This article was first published in 2004.
We first introduced you to Kevin Davis’ active exhaust butterfly valve in the
article Pure Pipe Perfection 2 - Introducing the Secret Weapon...
Nearly four years have passed since this encounter and the system has evolved
into a much more sophisticated package.
Note that the product is also now sold under the Active Exhaust Systems (AES)
banner – it’s no longer a product of Variflow Technology.
In this article we’ll look at the system’s latest specs.
The Principle of the AES
At the heart of AES kit is a variable opening butterfly valve that is fitted
as part a vehicle’s exhaust system. The butterfly opening is infinitely variable over a
90 degree range and is designed to control exhaust noise and backpressure. The
valve is typically used to reduce noise where a big-bore aftermarket exhaust is
When the butterfly valve is closed – or very near closed – it improves noise
suppression of mufflers and cat converters. The valve itself also has a
quietening effect. However, when the valve is open, it sacrifices its noise
suppression advantage to allow maximum engine breathing and power.
Put simply, the active exhaust valve provides the best of both worlds – noise
suppression and maximum exhaust flow when you want it.
Control over butterfly valve angle is critical. At any given moment, the
valve angle must provide the suitable balance of noise suppression and engine
performance. Typically, the valve is near closed at idle, cruise and part
throttle – this gives a quiet exhaust in all normal driving conditions. However,
at wide open throttle, the valve should be fully open for maximum performance.
These are ‘givens’ – but there are many grey areas in between...
In previous incarnations of this system, the butterfly angle was controlled
by an arrangement comprising various bleed and one-way valves and a large
control box (see photo). This pneumatic approach to butterfly angle was
relatively simple and effective, but – according to Kevin Davis - it was
relatively slow reacting and inaccurate.
In the years following our initial test of the active exhaust valve, Kevin
has made numerous enhancements...
Butterfly Valve Improvements
The butterfly valve and housing assembly are largely unchanged – the
butterfly is made from 316 stainless steel and the housing is cast alloy. Kevin
says this combination has proven 100 percent reliable in service. The valve is
installed to a vehicle’s exhaust system using a pair of steel flanges – these
allow easy removal and eliminate the problem of trying to weld dissimilar
The biggest change to the butterfly valve is the adoption of dual bushes. The
dual bushes let the valve rotate smoother and faster than the previous
arrangement, which employed only a single bush. Kevin says the new valve’s
bushes and shaft are made from the same sort of materials you’ll find in
external wastegates – their high temperature resistance makes them ideal.
The vacuum actuator – which pulls the valve closed for normal driving
conditions – remains the same as previously.
Note that the valve is available to suit 2 ½, 3, 3 ½ and 4 inch pipe
Control System Improvements
The most dramatic area of improvement is control of the exhaust
The latest AES system employs two parallel solenoid valves to control the
amount of vacuum applied to the butterfly actuator. Kevin says this improves the
precision of butterfly control compared to the previous arrangement.
Vacuum is made available to the solenoids via a hose from the engine’s intake
manifold. This hose is also fitted with a restrictor and one-way valve. The
one-way valve serves to store vacuum in the system (vacuum being necessary to
close the butterfly).
The AES butterfly valve can be controlled with a choice of strategies.
The most basic approach is to tie butterfly valve angle with throttle
position. At small throttle openings, the valve will be closed while at large
throttle openings it will switch to fully open. Once fully opened, the valve
will then return to its normally closed position after a user-definable period.
This period is adjustable from 6 to 16 seconds using a trim-pot on the control
Note that a dedicated micro-switch can be added to vehicles not already
equipped with a 0 – 5V TPS signal.
The next (optional) step is to incorporate an in-cabin switch that fully
opens the butterfly irrespective of other factors. This switch is teamed with an
in-cabin LED that indicates system status – the LED illuminates when the valve
is set to fully open.
If you don’t want to tie butterfly angle with throttle position, it is
possible to use engine rpm as the primary input. Note that the rpm input can be
used only where an aftermarket ECU is installed. (However, we imagine you can
add a shift-light based rpm switch to perform the same function).
For further improved results, the system’s rpm input can be employed together
with the TPS input.
Kevin tells us throttle position and rpm based butterfly control are the most
commonly used strategies. But there is another more sophisticated approach that
can be taken...
The latest AES system can be arranged in a so-called “self-regulating”
configuration. By tapping into the front section of exhaust you can feed a
backpressure input into the control unit. The control unit recognises when
backpressure reaches your predetermined value (which is trim-pot adjustable up
to 5 psi) and automatically opens the butterfly to allow maximum breathing. In
all other situations, the butterfly remains in a closed position for maximum
Here’s a real-world scenario using the self-regulating mode...
Let’s say your newly installed aftermarket exhaust causes up to 3 psi
backpressure at full power. If this is the case, Kevin suggests setting the
backpressure trigger value to around 2.5 psi. This will give a qiet exhaust note
at all times except for when you’re wringing the last bit of power out of the
engine. The downside of this approach is additional backpressure in normal
The new AES control system is contained in a relatively compact extruded
aluminium enclosure. The enclosure can be seen in this photo alongside the ABS
unit. Note that the latest production models are painted blue and are labelled
Where and How Much?
The latest AES system can be purchased through Active Exhaust Systems. The product comes backed by a 12 month warranty.
If you’ve got an exhaust that’s a bit on the loud side but you don’t want to
strangle it with extra mufflers, we highly recommend that you give it a go.
Active Exhaust Systems Australia