Motorcycle Exhaust Terms Explained

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January 14, 2010 | By: Scott Holton

Motorcycle Exhaust TermsHere at J&P Cycles, our Sales & Service and Technical Support teams have thousands of conversations each year with riders regarding motorcycle exhaust systems. Recently, I was asked the question, “What terms do I need to know as I begin researching aftermarket exhausts for my new Harley?

Having installed, maintained, and advised customers for years on motorcycle exhaust systems, I now realize how easy it is to take my knowledge for granted. Having good general knowledge of your motorcycle’s exhaust system will help you discuss, troubleshoot, or purchase a new exhaust system for your bike. Here is a basic list of motorcycle exhaust terms I feel every bike owner should know:

  • Exhaust pipe: A piece of tubing used to route exhaust gasses away from the engine, also called a “header” or “head pipe.”
  • Muffler: A device intended to reduce the noise produced by a running engine.
  • Baffle: A device intended to reduce an engine’s operating noise. A muffler typically uses packing; a baffle uses mechanical obstructions to achieve the same result.
  • Collector: Any exhaust system in which multiple pipes from the engine go into a single exit; the place where they converge is called the collector.
  • Port: A passageway in the cylinder head. There are 2 types of ports: The Intake port allows the air/fuel mixture in; the Exhaust routes spent gasses to the exhaust pipe.
  • Pulse: Every time the exhaust valve opens and closes in the cylinder head, an exhaust pulse is created. The pulses control when and where the pipe creates its power. When tuning, for example, the pulse should arrive at a specific time. Pulse is controlled by pipe length, pipe diameter, exhaust valve timing and other tuning characteristics.
  • Flow: The measurable movement of exhaust gas in an exhaust pipe
  • Backpressure: The amount of gas flow resistance within an exhaust system.
  • Torque cone: A cone-like device added at the head of the exhaust pipe to change tuning characteristics.
  • Reversion: The backflow of exhaust gases. Reversion can be created by low exhaust velocity or pulses arriving at the combustion chamber at the wrong time. A major deterrent of drag pipes.
  • Exhaust Velocity: The speed in which air flows through the pipes.  Ideal Exhaust Velocity is 300 feet per second.
  • Scavenging: The process that uses an exhaust pulse of one pipe to help pull the exhaust pulse from the other in a “2 into 1” system.

When it comes to Harley-Davidson motorcycles, there are two main categories of exhaust systems:

  • 2 into 2: An exhaust system that has 2 entrances and 2 exits. This is the standard exhaust on all factory Harleys. Consists of: head pipes, mufflers or baffles (optional), flanges, gaskets, support brackets and mounting hardware.
  • 2 into 1: A system that has 2 head pipes that exit into a collector. A 2 into 1 system uses exhaust scavenging to assist in generating power. A 2 into 1 will result in more power due to the use of exhaust scavenging.  Consists of: head pipes, collector muffler or baffle, flanges, gaskets, support brackets and mounting hardware.

Don’t hesitate to ask a question or add your insight. Happy Riding!

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Comments (3)

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Good Information.

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