Over the last 15-20 years, a butterfly valve was added in the center manifold of the exhaust system. Yamaha was the first to introduce it on production bikes; this F1-derived system was called the Exhaust Ultimate Power Valve (EXUP). Nowadays, this valve is found on many motorcycles, especially sport and ultra-sport models, although standard road bikes are now following the trend.

The electronically-controlled, variable-opening valve system regulates throttle flow while reducing backpressure at low- and mid-range. Yet, it maintains all the positive effects at higher revs. Usually made of titanium or ceramic alloy, this valve is found at the end of the exhaust manifold or just ahead of the muffler.

BMW exhaust system

Things get far more complicated with the muffler, actually. It's a matter of good and bad decibels -- and so much more. At Harley-Davidson, for example, engineers are concerned about the original sound and the muffler design: they strive to make it compact, stylish and effective, true to the corresponding model and, most importantly, capable of meeting the expectations of hardcore H-D enthusiasts.

In addition, the muffler has to comply with the decibel standards of each country where the bike is sold. That being said, things are likely to change over the next few years as the new emission regulations will make catalytic converters mandatory, thus increasing the interior and exterior dimensions of the muffler.

In order to achieve a good exhaust performance while meeting the decibel limit, manufacturers don't have much choice. You can't design a muffler without planning for mechanics, acoustics or a combination of both. You must also observe the law. In Quebec, for instance, exhaust systems cannot be designed in such a way that the acoustic value can be altered to exceed the legal standard. Only fixed, permanent systems are allowed... provided that they meet every other standard. However, you can replace your factory exhaust with a new one offering the same acoustic value or lower.

Stay tuned for my next feature on the efficiency of exhaust systems as seen on production motorcycles.