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Boost Controller Roundup - Part One

The characteristics you want in a boost controller - and the systems to achieve it!

By Michael Knowling

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At a glance...

  • First of two-part series
  • A look at the characteristics of different boost controllers
  • Details of Do-It-Yourself systems
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In the seven years of AutoSpeed we’ve covered all types of boost controllers. Everything from cheapy buy-it-online controllers to electronic controllers costing AUD$750. So what’s the deal? What boost control system is best for you?

In this article we’ll take a look at the characteristics of each boost control system we’ve covered.

Boost Control Considerations

Let’s assume you’ve decided to push some more boost through your turbo car. Congratulations – you’ve settled on one of the most cost-effective ways to improve performance. But let’s take a moment to consider things in detail.

The first question is how much boost your car can safely withstand. This is a difficult question to answer but there are two main factors that limit the amount of boost you can run – the point at which the engine starts to detonate and the point at which the fuel system can no longer maintain a suitably rich mixture. Detonation is most widely be detected by ear as a ‘tink-tink’ noise while mixtures are best monitored using an air-fuel ratio meter. Do an AutoSpeed site search under “detonation” and “air fuel ratio meter” and you’ll find plenty to read.

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Within these limits, you can now think about the boost characteristics you’re after. How quickly do you want boost to rise and do you want to hold max boost to the redline? The shape of the boost curve (ie how much boost there is across the rev range) has a major effect on the on-road feel of a car – for example, a car that comes onto boost quickly can be very exciting to drive on the road (especially in the wet!). Note that the rate of boost rise is sometimes referred to as wastegate creep – by reducing wastegate creep, boost pressure will build faster.

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A characteristic of many boost controllers is boost pressure fall off at high revs. This is often the case where the turbocharger is being pushed near its output limit and, as a result, the wastegate opeing must be reduced to hold steady boost pressure. In other words, when the engine is running at high revs and full throttle, the turbo may need to be run almost flat-out to provide constant boost pressure. It's most people’s aim to maintain constant boost pressure to the redline but be aware that running the turbocharger near flat-out will likely reduce its lifespan. It’s also not necessary to maintain constant boost in engines that provide ample mid-range torque – there’s no need to spin the engine to high revs.

Also be aware that certain types of boost controllers are more susceptible to pressure variations than others. A system that is set to provide 14 psi in normal temperature conditions might allow 18 psi in unusually cold conditions – a scenario that can easy cause a lean-out and one dead engine. For maximum safety it’s advisable to go for a controller that offers good consistency.

Finally, you should consider the price and installation difficulty of each boost controller. Obviously, the cheapest controllers are the most appealing but if you’re not confident installing and tuning such a device, it might be worthwhile to spend extra on an easy-to-use controller. You might also want to invest in professional installation and tuning.

DIY Boost Controller #1

One of the most tried-and-proven systems is the in-cabin boost controller discussed at Project EXA - Part 3 - DIY Boost Control. This system is suited to all turbo cars and is easy to install and calibrate.

The system uses two brass valves – a quarter inch needle valve (which becomes the in-cabin controller) and a quarter inch ball valve (which should be mounted under the bonnet and gives coarse boost control for set-up purposes). You’ll also need a quarter inch T-piece and a couple of metres of quarter inch hose. Total cost should be less than around AUD$75. Installation and calibration is discussed in our original article (Project EXA - Part 3 - DIY Boost Control)

So what are the characteristics of this system?

Well, the rate of boost rise is very fast but it's possible to achieve an even faster rise with other systems. Boost pressure stability also varies depending on the combination of turbocharger and engine. In some instances your newly set boost value will be held all the way to the redline – however, in some cars (particularly those with a relatively small turbocharger), it will fall off at high rpm. Note that this system is also susceptible to variations in ambient temperature and across different gears.

So, in summary, this system is reliable, easy to configure and cheap – but it doesn’t necessarily give the fastest rate of boost rise (if that’s what you want), it may allow boost to fall off near redline and it can’t compensate for changes in ambient temperature and across different gears. It may seem flawed but this system can work well – especially if you have an in-cabin boost gauge to keep an eye on.

DIY Boost Controller #2

One of the most discussed boost controllers is the system outlined in The Audi's DIY Boost Control - Part 1 and The Audi's DIY Boost Control - Part 2

This is the first Do-It-Yourself boost controller we’ve seen that incorporates adjustable wastegate creep – you can have a relatively progressive rate of boost rise or you can have it shoot up as quickly as possible. And, in contrast to the previous system, boost pressure is controlled using a pressure regulator rather than a bleed.

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The Audi DIY Boost Control comprises two main components – a pressure relief valve (which allows adjustment of wastegate creep) and a pressure regulator (which allows adjustment of boost pressure). The pressure relief valve is fitted in the wastegate hose and the regulator is connected downstream (towards the wastegate actuator). The pressure relief valve can be set to open at, say 10 psi, so absolutely no pressure will reach the regulator or the wastegate actuator at boost pressures lower than this. This means boost can build as quickly as possible. Then, once the relief valve opens, the pressure regulator steps into action to control the amount of pressure applied to the wastegate actuator - and therefore control boost pressure.

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In addition to these two main components you may also need a large diameter one-way valve which serves to improve boost response after gear-changes (by ensuring there is no residual pressure trapped against the wastegate actuator). The one-way valve is plumbed so that it connects the outlet end of the pressure regulator to the inlet end of the pressure relief valve. The valve is orientated so air can flow from the wastegate line to the input end of the pressure relief valve.

The biggest advantage of this system is the adjustment it gives over the rate of boost increase – you can have it exactly how you want. In terms of boost pressure consistency, the pressure regulator ensures the wastegate opens the same amount regardless of any other conditions. This might seem ideal, but in very cold conditions (where the engine can produce more power and, therefore, more exhaust gas than usual) we’ve seen this system allow over-boosting. Again, this is a good system to use in conjunction with an permanent in-cabin boost gauge.

At around AUD$100, the Audi Boost Control has been widely put into service with success. Its adjustability is the major drawcard, but be aware that – like a bleed - the pressure regulator does not ensure consistent boost pressure. Compared to the previous controller, it’s also slightly more difficult to install and tune.

DIY Boost Controller #3

The final Do-It-Yourself system we’ve devised is exclusively for use in cars with open loop factory electronic boost control - see Brilliant Boost and Bumped Up Boost. This is probably the trickiest Do-It-Yourself approach because, unlike the previous systems, the logic of the factory boost control system is retained. You’ll enjoy a ‘bumped up’ version of the factory boost curve along with all of the manufacturer’s safety strategies.

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Extremely easy to set-up, this system involves installing a quarter inch flow control valve in the factory hose between the compressor outlet and the OE boost control solenoid. Start with the flow control valve fully open and adjust it so that the desired boost pressure is reached. It couldn’t be easier.

So what boost characteristics can you expect from this system?

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Well, you’ll be able to set your maximum boost pressure but you can’t alter the factory boost curve – if the factory system brings boost up gradually and drops off toward the redline, then that’s what you’ll get. However, as a result of inserting the flow control valve in the wastegate hose, you can expect boost to rise slightly faster than standard. In addition, you’ll have the protection of OE failsafe strategies which, for example, might involve a reduction of boost pressure when the airflow meter signal is detected faulty.

For under AUD$50, this is an easy to install, safe and well integrated approach to boost control and should appeal to anyone not wanting to ‘throw the baby out with the bath water’.

Stick around for the second and final part of the series – we look at off-the-shelf aftermarket boost controllers...

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