Some people ask how it is possible that aftermarket exhaust systems can increase 6 to 10 percent more horsepower. The answer is inside the stock exhaust systems of most motorcycle manufacturers. Inside your standard exhaust is a device called a catalytic converter. Although you may have heard of one of these, and your probably have a basic understanding what the catalytic converter does, but do you know that the catalytic converter also limited your motorcycles power?
A catalytic converter is a lump of honeycomb metal with its surfaces coated in platinum and other materials. As hot exhaust gases pass through it, this coating creates a reaction that gets rid of nasty emissions. It's great for the environment, crap for motorcycle power.
Having a catalytic converter in you exhaust pipe is like trying to pour water down a blocked sink pipe. To run efficiently, an engine needs to get rid of about 900 cubic liters of exhaust gases a minute, but the catalytic converter creates a blockage and basically chokes the engine, sapping power. But that's not the end of it.
Having fought its way past the catalytic converter, the exhaust gases now hit the silencer, which impedes their progress even further in an attempt to deaden sound. All in all, just escaping the engine is quite an assault course for the gas. With most aftermarket can or complete exhaust system, it's a straight sprint.
By junking the catalytic converter and most of the silencing, the exhaust gases are given a virtually straight run out of the exhaust pipe, allowing the engine to run with better efficiency and giving you more power.
Such is the effect that by just swapping over the standard exhaust pipe for a sporty aftermarket exhaust system, you actually cause the motorcycle to run lean, which can potentially damage the engine. So for the best results, you should also look at the engine fueling.