Slip-Ons

Words Dean Mellor

Fitting an aftermarket slip-on muffler is one of the first modifications many riders make to their bikes, and for good reason. Firstly, fitting a slip-on is not overly expensive, secondly it’s generally pretty easy to fit and, thirdly, there are several benefits to be had by making this relatively straightforward modification.

The standard exhaust system fitted to many new bikes is not exactly attractive, especially if its finished off with an oversized alloy silencer that pokes out the side of the bike, ruining the lines of what you might think is an otherwise exquisitely designed masterpiece. Sure, many modern bikes have much prettier OE mufflers than those seemingly bolt-on afterthoughts of years gone by, but motorcycle manufacturers are still hamstrung by various rules and regulations that require them to limit potentially harmful emissions and engine noise outputs.

There are several weighty and noise-reducing components hidden within the housing of an OE silencer, such as steel tubes to direct gas flow back and forth, louvered baffles, stainless steel wool and fibreglass packing. While these components do a great job of minimising sound output, they restrict gas flow, so your bike’s engine is not able to breathe as freely as it could, which also limits performance. And chock-full of these components, OE silencers generally weigh a hell of a lot more than aftermarket slip-on silencers.

Slip-On Advantages

Fitting an aftermarket slip-on silencer will have several benefits, including reduced overall weight, performance gains, a tailored appearance and, of course, better sound output.

Aftermarket slip-on silencers weigh less than OE units for a couple of reasons. Firstly, as noise restriction is not as high a priority with a slip-on, the need for restrictive internal baffles and other packing materials is lessened, resulting in reduced weight. Secondly, the outer housing might be significantly smaller than the OE silencer’s, and also manufactured from materials such as titanium or carbon-fibre, which are lighter than the aluminium or stainless steel used in the construction of many OE silencer housings.

The reduced amount of packing material in an aftermarket slip-on silencer means it’s also easier for the engine to expel gases, which can result in significant performance gains. One bloke who knows more than most about this is ‘Dyno’ Dave Holdforth, from Sydney Dyno, who has been dyno-tuning motorcycles for close to 20 years. “Your average slip-on silencer provides more output across the entire rev range,” says Dave. “You might get a two, three or even four horsepower improvement with a slip-on, even without any other mods. It’ll breathe a lot better and it will certainly sound a lot better.”

Which brings us to one of the main reasons riders fit slip-on silencers: sound. No one wants to ride around on a bike that emits barely more than a wheezy murmur, and a well-designed and engineered aftermarket slip-on will liberate your engine’s voice without producing excessive noise. Think deep throaty burble at idle though to a racebike-like howl at redline and subdued banging and popping off-throttle… but not so raucous that it will attract the attention of the constabulary. In fact, many slip-ons are supplied with an easy-to-fit noise restrictor for ‘legal’ street use that can be quickly removed for track use.

Finally, many riders will opt for an aftermarket slip-on simply because they don’t like the appearance of their bike’s OE silencer. Most manufacturers of slip-ons will have several options for each make and model of motorcycle, so whether you opt for chrome, stainless steel, titanium or carbon-fibre will depend on what bike you ride and the look you’re after.

Breathing better

There are performance gains to be had simply by fitting an aftermarket slip-on silencer, but to get the most out of your investment you should consider the slip-on as just one component in the tuning puzzle.

While a slip-on will help your bike’s engine to more freely expel gases, the engine will also need to be able to suck in more air on the induction side to realise the full benefit, and the best way to achieve this is to fit an aftermarket high-flow air filter.

Original equipment paper air filters perform well when they’re new, but they can be restrictive when they’re dirty.

“The bike manufactures might claim that OE paper air filters flow better than aftermarket filters, but that’s only on day one,” explains Dyno Dave. “Think about what happens when you’re riding around the city, with the soot and the smog and the oil fumes and all those particles of rubber and diesel. Two weeks later your paper filter looks like grey muck, and if you were to put that filter back on to a flow bench, you’ll soon see that the performance drop-off is dramatic. If you test the same bike with a K&N filter or a DNA filter or a BMC filter, it will still flow as it originally did.”  

The air filters Dave mentions all use a pleated cotton media soaked in oil, and the benefit of this design is that airflow is not restricted even as the filter becomes dirty.

“Twelve months down the track, an aftermarket cotton air filter, even though it’s filthy dirty, will still offer virtually the same airflow as when clean… which is generally 20-30 per cent better than the original paper filter,” says Dave.

“Whether a bike has been fitted with a slip-on or a full exhaust system, anyone who’s having a dyno tune, I always make sure that they’ve got a K&N or a DNA or a BMC air filter fitted,” says Dave. “You really need a high-flow filter like this as the logic is the engine stays in tune for longer.”

Tune it up

While fitting a slip-on can result in a performance increase on its own, in some cases, particularly with older bikes running carburettors, an engine retune will also be required to account for the fact the engine will run leaner. Before the advent of electronic fuel injection, this often meant re-jetting the carbies. These days, with modern EFI engines, you might get away without a retune.

“The closed-loop O2 operation of the EFI will make minor adjustments for a small amount of enleanment… which is achieved when fitting a slip-on,” says Dave. “All the current bikes that have got the closed-loop O2 operation, they’ll cope with this enleanment.”

If an EFI bike needs a retune after fitment of a slip-on, however, Dyno Dave recommends fitment of a tunable system such as a Dynojet Power Commander. “With a Power Commander or other ECU piggy-back type system, you can do a custom tune,” says Dave.  “No two bikes are the same, so with the generic fuel maps, they don’t always work. You’re always going to come out better with something like a Power Commander, because you custom tune the bike as it’s being measured on the dyno.”

Full systems

A slip-on silencer is obviously only one part of the exhaust system and for those who want even greater performance gains there’s the option of flicking the entire OE exhaust system – from the header pipes through to the end-cap – in favour of full aftermarket exhaust system. But there are trade-offs.

“Your average slip-on silencer will give you a little bit more performance across the entire rev range,” says Dyno Dave, “because the factory header pipes keep the torque across the range.

“A full system, however, tends to move the goalposts. You might get a 10hp gain at the top of the rev range but you’ll almost certainly lose eight or nine horsepower off the bottom, whereas a slip-on will just give you two, three or maybe four horsepower overall.”

In other words, a slip-on silencer will provide a modest performance improvement without affecting real-world rideability, whereas a full system will better suit trackday and racing applications where peak output is of more importance.

* Thanks to Dave Holdforth at Sydney Dyno: 0418 224 000; sydneydyno.com.au