Tech Tips

Motorcycle Exhaust Systems 101

Written by  March 31, 2006

When you hear that loud rumble coming up behind you, it is a sound that you recognize, and you immediately know it can only come from that Harley with Screamin’ Eagle or drag pipes. But you wonder, do those pipes help your engine or just max out the decibel gauge? I have seen many different applications from H-D to Ducati, and they all do one basic thing that gives them an advantage over stock systems—they do make the engine breathe better with minor carburetor adjustments or fuel injection remapping.

Motorcycle exhaust pipes need to perform many different functions; however, I want to focus on only a few. They need to export gas away from the bike, improve the overall performance of the motorcycle, and regulate the emissions and the overall sound decibel spectrum.

One of the hidden secrets between a good exhaust and a great exhaust system is the amount of exhaust fumes or gasses and the overall heat that the system puts out. What separates the men from the boys is the amount of energy that the system refuses to take back into the system. This is where “scavenger power” comes into play.

Scavenger power is power that is released by your exhaust and creates an energy pulse which can improve performance; however, when this is out of sync, it can degrade performance of the entire rpm range. The exhaust system needs to correctly seal and match the engine for the best possible horsepower gains.

When it comes to mufflers there are two types:

Absorption type: This allows the gases to pass freely through the tube that is covered with a fiberglass packing and this is what absorbs the high frequency.

Baffled type: This type forces the spent gasses through a series of welded internal baffles which forces the low frequency sound waves to be channeled differently which lowers the decibels.

Newer type systems incorporate both types and cost a little more. Modern mufflers produce less noise and more horsepower. So consider these questions when looking at purchasing a new set of mufflers: Are you just buying for the look? If so, just about any muffler will do. Or is it horsepower you are after? If this is a true statement, then you have many options.

Years ago I got an earful from an instructor at the American Honda Training Facility in Georgia. He could not believe that we as technicians remove exhaust systems on which companies like Honda have spent millions of dollars doing research and development to make the most horsepower and torque available. This always comes back to the philosophy that the customer is always right, and we as consumers always want the latest and greatest things for the toys we have.

In the last year I have installed some of the most expensive and beautiful exhaust systems on the market today. One thing that may be overlooked is that the bike needs to be tuned up and the valves need to be adjusted, and if applicable, the carburetors must be synchronized, and the bike must be running correctly to give maximum performance.

I also want to add that slapping on a set of pipes will not be a cure-all, but it will make the bike more attractive and give it a sound that is all yours. So before your buddy talks you into spending a large chunk of change, just remember what the scoop is on the different types of mufflers and be aware the gains you actually get may be different than what you expected.

On my tough-o-meter scale this is a 1 because I am defining, not installing.

B-safe out there!

Dave Miller