Pressure is good for character

by Simon Hargreaves

Just when they develop flat torque curves, we want peaky ones back again

From the launch of Yamaha's new Super Ténéré: "The exhaust pipe connection between the two headers is discontinued to give more character to the engine. The link pipe gives a flat feeling to the curve, so removing it makes the engine a bit more peaky."

This is all about exhaust gas. When a four-stroke is tuned for power it means more revs, and so the cylinders have thousandths of second to fill with fresh mixture, compress it, burn it and pump it out again. To get enough mixture in and out, inlet valves open early and exhaust valves stay open for longer – so both are open at the same time. This is valve overlap and, generally, the more power and higher revs you want, the more of it you need.

With both valves open there's no physical impediment to exhaust gas getting pushed back into the inlet ports and obstructing fresh charge coming in. To help stop this, engine designers use exhaust pipe shape, diameter and length to control gas behaviour.

Imagine exhaust gas as a flow of water, released in big, high-pressure pulses rather than a continuous flow. It speeds up in a narrow pipe, slows when it's wide, becomes turbulent at corners etc. Furthermore, opposite low-pressure waves are created every time the high-pressure wave hits a pipe bend, a baffle, or changes momentum rapidly. By 'tuning' exhaust dimensions, an area of low pressure can be timed to arrive back at the exhaust ports at exactly the right moment to help 'draw' out spent mixture. This is scavenging.

Short of building exhaust pipes with variable lengths like trombones, exhaust dimensions are fixed and optimised for peak power at high revs. But because of the nature of the waves bouncing back and forth inside the pipe, an exhaust will also work well at other engine speeds, and correspondingly less well in between.

This is partly responsible for the lumpy shape of torque curves, and undesirable in a race engine where a predictable delivery is crucial. And so engineers spent a long time trying to smooth it out. Before sophisticated engine management allowed total control over ignition and fuelling, this was done in a number of ways, including tuned exhaust lengths, tapering headers, even servo-controlled exhaust valves.

Another method was to use short balance pipes linking the separate headers. Ducati, among others, used balance pipes in the 1970s. Apart from effectively giving each cylinder two silencers and therefore being quieter (anyone hearing a pair of Contis at full bore may disagree), the link pipe 'balanced' pressure so a negative wave would arrive at one exhaust port to assist in scavenging, in opposition to the other cylinder that was firing, smoothing out the torque curve. Later, the same thinking gave rise to the small crosspipes found between headers on many modern multi-cylinder bikes.

Including, for example, the 2010 Super Ténéré. However, this year, Yamaha clearly think they overdid it and want to inject a bit more feel to the XT1200's power delivery. So they've removed the link pipe and made the torque curve lumpier. That's progress.