Exhaust Terms Explained
Couresy of TODD CANAVAN, CRYSTAL PUBLICATIONS

As part of our annual exhaust mega test issue, we decided to clear the air on some of the venting vernacular you’ll see elsewhere in this issue. We quote below from “The Big Twin High Performance Guide,” by William Penish, which we used as a reference. Some of these terms may seem overly simplistic, but we wanted to cover all bases and help the greenhorns and old salts understand each other better.

Exhaust Pipe: A piece of tubing used to route exhaust gases out of the cylinder head and away from the machine, also called a header.

Flange: A metal collar used to facilitate fastening the exhaust pipe to the cylinder head, especially by providing a surface perpendicular to the pipe suitable for fastening with bolts or studs and nuts.

Gasket: Material used to seal two or more components together, typically by compression and deformation of the gasket.

Clamp: Device used to constrict one or more parts in the interests of fastening and sealing. Exhaust clamps are usually cylindrically shaped and are a popular method to attach mufflers to exhaust pipes.

Muffler: A device intended to stifle, subvert or lessen the noise produced by a motor’s combustion. A simple muffler consists of an outer body and a perforated core wrapped with sound damping material. Exhaust gas flows through the core, lessening the severity of the noise at the exit of the muffler. Mufflers are almost always attached to the end of the exhaust pipe.

Baffle: A device intended to lessen the noise produced by a motor through physical implementation. While muffler typically use some sort of packing, baffles use mechanical obstructions to achieve the same goal.

Collector: On any exhaust system with multiple pipes from the engine and a single exit, the point where the pipes merge into one is the collector. The shape of the collector and the length at which it is placed affects the power characteristics of the motor.

Port: A passageway in a cylinder head. Most V-Twin cylinder heads have two ports - one intake port and one exhaust port. The exhaust pipe connects to the cylinder head at the exhaust port’s outlet.

Pulse: An exhaust pulse is created each time the exhaust valve is opened and closed in the cylinder head. Exhaust pipes can be tuned to favor a small rpm band or exhaust pulse widths.

Backpressure: The amount of resistance within the exhaust system that can reduce net exhaust flow. Small diameter exhaust pipes, mufflers and baffles all contribute to backpressure.

Flow: The measurable movement of exhaust gas. Stepped Pipes: Pipes that change in diameter to take advantage of the scavenging properties of pulses.

2-1, 2 into 1: An exhaust system in which the two head pipes merge at a collector and exit through a single muffler, or pipe.

2-2, 2 into 2: An exhaust system in which the two head pipes each have their own exit. Drag Pipes: A popular term for any 2-2 system without mufflers or baffles. Also called straight pipes )in that there is nothing inside the pipes).

Torque Cone: A conical device slipped inside an exhaust pipe near the flange that reduces the initial inside diameter of the pipe to create backpressure. Packing: Material, typically fiberglass, used to pack a muffler between the core and muffler body for sound damping purposes.

Reversion: The backward flow of exhaust gas through the exhaust port due to low velocity in the exhaust pipe.

Scavenging: The process in which a column of fast moving exhaust gas or supersonic energy pulses helps extract exhaust gas from the cylinder.

Source Box: Crystal Publications Dept. HRB05 P.O. Box 21869 Mesa, AZ 85277 800-945-4890