Air Intake and Exhaust Design | Hot Stuff | Drawing the Line

Putting all of the many components of a motorcycle together in the small volume that’s available, and then making it all work efficiently while looking good, is surely more art than science.

Two components in particular complicate the motorcycle design picture these days: the air intake and the exhaust. And two trends multiply the difficulty further: adding power and regulating noise and emissions.

To make more power, airboxes generally need to be larger, both in volume around the intakes and in the space taken up by devices such as ram-air ducts. More power means more intake air, which means more intake noise. But the airbox is generally surrounded by frame and bodywork components. Often the fuel tank covers the airbox and the nose of the fairing incorporates the ram-air system, so there are double walls and surfaces that can be used to deflect and absorb sound.

Increasing the volume while decreasing the noise emitted by the intake isn’t easy, but it’s not as difficult as the exhaust system. The exhaust must also gain volume and silencing efficiency if power is to be increased, but the latest emissions regulations throw a new variable into the mix. Catalysts (a.k.a. catalytic converters or “cats”) are or soon will be needed on most bikes.

Cats run much hotter than a muffler alone, and this means re-thinking a lot of motorcycle packaging. We still see underseat mufflers on a few sportbikes like the Yamaha YZF-R1 and MV Agusta F4, but those layouts will probably soon be phased out as exhaust heat becomes more of a problem.

The muffler/cat position of choice is under the engine, as far from the rider/passenger as possible. On inline four-cylinder bikes, this makes sense as the four header pipes are already in front of the engine. The pipes can combine under the engine to enter the catalyst while the exhaust is still hot, making exhaust cleaning more efficient. A cat/muffler “box” can be placed under the engine/transmission with a tailpipe exiting to one or both sides.

The cat/muffler box needs to be fairly large, though. How to find the space? The Kawasaki ZX-10R’s new rear suspension linkage with horizontal shock is in part a response to this need for space under the bike, moving the shock away from the hot stuff.

The most interesting approach to under-engine mufflers is the clean-sheet Ducati 1199 Panigale’s design. The oil sump is now only a small downward central projection that allows a muffler/cat box on each side. Each box has exhaust flow in both directions: The headers enter both mufflers at the rear and exhaust flows through the cat toward the front, where it makes a 180-degree turn and flows rearward, exiting just under the muffler entry. Pretty complex, but the bike makes big power numbers that indicate it works well.

Underside views of some of the bikes using under-engine hardware are not pretty—Motorcyclist recently commented on the Honda CB1000R and Kawasaki Z1000 in that regard—but, again, the Ducati leads the way here as the underside view is very clean. Fortunately, under-engine mufflers can be covered with fairing panels to provide additional sound control.

A muffler box—especially if it is flat-sided like some of the ones we are starting to see—will have some wall flex as pressures fluctuate and resonances vary, and that flex transmits sound. Some Honda models like the CBs and CBRs now have panels that mount directly to the muffler to control noise and hide some of the less attractive joints and welds.

The new exhaust hardware still presents some packaging and aesthetic rough edges, but we’ll see better design solutions soon. As an example, Ducati’s aftermarket design partner Termignoni already has a racing exhaust for the Panigale, and it’s a beautiful concoction of stainless steel and titanium. The new exhaust systems will still be an important part of what makes a motorcycle interesting and beautiful. MC

Arguably the most difficult component to design on any motorcycle, the exhaust system must combine form and function in a way that fits everything around it.