Exhaust Systems Glossary

Back Pressure

Back pressure is the plague of an inefficient exhaust system. When spent gases cannot flow freely through the pipes because of restrictions, they cause gaseous gridlock in your engine that goes all the way up to the cylinder. Since the gases cannot escape at the proper velocity, your engine has to work harder without yielding greater power. In effect, you lose horsepower, torque, and fuel economy.

CARB

An acronym for the California Air Resource Board, which is California's regulatory agency tasked to set the state's acceptable automotive emissions levels. Most performance exhaust systems are built to meet CARB's strict requirements, and some premium manufacturers, like JBA, weld a CARB acceptance number directly onto their products to guarantee their legality.

Carbon Monoxide

A colorless, odorless, highly toxic gas that is formed by the incomplete combustion of carbon, especially fossil fuel combustion. It is a key ingredient in smog formation.

Cat Back / Catalyst Back

When applied to a performance exhaust, this term means that the system will hook up after your catalytic converter and run all the way back to your tailpipe. By leaving the catalytic converter connected, cat-back exhaust systems stay street legal.

Catalytic Converter

A catalytic converter reduces the amount of pollutants in your vehicle's exhaust by causing a chemical reaction that transforms once harmful particles into more benign elements. It looks like a muffler, and is filled with platinum and palladium-infused ceramics, the catalyst for the transformation.

Combustion

Burning of the air/fuel mixture in the combustion chamber that generates the energy necessary to run your engine. The mini-explosion in a combustion engine is triggered by a spark from the spark plugs that's added to the compressed air/fuel mixture at the top of the piston cycle. Timing and a proper air/fuel ratio are critical to the power gained during combustion.

Cylinder

Cylinders are the core of your engine, and they are the location where combustion occurs. There are a number of key components that work together inside each cylinder to create the power that propels your vehicle down the road. At the top, there are intake valves that feed fuel and oxygen into the chamber, and exhaust valves that vent waste gases from the cylinder. At the base of the cylinder is a piston that is being driven up and down inside by the crankshaft, which, in turn, is spinning power out to the transmission and then to your tires.

Diameter

A straight line segment that passes through the center of a circle or sphere and connects two opposite points. Mathematically, the diameter of a circle is expressed as 2r, with "r" representing the radius of the circle or sphere.

Engine Scavenging

The vacuum created by the flow of exhaust through the exhaust manifold. When a system is tuned properly, engine scavenging can greatly increase efficiency and horsepower because it helps pull exhaust out of the other cylinders.

Hydrocarbon

An organic compound that contains only hydrogen and carbon. In vehicle emissions, these are usually vapors created from incomplete combustion or from the vaporization of liquid gasoline. Emissions of hydrocarbons contribute to ground-level ozone.

Make-Years

There are two ways of determining the age of your vehicle: the year it is built in (build-year) and the year it is sold as (make-year). Auto manufacturers need to have plenty of vehicles in stock before they start selling them, so they begin production about a year ahead of the actual release date. In other words, most 2005 models were actually made in 2004. However, the DMV dates the age of a vehicle by the make-year, which is the year the car, truck, or SUV is sold as. This distinction is important because many states offer a smog certification waiver to automobiles that are 30 make-years old or older, not just 30 years from the build date. Moreover, when you are choosing parts, make sure you go by the make-year of your vehicle rather than the year it was built, or that new set of performance pipes may not quite fit. If you are unsure about your vehicle's year, check your registration, which should be in your glove box.

Mandrel Bending

This type of bending is the least restrictive and will give you the most performance because the pipe's diameter remains the same throughout the bent areas. To keep a pipe's size uniform, a flexible rod called a mandrel is inserted inside the pipe before it is bent. With the mandrel inside, the pipe can be bent without crushing in on itself. This is critical, as a crushed pipe will diminish airflow.

Portmanteau

A portmanteau is a large suitcase with two hinged sections that open and close like a big, leather clam. When a word is created by sandwiching two other words together (e.g. "breakfast" + "lunch" = "brunch"), linguists call them portmanteau words.

Press / Crush Bending

An old-fashioned, cost-effective and substandard method for shaping and curving pipe. The piece of pipe is positioned in front of a stationary post, which has a curved shape to match the desired angle. The ends of the pipe are pulled forward, and the pipe wraps around the shape of the stationary post, forming the angle. The problem with this type of bending is that the diameter of the pipe constricts at the point of the bend, and this constriction causes restrictions that rob you of horsepower, torque, and fuel efficiency.

Slag

During iron, copper, and steel smelting, a fluxing agent (usually limestone) is mixed in with the heated ore to help separate the pure metal from the rock it's found in. The result is that the fluxing agent fuses with the rock, and they combine to form a waste material, called slag.

Smog

Smog is a portmanteau constructed from the words "smoke" and "fog," and it is used in our times to refer to heavy air pollution. Generally, smog is created when two types of pollutants (nitric oxides from engine exhaust, and volatile organic compounds from paint, pesticides, and solvents) mix together under sunlight to form ozone. Not only does smog look like a hideous brown cloud looming over urban areas, it also causes serious respiratory distress, especially in children and the elderly. In an attempt to reduce smog, state governments have passed laws that define the acceptable amount of pollution that an automobile can produce, and smog tests are used to determine whether cars, trucks, or SUVs are in compliance.

Street Legal

When a performance automotive part claims to be street legal, it means that the part will not alter your emissions levels so much that your vehicle cannot pass a state's mandatory smog test.

Tenth Amendment

The final amendment in the original Bill of Rights, the Tenth Amendment declares, "The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people." In other words, individual states have the power and discretion to make and enforce their own laws, so long as those laws do not conflict with other constitutional provisions.

Torque

Like horsepower, torque is a measurement of an engine's power, but it is a more specific kind of power. Namely, torque refers to the force that rotates or turns things, like the force used to tighten a wheel's lug nuts. Engines create torque to turn their crankshafts and spin tires.