to the pressure reg (and so also the wastegate) until the preset pressure is overcome. In other words, the pressure relief valve can be set so that wastegate creep is zilch.

And that's the total system - there's just the pressure relief valve and the pressure regulator.

But confused as to how it all works? Here's a simple summary. Let's say that the pressure relief valve is set to 10 psi and the pressure regulator is set to keep the wastegate hose pressure at 11 psi. You get into the car and cane it. Boost rises in first gear - the wastegate is completely shut so the turbo spins up very fast. Why is the wastegate shut? Cos the pressure relief valve hasn't opened yet and so no boost pressure can get to the wastegate. Boost rockets to 10 psi and at this pressure the pressure relief valve opens, passing boost along to the regulator and then to the wastegate. Boost keeps rising, and when it reaches 11 psi, the pressure reg starts to operate, holding it at that level. Bingo! You've spooled the turbo up as fast as possible, then boost has then been kept at a constant level by means of the pressure regulator.

You can think of the pressure relief valve as setting how fast boost comes on, and the pressure reg in setting how high the boost level goes.

The Parts

The parts were sourced from the German company IMI Norgren Pty Ltd - branches in most states and countries. The parts that were used were:

• 1 x R07-200-RNKG Inline 07 ¼ Regulator Relieving 7Bar;

• 1 x V07-200-NNKG Inline 07 ¼ Relief Valve 7 Bar;
• 2 x 2069-02 07 Spring 0.7 Bar (brown);

plus mounting brackets and four ¼-inch barbed hose fittings to suit the regulator and relief valve. The total cost was A$83 (including GST).

Note that the pressure reg and relief valve are nominally 7 Bar (ie are designed to set pressures up to ~100 psi), and so you need to buy the softer springs (rated at 0.7 Bar - 10 psi) to go inside each of them. The bodies easily unscrew and spring replacement takes only a few moments. If you are going to be running very high boost, retain the higher rated springs as well - for 1 Bar (14.5 psi) boost, the softer springs worked fine, despite their nominal 10 psi setting. In addition, you will need a metre or so of ¼ -inch fuel hose (or similar high temp hose) and 6 hose clamps. In fact I used 5/16^th ID hose - the ¼ inch fittings seemed a little large for ¼ inch hose.

Installation

As indicated earlier, the Audi runs electronic boost control. I removed the solenoid (leaving it plugged into the wring loom to fool the ECU that all was well) and plugged the now unused solenoid bleed port that is present in the pre-turbo rubber intake duct. The pressure line from the turbo compressor was then connected (via a new hose) to the pressure relief valve. You need to make sure that the pressure relief valve is orientated the right way around - the directional arrow on its body should face towards the wastegate.

The other side of the pressure relief valve was then connected to the pressure reg. The pressure reg is also directional - again the arrow should face towards the wastegate. Finally, the other side of the pressure reg was connected to the wastegate.

If you buy both the pressure relief valve and the pressure reg from Norgren, the pressure relief valve can be identified by its grey knob - otherwise the two units look externally identical. (Note also that these components will probably come with their two gauge ports unplugged - you'll get two Allen-key threaded plugs that fill these smaller diameter holes.)

Tuning

Properly setting up the system is absolutely critical to success. As a guide, it took a full hour of on-road testing to set the system up to give exactly the response I wanted.

1. Screw the pressure relief valve fully anticlockwise, so air can pass through it without restriction.
2. Screw the pressure reg fully clockwise, again so air can pass through it without restriction.
3. Road test the car - boost should be standard (or below standard in the case of cars using electronic control - they have a weaker wastegate spring.)
4. Unscrew the pressure reg step by step, testing the car each time an adjustment has been made to see what the peak boost holds at.
5. When the boost level rises to the correct value, push the knob downwards, which locks it in place.

If you cannot adjust the boost make sure that:

• You have pulled the knob upwards, which releases it.
• You are turning the knob in the right direction (anticlockwise for higher boost).
• The pressure reg and pressure relief valve are both connected around the right way.
• The hoses are properly connected.
• The pressure relief valve is fully open (knob turned anti-clockwise).

Once you have set the pressure reg to give you the right max boost level, you can set the wastegate creep control - that's the pressure relief valve.

1. Screw clockwise the pressure relief valve step by step, testing the car each time an adjustment has been made.
2. As you screw down this valve, the initial boost level will rise more and more quickly.
3. Set it so that boost rises as quickly as possible to the max level that you want (ie whatever you have set the pressure reg to give you) but does not overshoot this level.
4. When this valve is set correctly, push the knob downwards, which locks it in place.
5. You may have to make a minor adjustment to the pressure regulator once you have finished adjusting the pressure relief valve.

Note that if you wish, you can set the pressure relief valve to give an initial over-boost, or conversely you can adjust this valve so that the onset of boost is gentler. The length of hoses in the system should be kept as short as possible if the best throttle response is to be obtained.

The Results

With the new boost control system fitted, the Audi was absolutely transformed in its on-road responsiveness. The factory obviously set the car up to bring on boost in a very gentle manner - and not to lift boost level to the max until 100 per cent throttle. With the new boost control, peak boost could be achieved on half throttle, making the car feel enormously torquey. Boost actually rose substantially faster than the (in this respect, still very good) factory system. However, boost regulation wasn't quite as stable as I had hoped - it varied a little depending on gear and load. For example, it would rise to 1.1 Bar (16 psi) in sixth gear if the engine was really loaded up (eg full throttle from 75 km/h in sixth) while reaching 1 Bar (14.5) in the other five gears.

The two-stage control system could be clearly heard working - the turbo developed a characteristic 'wastegate-closed' faint scream until just before peak boost was developed, whereupon it changed in note noticeably as the pressure relief valve opened, the pressure regulator started to operate, and then the wastegate opened.

The measured acceleration before and after the fitting of the new boost control shows clearly how much harder the car came on boost. At 4000 rpm the acceleration in second gear was increased by massive 17 per cent, showing an enormous lift in mid-range torque. And remember this was with identical before/after peak boost - the boost wasn't "turned up" at all!

Contact:

IMI Norgren Pty Ltd

(branches in Melbourne, Sydney, Adelaide, Brisbane, Perth and worldwide)

Next week: Giving the system even better boost regulation and faster after-gears boost response.