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.
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.
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.
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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.
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.
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.
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?
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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