I’ve got the power bug… what can I add?
Power Commander can alter the fuelling to help eradicate flat spots
Yoshi connecting pipe has a small silencer built in yet still makes more power than standard
■ DE-CAT PIPE Simply a pipe that is put in place of the catalytic converter on some bikes. It has a questionable impact on performance, but saves weight. Expect to pay £70-£100. ■ AIR FILTER A freer flowing air filter is an easy modification, but with modern bikes being so efficient it won’t add lots of power. They cost around £50 but can be cleaned instead of being replaced. ■ DOWNPIPES Changing the downpipes and exhaust for a full race system could net more power but will need to be set up properly – fuel-injected bikes will benefit from a Power Commander and carbureted bikes will need rejetting to suit. Budget £1000 for the system and around £500 for a Power Commander and dyno time. ■ VELOCITY STACK Changing the velocity stacks, the bell-shaped devices that supply air to the injection throttle bodies in the intake system, can lead to a high-end performance boost, but this comes at the expense of mid- and low-range power. ■ INSURANCE You will need to tell your insurer if you have modified your bike in any way. An exhaust will rarely affect the policy, but any further mods may well increase the cost.
even from a lowly 4500rpm. With the baffle in place, the Yoshimura keeps a steady 3bhp stronger than standard from 7000rpm. Remove the baffle, and power rises a hearty 8-10bhp above stock, from 8000rpm up. Air-fuel ratio is leaner. An average of 13.1 AFR is closer to the optimal 13.2 than is the standard can (12.7). The unbaffled Yoshi adds an impressive power boost at the top of the rev range. This is good news, but while we install the Remus Okami, Steve tells us there’s another side to the story: “When tuning a bike, it’s important not to focus solely on chasing more top-end power. Flat spots will hinder good control at lower revs and on the road. After all, road bikes spend the majority of their time below 7000rpm, so the low and midrange of the engine should be reliably smooth and not over-responsive.” The Remus end can is also much easier on the eye than the dirty great lump of a stock exhaust, but we agree that the black plastic end cap loses out to the Yoshimura’s stainless steel offering. Its deep bass idle is stronger than the Yoshi’s, but we think it lacks fizz up in the heady heights of five-figure revs. After a few trips to the red line and back, Steve brings up results for our second slip-on. This time the shapes of the power curves are strikingly similar to stock performance. Baffled, the Remus delivers 2-3bhp more than the standard exhaust from 6800rpm, widening out to a 5bhp difference between 10,000 and 11,000rpm. The unbaffled exhaust follows this pattern almost perfectly, maintaining a 2-3bhp lead above the baffled Remus from 6800rpm. However, with this power increase comes a lean 13.6 average air-fuel ratio in lower revs that could lead to a snatchy throttle response at normal road speeds.
WHAT POWER DIFFERENCE DO THEY MAKE?
Based on our dyno results, the unbaffled Yoshimura produces the most power, with 138.2bhp at 11,500rpm, and 71.4lb.ft torque at 7700rpm. The unbaffled Remus is not far behind, with 137.5bhp at 11,500rpm, and 70.1lb.ft torque at 7800rpm. You can expect similar results from other manufacturers’ exhausts. Both exhausts are noticeably louder without their baffles. This may or may not bother you but on the track it could lead to the bike failing the noise test and if used for commuting, or kept at home within hearing distance of neighbours, a loud exhaust could be seen as anti-social. Both baffles do a good job of cutting down the decibels without losing the characterful exhaust notes, and we’d keep them in.
It’s important not to focus too much on chasing more top-end power. Flat spots will hinder control on the road HOW MUCH DO THEY COST? A regular round or oval exhaust in stainless steel can cost as little as £100, but look at the quality before you buy. This is easy as there are several very good UK manufacturers you can visit (see below). If you start looking at titanium and carbon fibre exhausts or fancy shapes then expect to pay more. Beware of unbranded exhausts; we’ve seen one from China shoot its innards out of the back after a couple of hours’ use. The tests involved in getting an E mark are prohibitively expensive, which stops some makers getting them tested. So they may be quiet, but without the stamp they’re illegal all the same.
WHAT IS A POWER COMMANDER? Our Fazer gives textbook results. As standard it has a smooth power curve. A baffled aftermarket exhaust gives more power and an unbaffled one gives yet more, but it neither introduces any flat spots nor leans the mixture to a dangerous level. Be aware that some bikes may behave differently and adding a new exhaust could change the power curve or even introduce flat spots. If this is the case, you may need to go to a Power Commander or similar device. These cost around £350 and piggyback the ECU, adjusting the messages going from the ECU to the injection system and allowing you to download modified fuel maps for your modifications or to build your own custom map for your bike. It can iron out inconsistencies in the power curve that an aftermarket exhaust system could create. However, it’s an expensive piece of kit, and usually not needed for bikes with only a slip-on exhaust.
Useful contacts
Yoshimura www.performanceparts-ltd.com Remus www.performanceparts-ltd.com Scorpion www.scorpion-exhausts.com Beowulf exhausts www.beowulfuk.com Power Commander www.dynojet.co.uk Juicebox fuelling, TBR exhausts www.juicypipes.com Arrow exhausts, Pipercross air filters www.bandcexpress.co.uk