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Evaluating Dyno Charts

Cam Basics

Porting: The Black Art

Power Transfer to the Rear

Electrifying...Explosive..
Enigmatic

Exhaust Systems

Corrected Compression Ratio

Exhaust Systems

These days, bike enthusiasts have many exhaust systems to choose from. Some typical determining factors in replacing a stock exhaust system are the aesthetic looks of a set of pipes, the sound, cost considerations, performance, and "just because my brother-in-law says 'This is the best one.'"

Outside of the aesthetic value of enhancing the look of your bike, the main purpose of an exhaust system is the releasing the spent combustion gases from the motor in an orderly, controlled manner. When an exhaust system works in harmony with the rest of the motor design, the bike's performance is improved. But when the exhaust systems does not work with the existing package, your efforts to have a power band that you can enjoy is gone. For example: Installing non-restrictive exhaust pipes on your two cylinder Harley that are larger than you would find on a big block Chevy is not going to be beneficial because it causes you to give up horsepower and torque in the lower RPM range.

Your choice of cams can make picking the proper exhaust system more difficult. Very mild cams (such as those found on factory installed California emissions bikes) are not particularly picky about which after-market exhaust system you choose. If you like the way it looks, it should adequately serve the main purpose of removing exhaust gases. As you move to more aggressive street/strip type cams, extracting maximum performance from your motor limits your selections. One reason for this is that mild cams have very little, if any, overlap (the time when both the intake and exhaust valves are open to any degree during the exhaust stroke.) Overlap increases as you move toward higher performance cams. The more overlap a cam has, the greater the window of opportunity for reversion becomes. Reversion is the shock wave caused when the vacuum created by the velocity of the exhaust gases sucks those gases back toward the exhaust valve. This pulse can then contaminate the new air/fuel mixture charge filling the cylinder. You feel reversion as a lull in acceleration during a given RPM range. The bike may have good initial acceleration, and then it hesitates before "recovering" and begins to accelerate again. Reversion can also be easily detected on a dyno chart, appearing as a plateau or even a drop of power or torque on the graph.

There are many ways to deal with unwanted reversion. You can install some type of baffling system in the pipes. This can vary from specifically manufactured items such as cones or baffles, to something as simple as a bent washer and a bolt placed toward the end of the tail pipe restricting a pipe's diameter 25-30%. Shortening or lengthening your pipes is another option you might hear about, although this may only change the RPM range of the reversion pulse. Or you can replace the exhaust system with a more appropriate package to suit your requirements.

Basically, if your priorities are to have the aesthetic look of a certain system, you may not be able to achieve the performance achieved by your fellow riders using a different exhaust system. If you want the most from your motor, you'll need to choose a system that works with your package and allows you to reach the optimum performance possible.

Do you have a subject you'd like to see covered in a future column or a technical question you'd like answered? Drop John a line at jhfxr@aol.com.