Know This – Motorcycle Exhaust Systems

Replacing the exhaust is the most popular motorcycle performance modification and one which is well within the scope of the home mechanic. The popularity of pipe-swapping is no surprise, because fit a new system and you can have a lighter bike with better looks and more performance. But costing anything up to a grand, a new exhaust system is a serious investment, so it pays to understand the subject so you can pick the right pipe for your bike.

Know the basics – removal and replacement

The very first thing to do when approaching exhaust removal is to get hold of some new exhaust header gaskets, so you can put the thing back together. Next priority is to make a plan: what fairings need removal for access, will you need to drain and remove the radiator? It may be possible to simply unbolt the radiator and move it to one side, but be very careful it doesn’t get damaged as you wrestle the exhaust free.

To dissemble the system, start at the back, removing everything rearward of the collector box. Get handy with some penetrating oil on joints and fasteners, as this will not only help pulling apart, but protect the system for storage. If the whole lot is held together with springs, then it’s a straightforward matter to knock up a T-handle spring-puller in a vice, using stiff wire wrapped around a piece of wood with the free end fashioned into a hook.

Whether the exhaust you’re fitting is a new road legal version, or a noisy race pipe, store the old exhaust carefully – assuming you’re not changing it because it’s knackered. Wipe the metal over with an oily rag, put all the fasteners in a plastic bag with a squirt of WD40 and then wrap the lot in bin liners. When you come to sell your bike it will shift easier with the standard pipe and you can sell the trick pipe separately.

Before refitting or replacing, lubricate all fasteners and joints with copper grease and do a practice build of the system off-bike. This grease can also be useful for holding the new header gaskets in place as you undertake the first step of the rebuild – fitting the header pipes.

It may be it’s a simpler job to get the whole exhaust on in one piece, or that you’ll need to build as you fit. Whichever, a spare pair of hands and a good dose of patience will help thing go more smoothly. While fitting the system, don’t fully tighten clamping bolts until it’s all on, so you can give it a good wiggle around.

Once done, replace the radiator and coolant and run the bike for a couple of minutes, checking for gas leaks, especially at the headers. Now retighten the header bolts (you’ll want a tool that allows tight access behind the rad’ – or may need to leave it loose), being very careful not to burn your hands. Wipe off any excess lube, stick bodywork back on and roar away for a test ride, before having a careful listen to make sure everything is gas-tight.

Become an expert – material benefits

Mild steel

Terribly prone to rusting unless chromed, but when used very thin is fairly light and suitable for prototypes and racing one-offs. Baffles tend to rot out. Steel is cheap, but best left alone when looking for an aftermarket pipe.

Stainless steel

An excellent material, that lasts years without rotting (though check the internals are also stainless). Relatively heavy when you start looking at…

Titanium

Space age kit that’s light, strong, pretty and pretty damned expensive, with full four-cylinder systems costing over a grand. Just replacing a steel end-can with titanium can save around 2kg (about £300 for a top quality item).

Carbon fibre

Is unsuitable for main pipework – as it won’t deal with the heat or vibration stresses, but is often used for end-can construction. If going carbon, go for a top-end brand (Akrapovic titanium/carbon sleeve £350) as cheapos will use heavy internal materials and poor quality carbon will deteriorate at riveting points and crack up.

The art of noise

One of the biggest decisions when buying a new exhaust system is whether to go for something road legal, or buy a bit of extra noise. While there are arguments around noise and safety, the fact is that a loud pipe can draw unwanted attention from the law (if only by telling them you’re heading their way) and annoy the neighbours. Even on track days there are likely to be noise restrictions and many riders are disappointed when they turn up and aren’t allowed to ride (and won’t get a refund).

But there is an answer: end-cans that allow you to change the noise level of your pipe in minutes, by removing and replacing the baffle. Such an item is the Remus Grand Prix Revolution which can be bought in carbon, aluminium or titanium and typically costs £250-£400, depending on model.

These cans are a great idea, but the fuelling requirements will be slightly different for the two set-ups. If you’re fitting a Dynojet Power Commander in conjunction with the pipe, then you can download two fuelling maps onto your laptop to suit the requirements of the baffled and unleashed variants.

Be a boffin

System design

The way an exhaust is designed, in terms of the way the header pipes are joined (the collector) and the number of silencers, has a big effect on the resultant power characteristics. A four-into-one, for instance, tends to improve peak power, but moves it up the rev-range at the expense of mid-range driveability.

This happens because an exhaust system isn’t simply a way of sticking waste gases into the air. It also acts as a pump, using pulses from one cylinder to help draw exhaust gas from another and to help in the process of refilling the cylinders. When these pulses are not in tune with an engine’s valve timing, then ‘back pressure’ is created, working against engine efficiency. We feel this period of poor torque production as a ‘flat-spot’ – a dip in the torque curve.

When an exhaust valve opens after combustion, a pressure wave travels down the header pipe to the more voluminous collector box. Here it expands in all directions, including back up the header. This pulse is negative pressure – suction.

If this suction wave arrives back at the cylinder during the period of ‘valve overlap’ (a short period when both intake and exhaust valves are open) then it will help empty the cylinder of residual exhaust gases, while drawing in mixture before the intake stroke has fully commenced. At this point in the rev-range the engine is at its most efficient.

But this sweet spot is short lived because the power pulses move at a constant speed, while engine speed changes. So there will only be a couple of hundred rpm where everything is sweet, dictated by the relationship between pipe-length (the ‘tuned length’) and valve timing.

Mid-range-boosting Exhaust valves

Systems in the mould of Yamaha’s Exup system use a valve driven by an electric motor to change the timing of pulses within a system, according to where the engine is in the rev-range. This means the engine works efficiently over a wider range of revs than would be the case without it, so it can be tuned for more top-end, without sacrificing mid-range punch.