Sports Cycle

First introduced over eight decades ago, the megaphone exhaust revolutionised engine gas-flow...

There was a time when an exhaust’s purpose was so very simple – to keep debris from entering the exhaust port and to redirect noxious, burnt gases to the rear of the motorcycle. Not much thought was given to performance, especially at the turn of the 20th century when people knew very little about the importance of gas flow, or valve overlap, for that matter.

Most motorcycle manufacturers equipped their machines with constant circumference cylindrical steel piping, concerned most that their shapes weren’t unduly prone to compromising ground clearance or burning the legs of the rider.

Getting Rid Of Gases

The 1920s were perhaps the most innovative years of engine development, with an abundance of manufacturers and fantastic engineers constantly developing new and exciting methods to increase performance figures. Surprisingly, especially when you consider all the experimentation that was going on, very few people turned their sincere attention to tuning exhausts, quite simply because they didn’t know how to tune them or what they should be tuning them for.

Engineers began to appreciate the restrictive effects poor exhaust gas flow was having on motors, actively hindering their ability to omit spent gases and restricting the following pulse of exhaust gases from making it down the pipework. To combat this, many manufacturers tried many different things to enhance the flow, including AJS, which came upwith the 350cc Big Port, which had, as the name suggests, a big exhaust port and an accompanying larger-than-normal exhaust pipe diameter.

Although the idea was theoretically sound and the motorcycle achieved significant sporting acclaim when Howard Davies won the 1921 500cc Senior TT on his Junior TT capacity bike, the reality was that slotting a larger bore pipe on an exhaust was not the answer to the problem of gas flow. By doing this, it is likely to reduce the velocity of the gas being sent down the pipe to such an extent that the flow will essentially lose energy and stagnate, creating turbulent eddies in the pipework that, in turn, compromise the next charge of gas pulse being fired down the system.

Rudge was one of a number of other companies that also tried to improve gas flow, opting for a four-port cylinder head (two-inlet, two-exhaust), as was readily being used in the aeronautical industry. The thinking was that by having two exhausts, one from each exhaust port, twice the volume of gases would be emitted at any one time. The reality was that the split system compromised the velocity of flow down both exhaust systems, to such an extent that some people would block one of the two ports to focus the gas flow down a single exhaust.

Many other fantastical ideas were tried but, as was later to be discovered, the secret to efficient waste gas dispersionwas far more complex and necessitated the understanding of many different factors, such as turbulent and laminar flow, pressure pulses moving at the speed of sound and the effect of internal steps, ridges and bifurcations.

Shaping the Future

When exhaust gases exit a system’s pipework, they are not at a constant pressure. They contain the pulse signature of the cylinder(s) combustion gas as released by the exhaust valve(s). As these pulses escape from the pipe, the opposite sign is then ‘reflected’ back upstream to the exhaust valves; A positive pressure pulse returns as a negative pressure pulse and a negative pressure pulse returns as a positive.

Most motorcycles came equipped with relatively similar sized exhaust pipe bores well into the 1930s, retaining the same internal diameter from the inlet of the system to the outlet. Generally speaking, a smaller pipe bore will encourage a faster gas velocity, moving at higher pressure than a larger one. But only to a point – an overly large diameter exhaust pipe will introduce turbulence into the exhaust gas, robbing the whole system of the ability to utilise pressure pulses and reducing overall flow. So the challenge was to reduce the internal flow pressure of a system, while maintaining sufficient velocity to ensure the gas pulse had the energy it needed once it reached the pipe’s outlet.

By the 1930s, pioneering designs were everywhere, most of which had been influenced by aircraft development projects, an arena which enjoyed far greater, government-backed financial investment than the humble motorcycle industry. This being the case, aero and motorcycle exhaust orientations had very different objectives and consequent directions. But it’s very likely that Norton’s ‘works’ racing manager, Joe Craig, took inspiration from aircraft exhaust designs when he began experimenting with conical-shaped exhaust systems on his team’s race bikes.

In 1934, Norton competed in the Belgium TT with a bizarre-looking race pipe, shaped similarly to a megaphone. The performance results were positive, so over the following few years the British manufacturer, along with several other firms including the German brand DKW, were to experiment with megaphone designs, culminating with Norton winning the 1938 Senior TT with its megaphone-equipped 500cc Manx racer.

While the exhaust headers remained constant in circumference, the tail end of the pipework – the megaphone section – featured a 1:8 expansion ratio that designers found aided exhaust gas flow and improved peak horsepower by a claimed 7-8%. The unique flared-shape of the system encouraged the gas stream to gradually expand as it exited the exhaust, rather than ‘bursting’ from a plain hole.

The upshot being that the magnitude of the reflected pressure pulses induced by the pipe exit were reduced and spread over a broader frequency. So rather than having an exhaust pipe that worked perfectly at one single rpm, but poorly at all others, the system worked reasonably across a broader rev-range.

Megaphonitis

For all its many advantages, the megaphone design was far from perfect. As the new innovation became the norm in racing circles, with most major manufacturers adopting the flared ending to their exhaust systems, more and more people would come to complain of poor low down performance – ‘until it came on the mega’.

Dubbed ‘megaphonitis’, the problem with the broad mouthed mega was its susceptibility to creating the wrong kind of pressure waves at low revs, which would bounce back down the pipework and impede the flow of exiting gases. By persisting with the throttle held open, in most cases the gas flow would eventually improve at higher revs and the symptoms would vanish. For this reason megaphones lost favour on tight and technical circuits, where it was impossible not to depend on the lower scale of an engine’s rev-range, meaning you had to ride through an area of rough engine performance before the gas flow righted itself in the top-end.

Many different ways were considered to cure the problem, including making changes to the jetting and air slide gap, but there was no magic cure. Of course, on circuits such as the TT, where the core of the racing witnesses fantastic speeds and a much higher dependency on top-end revs, the megaphone was still favourable. But in other environments, the traditional constant radius systems proved more favourable.

The Peashooter

In a bid to improve the situation, megaphones were then produced with a ‘reversed-mega’ cap at the very end, such as the 1957 AJS 7R, which saw a 1:9 ratio of megaphone, with a reverse cone exit that was around 40% smaller than the diffuser. The change of shape altered the backpressure drastically and is claimed to have significantly helped in eradicating megaphonitis.

As a direct consequence, the peashooter design became massively popular and went on to influence many manufacturers’ exhaust designs over the next few decades. Evolving as it went, another change made to the peashooter system was to extend the length of flare quite significantly, while simultaneously reducing the exhaust’s outlet diameter.

Today's Designs

Huge advances in gas-flow diagnostics, plus the introduction of much stricter noise and emissions legislation, has seen a whole new orientation in exhaust profile and thinking.

This being the case, the ‘mega’ exhaust still holds a firm place in the racing paddock, being the chosen sting-in-the-tail design on most Moto GP factory exhaust systems. Naturally, the design of today’s megaphones is significantly different to the likes of what came about in the 1930s, but the principals remain the same. History has a funny habit of repeating itself.