Motorcycle Exhaust Systems Explained

It’s So Much More Than a Pretty Pipe!

exhaust systems, tech
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In the early days of internal combustion, an engine’s exhaust system was little more than a pipe put in place to route hot, noxious fumes away from the site of ignition. It wasn’t long, however, until engineers discovered that the pipe’s length and diameter affected the engine’s power and efficiency, and from that point forward exhaust plumbing became an important factor in the overall design of the powertrain.

The exhausts on today's high-performance bikes are finely tuned and surprisingly complex assemblies, appropriately referred to as exhaust "systems." They are not designed as an afterthought and in fact represent "one of the more expensive and involved parts of the drivetrain to develop," says BMW's Head of Drivetrain Development, Uli Blüemelhuber. From the header pipes on back to the muffler opening, every component is carefully selected to fulfill a specific purpose, whether it's noise or emissions compliance or the manipulation and harnessing of pressure waves traveling at the speed of sound.

Take a look at any modern exhaust system and you’ll see header pipes that snake down under the engine and convene at a collector. This collector can vary greatly in size, with the recent trend favoring a larger box-like contraption (often containing the catalytic convertor and primary baffling for sound suppression) under the engine that permits the use of a smaller, lighter muffler on the bike’s flank. Beyond the collector there may be a servo-controlled butterfly valve, and somewhere along the line—typically just upstream from the collector but sometimes near the exhaust port—there will be one or more lambda (oxygen) sensors. Occasionally you will see crossover pipes linking adjacent headers.

exhaust systems, info
The exhaust valve in the latest Yamaha YZF-R1 not only regulates back pressure, but it actually reroutes exhaust flow based on engine speed and load. The low-speed routing shown here is longer and uses narrower tubing for better midrange performance.©Motorcyclist

There are two parts to the exhaust event: the evacuation of slow-moving exhaust gasses and the escape of pressure (sound) waves moving at up to 1,700 feet per second. The exhaust gases travel down and out the pipe, but as the pressure pulse moves through the exhaust plumbing it encounters various design features that influence its behavior in very interesting ways. A change in pipe diameter will affect its velocity, and it will also reflect part of the wave back toward the exhaust port, like ripples in a pool ricocheting off a wall. Not only that, depending on the change in the pipe’s cross section, the wave’s sense may reverse from a compressive (positive) force to an expansive (negative) force. If timed right, this negative pressure wave will arrive back at the exhaust port and help scavenge the cylinder of residual exhaust gasses as well as help draw fresh fuel charge into the cylinder, improving power and fuel efficiency. Two-stroke expansion chambers are an example of this tuning technology taken to the extreme.

As if all that weren't complicated enough, engine rpm (among numerous other factors) influences the timing and strength of these crucial pressure pulses, restricting a pipe's optimal tune to a certain engine speed. Similar to the way electronically controlled throttle plates are used to meter intake-air velocity to match engine speed, a butterfly valve in the exhaust system "reduces or increases flow to make effective use of the pressure waves," explains Yamaha's Media Relations Manager Marcus DeMichele. This dynamic manipulation of exhaust pressure helps flatten the engine's torque curve and in some cases "may also be used to alter sound at a certain rpm to meet government sound regulations," says Sales Manager Jeff Wells at SuperTrapp Industries. Think the valve is there solely to make the sound-testers happy? Think again. The technology is currently in use on Aprilia's new RS-GP MotoGP racer and has been employed by other teams as well.

Besides enhancing the performance of the engine, the exhaust system must also reduce sound to an acceptable level and scrub hazardous compounds from the exhaust gas, all while satisfying the marketing team’s aesthetic desires, the accountants’ cost demands, and meeting the packaging requirements imposed by the bike. It’s no easy task and represents a huge balancing act and myriad compromises.

And yet, “It’s very hard for an aftermarket exhaust manufacturer to produce something that is better over a broader range than many of the stock systems on today’s bikes,” Wells says. That’s not to say a well-developed aftermarket pipe won’t improve power or offer other benefits (see the “Why Buy Aftermarket?” sidebar), but it does go to show that most manufacturers do their homework and equip their machines with the best exhaust setups possible.

aftermarket exhaust, pipes
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PECULIAR PLUMBING
Tube length and diameter are critical factors in exhaust design. As a general rule, if you're tuning for low-end torque, you want longer tubes of a smaller diameter, while an exhaust tuned for peak power will have shorter, larger-diameter piping. Equal pipe length on all cylinders is almost always desired, which is why circuitous routing is often employed on the rear cylinder(s) of V-layout engines, such as Ducati's Panigale or the Honda RC213V MotoGP machine pictured here.

yoshimura r&d, exhaust
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WHY BUY AFTERMARKET?
Twenty years ago you could gain big power by dumping your bike's stock exhaust and bolting up an aftermarket system. OE pipes weren't as well developed as they are today, leaving the door open for companies like Yoshimura R&D to build really bitchin' systems.

Nowadays, as we’ve shown, things are different. “Modern exhausts systems are extremely good,” says Yoshimura R&D Technical Sales Manager Tim Welch. 
 If that’s the case, why buy an aftermarket exhaust? Power gains are still a possibility, but it’s no longer the principal attraction. Weight savings, a more appealing appearance, and better sound are the three primary reasons most consumers buy a pipe for their bike.

Since the vast majority of aftermarket pipes are for “closed-course use only” (Yoshimura’s CARB/EPA-compliant Signature Series is a notable exception), they do without a heavy and restrictive catalytic convertor, thereby saving serious weight. Fancy finishes like carbon fiber and titanium are rarely available from the factory but readily available in the aftermarket, and the deeper, louder tone emitted by a non-compliant exhaust is appealing to some.