Motorcycle Exhaust Glossary
Bikers, like many enthusiasts, have a tendency to slip into jargon when talking bikes. So you know your header from your outlet we thought we'd compile a list of frequently used terms when it comes to motorcycle exhausts and their meanings. If we've missed any, which we undoubtedly have, do let us know and we'll add them to the list.
Aluminium – Another very lightweight metal, used extensively in aeroplane design. It shares many properties with steel though it is significantly lighter, though typically not as strong.
Anodised – a process used to protect metals from abrasion and corrosion. In motorcycle exhaust manufacture many titanium and aluminium exhausts are anodised in colours to match the paintwork on the bike.
Baffle – baffles act a bit like a silencer on the end of a gun. They are inserted into the end can of an exhaust and reduce the amount of noise emitted. Many performance exhausts now come with removable baffles, which can be removed when you want maximum performance and sound and replaced for MOTs or race scrutineers.
Bolt-on – describes the means in which the silencer attaches to the link pipe. On some bikes the silencer literally bolts on. Replacing the silencer simply entails unbolting the original and reattaching the aftermarket end can.
Carbon Fibre – is a fibre-reinforced plastic that is extremely strong and light. Typically a gloss black in colour with visible filaments of fibre within it it is oftern used to fabricate the outer sleeve of end cans. It is used by, among other manufacturers, the Carbon Can Company who’s name is synonymous with their product.
Collector – the header pipes that exit the exhaust ports on the cylinder head then merge into one or in some cases two silencers. The collector, or collector box is the point where the header pipes merge.
DB Killer- Decibel Killer, proprietory name for a removable baffle as used by many manufacturers. See also Baffle and Sound Killer.
Downpipes - Downpipes are quite simply the pipes that head down from your engine before meeting the collector box (if a twin, tripe or four cylinder engine) and then heading up through the link pipes (see below) into the silencers. As standard downpipes are made out of either mild steel or stainless steel, the former of which has a tendency to rot and discolour. Aftermarket downpipes are typically made from stainless steel or titanium.
End can - Another name for a silencer, see below.
Damping wool – see separate entry for repacking.
De-coke – exhausts, especially on two-stroke motorcycles running rich on oil, have a tendency to ‘coke-up’. This is an accumulation of carbon deposits in the pipework, collector box and end can. Decoking is a process of pouring an acidic substance into the exhaust that dissolves these deposits.
Decibels – are the units used to express the intensity of a sound and the loudness of your exhaust! Many race tracks have decibel limits and exhaust notes may not be louder than these limits if a bike is to be permitted to compete on the track.
Dynometer - A dynometer is a very sophisticated device that allows a mechanic to measure and analyze the horsepower, torque, and exhaust temperatures of a racing engine. A motorbike is placed on a rolling road in a dyno room and the engine is started and pushed through the rev range. Using a computer the mechanic can measure the power and torque the engine produces (see other entries) and also the air/fuel mixture (see rejetting entry).
Full-system – a full exhaust system including headers, collector where applicable, link pipe and silencer. Replacing the full exhaust system on a motorbike will result in the maximum power and performance gains.
Gaskets - A semi-pliable material used between the exhaust ports on the engine block and the headers to ensure a good seal. Exhaust gaskets should be replaced when the exhaust is upgraded to ensure a good fit, exhaust gases leaking at the gasket will decrease power and can damage the engine.
GP-Style – In the 06/07 MotoGP season we saw a number of teams running stubby exhaust silencers exiting just behind the riders foot. In some cases this was one of two silencers, the second exiting underseat (see separate entry). Most of the aftermarket exhaust manufacturers now offer exhausts emulating this routing. This style of pipe saves weight, boosts power and also helps keep the mass of the exhaust low down and centralised for quicker handling.
Headers - Exhaust headers are the pipes that immediately exit the cylinders of your engine. Each cylinder has its own header and they are carefully designed to ensure they are the same length so that the exhaust gases from each cylinder hit the collector (see above) at the same time, reducing back pressure and increasing exhaust efficiency. This is a useful link for calculating the ideal header length http://www.nightrider.com/biketech/calc_exhaustlength.htm if you're really interested.
Horsepower – Horsepower is a measure of the power of an engine. Brake horse power is that power measured at the rear wheel, a more accurate measure of power than when measured at the crank as most manufacturers tend to do to inflate their power figures on paper. As a general rule the more horsepower a bike has the faster it will accelerate and the greater its top speed will be. Most aftermarket exhausts will increase the horsepower your bike produces, some by as much as 15% against the standard power output of the bike.
Hydroforming - several exhaust manufacturers are now using this technique to shape their pipework. Essentially it means forcing extremely high pressure liquid through the pipe forcing it into shape without the need for cutting and welding. This means a smoother finish inside and outside the pipe, allowing free movement of the exhaust gases and an attractive finish. It also means more organic shapes, like the D-shaped headers described above are achievable.
Lazer – Common misspelling of Laser Exhausts, a famous exhaust manufacturer.
Link pipe – the pipe that connects the collector to the silencer. Many exhaust manufacturers supply custom link pipes for your bike.
Muffler - American name for the silencer, see below.
Oval – a common shape of silencer. Popular because it offers maximum capacity in the silencer chamber, but does not affect ground clearance as much as a round can when the bike is banked over.
Packing/repacking – exhaust silencers are packed with a fibrous damping wool that helps dampen the sound. Exhausts that are bolted together, rather than welded, can often be disassembled and the repacked with new wool.
Powercommander - Manufactured by Dynojet a powercommander is a small computer that allows you to remap the fuelling on fuel-injected bikes. If you visit the dynojet site you can download a map for your bike (choose your bike and the type of exhausts you have fitted) and simply upload to the Powercommander which you then plug into your bike following the instructions provided.
Race use only – the opposite of road legal, see below. An exhaust who’s sound and performance increases mean it is only suitable for us on the race track.
Rejet - Carburettors contain jets, tiny needle like tubes that squirt fuel. When fitting an aftermarket exhaust to a motorbike this can alter the characteristics of the engine meaning you need to adjust these jets to ensure the best possible performance. Engines run either ‘lean’ (too much air and not enough fuel) or ‘rich’ (too much fuel and not enough air). This can vary throughout the rev range, so a bike can be running rich low down and lean at the top of its rev range. Adjusting the needles (there are three the idle, pilot, and main jet) can work to correct poor fueling following a change of exhaust (which you will notice as flat spots in the bike’s acceleration).
Road-legal – this is a phrase used by exhaust manufacturers to describe exhausts that will pass an MOT and not attract the attentions of a police officer. They come with the appropriate road-legal stamp on the body of the exhaust to avoid any confusion. The distinction is somewhat blurred if the exhaust has a removable baffle (see above).
Silencer - the silencer is the large can at the end of a bike exhaust, hence the fact they are also referred to as end cans. Silencers help to reduce the noise emitted by an highly tuned motorcycle engine. They do this by slowing down the sound waves emitted by explosions taking place in the cylinder(s) of your bike. Silencers dampen this sound using baffle plates (thin plates within the cylinder with perforated with hundreds of tiny holes surrounded by packed muffling wool) and by the volume of the canister itself which helps to slow down the air-flow. Silencers vary in shape, size and the material they are made from. Motorcycles can have anything from one to six silencers and these are either bolted to the sides of the bike at the rear, underneath the rear seat (see Underseat below), or more recently exiting under the bike itself (see GP style below).
Slip-on – describes bikes which are not bolt-on (see separate entry). With a slip-on bike you may need to cut the link pipe to remove the old end can. In some cases you can simply loosen a steel collar. Either way, once the stock exhaust is removed the replacement ‘slips-on’ over the link pipe and a collar tightens to form a seal.
Sound killer – another name for a baffle.
Stubby – Another name for the GP style silencer (see separate entry).
Titanium – is a type of metal allow renowned for its strength and lightness, qualities that make it very popular as a material for fabricating exhaust end cans and pipework. Typically it is 30% stronger and 50% lighter than steel. It is however also more expensive.
Torque – is a measure of rotational force. For example when tightening a nut with a wrench torque would measure how difficult it is to turn the wrench when tightening the bolt. Torque is measured in pounds per foot (lb-ft or lbs) or Newton metres (Nm). Torque on a bike engine gives more low down power and driveability, hence typically increasing torque is a good thing in improving your bike’s performance, hence the claims many exhaust manufacturers make for increasing torque.
Tri-oval – an alternative to the oval shape (see above), featuring three sides all slightly rounded. First introduced and made famous by Yoshimura.
Underseat – Underseat exhausts became increasingly popular when Ducati introduced them to the mainstream on the 916 which powered Carl Fogarty to so many famous WSB victories. Quite literally the exhaust silencers exit under the rear seat. This tidys up the rear of the bike improving aesthetics. The design is better suited to V twin engines, like the Dukes, but is increasingly being used on in-line fours either as an aftermarket modification, particularly popular on Yamaha R6 and R1s for a spell. Recently many manufacturers have pulled away from underseat exhausts on their race replica model instead opting for GP-Style silencers (see entry above).
Zorst - Phonetic nickname for an exhaust.
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