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Back Plane

The main circuit board of a system, containing edge connectors or sockets so other printed circuit boards can be plugged into it.

Back Pressure

See backpressure.

Back Pressure

A buildup of pressure, usually in an exhaust system, that reduces the amount of flow leaving an engine. A number of freer-flowing exhaust components have been designed to reduce back pressure. This includes larger-diameter pipes, high-flow catalytic converters and high-performance mufflers. However, a certain amount of back pressure is required in a street engine below the engine's torque peak, or you can reduce its performance at lower engine speeds.

Back Pressure EGR

Some emissions-control systems use a back-pressure sensor or diaphragm to monitor back pressure so that exhaust gas re-circulating flow can be increased when the engine is under maximum load, and producing maximum back pressure.

Back Pressure EGR Valve

A back-pressure-dependent EGR valve. See the specific application; negative-back-pressure EGR valve or positive-back-pressure EGR valve, as applicable.

Back Seat

The position of a valve stem when turned to the left (CCW) as far as possible back seating a two-seat service valve.

Back Spacing

See backspacing.

Back Staging

Placing a competition vehicle at the start of a drag race behind the usual staging position. Also referred to as shallow staging.

Backbone Frame

A chassis structure having one boxed member running down the center. It is usually divided into two parallel members at each end to support the power train and suspension system.

Backfire

1. An explosion in the exhaust system of a motor vehicle caused when an unburned air/fuel mixture is ignited, usually upon deceleration. 2. An explosion of the air/fuel mixture in the intake manifold, which is evident at the carburetor or throttle body and may be caused by improper ignition timing, crossed spark plug wires, or an intake valve that is stuck open. 3. The momentary recession of the flame into the welding tip, cutting tip, or flame-spraying gun, followed by immediate reappearance or complete extinction of the flame.

Backfire Suppression Valve

An anti-backfire valve used in the air-injection system of an exhaust emission control.

Backfiring

1. The pre-explosion of an air/fuel mixture so that the explosion passes back around the opened intake valve, through the intake manifold, and through the carburetor. 2. The loud explosion of over-rich exhaust gas in the exhaust manifold that exits through the muffler and tailpipe with a loud popping noise.

Background

Multitasking computers are capable of executing several tasks, or programs, at the same time. In some multitasking systems, one of the processes is called the foreground (the one on the monitor ) and the others are call processors in the background.

Backing Plate

Stamped steel plate upon which the wheel cylinder is mounted and the brake shoes are attached; a metal plate that serves as the foundation for the brake shoes and other drum brake hardware.

Backlash

The excessive clearance between the meshing teeth of two gears.

Backlight

A side window, closest to the rear of a vehicle, which typically cannot be opened.

Backlighting

A technique used to make flat-panel displays easer to read

Backlite

A heat resistant plastic insulator material commonly used in printed circuit boards and transistorized components

Backpressure

Restrictions in the exhaust system that slow the exit of exhaust gases from the combustion chamber

Backspacing

When a wheel is installed on a vehicle, the backspacing is the distance between the innermost edge of the wheel and the mounting surface where the wheel bolts to the axle hub. To measure backspacing, you need to remove the wheel and lay it face down on a flat surface. Place a straightedge across the inside edge of the wheel. Then use a ruler to measure the distance from the straightedge down to the wheel mounting surface. When you order custom wheels, you'll need to be sure they have sufficient backspacing to avoid clearance problems with suspension components and wheel wells. See also offset.