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Reshaping exhaust design

Published: 25 March 2001

Updated: 19 November 2014

Exhaust downpipes may not be as sexy as the cans themselves, but their design is vital if you want the most power from your engine.

Until now, they’ve largely been produced in the same way – using lengths of pipe of fixed diameters, welded together in compromise.

Optimum design demands far more than that, but it’s costly, tricky to perfect and, until now, no-one has been prepared to put a system utilising all the possible tricks of the trade into mass production. Enter Micron and its revolutionary serpent-headed downpipes.

Contrary to popular belief, your bike doesn’t get quicker just because the exhaust is bigger and louder – that can actually reduce power. A properly-tuned exhaust system not only lets the engine’s gases flow quickly, it actively encourages them. This effect is known as scavenging.

Let’s isolate one pipe – whether it’s for a single, two or multi-cylinder engine doesn’t matter for the moment – and imagine the piston is being forced down after igniting a fireball of petrol mixture. As the burn loses power and the piston reaches the end of the stroke, the exhaust valve opens and the spent gases rush out at speeds of up to 200mph – this is the first stage in the process.

The second stage is the " pulse " , or pressure wave, which travels at almost twice the speed of sound. Normally the speed of sound is a mere 760mph, but being 900°C and nearly 30psi inside the pipe, this speed increases to roughly 1150mph thanks to the wonders of added pressure. Both the gas and the faster wave speed down the first section of exhaust.

As a change in pipe diameter is hit, it reflects a low-pressure wave back the way it came. This negative wave " sucks " the slower gas down the pipe and leaves the way clear for the next charge.

The next items to be encountered by the waves are the balancing pipes and the collector box. At each of these points, a scavenging negative pulse is sent careering back up to a different cylinder. By placing these changes in different positions, you can " tune " the pulses to work at different engine speeds. This is why a system that produces more peak power will almost always sacrifice bhp in different areas.

So manufacturers have to achieve the correct pressure and speed at exact distances from the valve and the end of the can. This means specific diameters of pipe at specific points are required for the perfect power at a given rpm.

It sounds ridiculous, but piping is only sold in specific sizes. So if the ideal diameter is 37mm, you have to make do with a section of 34mm welded to a section of 40mm. This is how exhausts have always been, until now. The manufacturers have never invested money in either the manufacturing technology to build their own pipework or the computer modelling required to come up with these designs. But Micron has coughed up for both.

To predict the gas and pressure waves, it uses British engineering firm Ricardo’s " Wave " simulation software. This is a seriously complicated piece of kit that allows Micron to accurately model the movement of exhaust gases from the moment they are generated to the point at which they leave the can. The software can also model the scavenging pulses so vital to high-performance exhaust systems.

Ricardo’s other clients include Formula One teams, but Micron is one of the first to use its technology for bike exhausts.

Combined with a new pipe manufacturing process called Hydratech, Micron claims to be able to produce the perfect exhaust shape and size. The welded, overlapping pieces of many different pipes are replaced with a simple single length of pipe, which is the exact diameter calculated by Ricardo to offer optimum performance. There are no welds or overlaps to disrupt the gas flow. This not only does a better job, but also cuts weight drastically. Micron claims to save up to 48 per cent over more conventional systems. That’s no mean feat.

But Micron didn’t stop there. One of the big problems with a motorcycle is the way the exhaust gases have to double-back on themselves before they can finally point in the right direction. An in-line four-cylinder exhaust only has a couple of inches to make 200mph gases change direction, and that kind of internal buffeting or turbulence is not good.

As gases move, they have to flow smoothly and snugly against the walls of the pipe. But as they hit the 90° bend found at the front of a motorcycle system they leave the sides and break the flow (see diagram far left). It’s the gases on the inside of the turn that cause the problem. Their momentum carries them clear of the sides – as they try to go straight on – and bashes them against the outside of the turn and the rest of the gases.

But using computer-modelling, Micron came up with its Serpent headers (see diagram left).

These are claimed to eliminate turbulence by allowing the gas to stay snug with the pipe walls while still turning the corner. The inside of the corner is where the turbulence occurs anyway, so that part is cut away. But the sides are stretched out to increase the actual volume of the section to the same as before. All the changes in direction have to be subtle enough for the gases to follow without breaking clear of the pipe’s walls.

It’s unclear who will follow Micron’s lead. Certainly the original manufacturers have the resources and already possess the technology to produce these systems.

And, with emissions laws set to tighten dramatically over the next few years, it’s a safe bet to assume power-liberating designs like this will begin to appear as standard.

Smaller firms will also be forced to follow suit. Otherwise, their aftermarket systems could lose power compared with the original.

Micron’s serpent headers are in production now and should go on sale later this summer for most modern sports bikes, costing from £699.

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