Butterfly Effect

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 Butterfly

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 fitted.

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.

Latest Developments

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 metals.

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 diameters.

Control System Improvements

The most dramatic area of improvement is control of the exhaust butterfly.

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 box.

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 noise suppression.

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 driving.

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 AES.

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.

Contact: Active Exhaust Systems Australia 0448139302 kevintdavis@bigpond.com