The Rest of the Main Terms

  • Differential: A special gearbox designed so that the torque fed into it is split and delivered to two outputs that can turn at different speeds. Differentials within axles are designed to split torque evenly; however, when used between the front and rear axles in four-wheel-drive systems (a center differential), they can be designed to apportion torque unevenly.
  • Disc Brakes: Properly called caliper disc brakes: a type of brake that consists of a disc that rotates at wheel speed, straddled by a caliper that can squeeze the surfaces of the disc near its periphery. Disc brakes provide a more linear response and operate more efficiently at high temperatures and wet conditions than drum brakes.
  • DOHC: Double Overhead Camshaft: a DOHC engine has two camshafts in each cylinder head; one camshaft operates the intake valves, the other actuates the exhaust valves.
  • Downforce: A vertical force directed downward, produced by airflow around an object: such as a car body.
  • Drivetrain: All of a car’s components that create power and transmit it to the wheels; i.e. the engine, the transmission, the differential(s), the hubs, and any interconnecting shafts.
  • Engine Control System: A computerized brain—often called the ECU, for Engine Control Unit—that regulates an engine’s operation by monitoring certain engine characteristics (rpm, coolant temperature, intake airflow, etc.) through a network of sensors and then controlling key variables (fuel metering, spark timing, EGR, etc.) according to preprogrammed schedules.
  • EPA Fuel Economy: Laboratory fuel-economy tests administered by the Environmental Protection Agency using simulated weight and drag to re-create real driving conditions. The tests were updated for the 2008 model year to better reflect current driving conditions.
  • Final-Drive Ratio: The reduction ratio, found in the gearset of a drivetrain, that is furthest removed from the engine. Typically, the differential ratio.
  • Flywheel: A heavy disc attached to an engine’s crankshaft to increase its rotary inertia, thereby smoothing its power flow.
  • Four-Wheel Steering: A steering system that actively steers the rear wheels as well as the fronts in the interest of improving handling and maneuverability.
  • Fuel Injection: Any system that meters fuel to an engine by measuring its needs and then regulating the fuel flow, by electronic or mechanical means, through a pump and injectors. Throttle-body injection locates the injector(s) centrally in the throttle-body housing, while port injection allocates at least one injector for each cylinder near its intake port.
  • Handling: A general term covering all the aspects of a car’s behavior that are related to its directional control.
  • Hemi: A term used to describe any engine that has hemispherical combustion chambers in its cylinder head. Although a four-valve design is more efficient, a hemi head provides room for a pair of large valves and offers good breathing characteristics.
  • Horsepower: The common unit of measurement of an engine’s power. One horsepower equals 550 foot-pounds per second, the power needed to lift 550 pounds one foot off the ground in one second: or one pound 550 feet up in the same time.
  • Intercooler: A heat exchanger that cools the air (or, in some installations, the intake charge) that has been heated by compression in any type of supercharger. An intercooler resembles a radiator; it houses large passages for the intake flow, and uses either outside air or water directed over it to lower the temperature of the intake flow inside.
  • Lockup: The juncture at which a tire starts to skid during braking. A tire’s maximum braking force is developed when it is on the verge of lockup, so a car’s shortest stopping distances are produced when its front and rear tires approach lockup simultaneously. This is very hard to achieve under varying conditions of load and traction, so one end typically locks up before the other. Front-wheel lockup is inherently more stable than rear-wheel lockup.
  • Mid-Engine: A chassis layout that positions the engine behind the passenger compartment but ahead of the rear axle.
  • Overdrive: Any gearset in which the output shaft turns faster than the input shaft. Overdrive gears are used in most modern transmissions because they reduce engine rpm and improve fuel economy. Occasionally, a separate gearbox with an overdrive gearset is coupled to a conventional transmission.
  • Overhead Cam: The type of valvetrain arrangement in which the engine’s camshaft(s) is in its cylinder head(s). When the camshaft(s) is placed close to the valves, the valvetrain components can be stiffer and lighter, allowing the valves to open and close more rapidly and the engine to run at higher rpm. In a single-overhead-cam (SOHC) layout, one camshaft actuates all of the valves in a cylinder head. In a double-overhead-camshaft (DOHC) layout, one camshaft actuates the intake valves, and one camshaft operates the exhaust valves.
  • Psi: Pounds per square inch, the common unit of measurement for pressure. Normal atmospheric pressure at sea level is 14.7 psi.
  • Redline: The maximum recommended revolutions per minute for an engine. In cars equipped with a tachometer—an instrument that measures engine rpm—the redline is usually indicated by, surprisingly enough, a red line. Some tachometers mark the redline with a colored sector. Others have two lines: the lower one marking the maximum allowable sustained engine rpm, the higher line indicating the absolute maximum rpm.
  • Supercharger: An air compressor used to force more air into an engine than it can inhale on its own. The term is frequently applied only to mechanically driven compressors, but it actually encompasses all varieties of compressors—including turbochargers.
  • Traction Control: An electronic control system that prevents wheelspin by detecting when a driven wheel is about to break traction, and then reducing engine power and/or applying the appropriate brakes to prevent it.
  • Turbocharger: A supercharger powered by an exhaust-driven turbine. Turbochargers always use centrifugal-flow compressors, which operate efficiently at the high rotational speeds produced by the exhaust turbine.
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Automotive Terms that begin with C

  • C-pillar
    The roof support between a car’s rearmost side window and its rear window. On a vehicle with four side pillars, the rearmost roof support may be called a D-pillar.

    Cam Profile
    The shape of each lobe on a camshaft. The profile determines the amount, or “duration,” of time the valve is open; it also largely determines the valve’s maximum opening, or “lift.”

    Camber
    The angle between the plane of a wheel’s circumference and a vertical line, measured in degrees and minutes. The tops of a car’s wheels tilt inward when the camber is negative, outward when it is positive.

    Camshaft
    A shaft fitted with several cams, whose lobes push on valve lifters to convert rotary motion into linear motion. The opening and closing of the valves in all piston engines is regulated by one or more camshafts.

    Carbon Fiber
    Threadlike strands of pure carbon that are extremely strong in tension (that is, when pulled) and are reasonably flexible. Carbon fiber can be bound in a matrix of plastic resin by heat, vacuum, or pressure to form a composite that is strong and light—and very expensive.

    Caster
    The angle between a vertical line and the car’s steering axis when viewed from the side, measured in degrees and minutes.

    Catalytic Converter
    Often simply called a “catalyst”, this is a stainless-steel canister fitted to a car’s exhaust system that contains a thin layer of catalytic material spread over a large area of inert supports. The material used is some combination of platinum, rhodium, and palladium; it induces chemical reactions that convert an engine’s exhaust emissions into less harmful products. So-called three-way catalysts are particularly efficient; their operation, however, demands very precise combustion control, which can be produced only by a feedback fuel-air-ratio control system

    Center Differential
    A differential used in four-wheel-drive systems to distribute power to the front and rear differentials

    Chassis
    A general term that refers to all of the mechanical parts of a car attached to a structural frame. In cars with unitized construction, the chassis comprises everything but the body of the car.

    Coil Spring
    A bar of resilient metal wound into a spiral that may be compressed or extended without permanent deformation. Coil springs have many automotive applications but are particularly important as suspension springs.

    Combustion Chamber
    The space within the cylinder when the piston is at the top of its travel. It is formed by the top of the piston and a cavity in the cylinder head. Since most of the air-fuel mixture’s combustion takes place in this space, its design and shape can greatly affect the power, fuel efficiency, and emissions of the engine.

    Compliance
    A slight resiliency, or “give,” designed into suspension bushings to help absorb bumps. Good compliance allows the wheels to move rearward a bit as they hit bumps but doesn’t allow them to move laterally during cornering.

    Composite
    Any material that consists of two or more components, typically one or more of high strength and one an adhesive binder. The most common composite is fiberglass, which consists of thin glass fibers bonded together in a plastic matrix. The structural properties of composites can be altered by controlling the orientation and configuration of the high-strength components.

    Compression Ratio
    The ratio between the combined volume of a cylinder and a combustion chamber when the piston is at the bottom of its stroke, and the volume when the piston is at the top of its stroke. The higher the compression ratio, the more mechanical energy an engine can squeeze from its air-fuel mixture. Higher compression ratios, however, also make detonation more likely.

    Connecting Rod
    The metal rod that connects a piston to a throw on a crankshaft

    Constant-Velocity Joint
    A particular kind of universal joint designed so that there is no cyclic fluctuation between the speeds of its input and output shafts.

    Control Arm
    A suspension element that has one joint at one end and two joints at the other end, typically the chassis side. Also known as a wishbone or an A-arm.

    Cornering Limit
    The maximum speed at which a car can negotiate a given curve.

    Coupe
    A closed car with two side doors and less than 33 cubic feet of rear interior volume, according to measurements based on SAE standard J1100. A two-door car is therefore not necessarily a coupe.

    Crankshaft
    A shaft with one or more cranks, or “throws,” that are coupled by connecting rods to the engine’s pistons. Together, the crankshaft and the con rods transform the pistons’ reciprocating motion into rotary motion.

    Cylinder
    The round, straight-sided cavity in which the pistons move up and down. Typically made of cast iron and formed as a part of the block.

    Cylinder Head
    The aluminum or iron casting that houses the combustion chambers, the intake and exhaust ports, and much or all of the valve train. The head (or heads, if an engine has more than one bank of cylinders is always directly above the cylinders.

    Cylinder Liner
    The circular housing that the piston moves in when the cylinder is not an integral part of the block. Also known as a “sleeve.”

Automotive Terms that begin with A

  • A-pillar: The roof support on either side of a car’s windshield.
  • Active Suspension: An extremely sophisticated, computer-controlled suspension system that uses powered actuators instead of conventional springs and shock absorbers. The actuators position a car’s wheels in the best possible manner to deal with road disturbances and handling loads.
  • Aerodynamic drag: Drag produced by a moving object as it displaces the air in its path. Aerodynamic drag is a force usually measured in pounds; it increases in proportion to the object’s frontal area, its drag coefficient, and the square of its speed.
  • Air Dam: A front spoiler mounted beneath the bumper and shaped to reduce the airflow under the car. Air dams can increase the airflow to radiators, reduce aerodynamic, and/or reduce lift
  • Anti-Dive: A tuned-in front suspension characteristic that converts braking-induced forces in the suspension links into a vertical force that tends to lift the body, thereby reducing dive under braking.
  • Anti-Lock Braking System: A braking system that senses when any of the wheels have locked up, or are about to and automatically reduces the braking forces to keep the wheels rolling. Commonly called ABS, such a system can control all four wheels or only two.
  • Anti-Roll Bar: A suspension element (used at the front, the rear, or both ends of a car) that reduces body roll by resisting any unequal vertical motion between the pair of wheels to which it is connected. An anti-roll bar does not affect suspension stiffness when both wheels are deflected equally in the same direction. Often incorrectly called a sway bar.
  • Anti-Squat: Similar to anti-drive, this suspension characteristic uses acceleration-induced forces in the rear suspension to reduce squat.
  • Apex: The point(s) or region on the line through a corner that touches the corner’s inner radius.
  • Aspect Ratio: Generally the ratio between two dimensions of an object. In tire terminology it applies to the unloaded sidewall height of the tire divided by its overall width. A lower aspect ratio implies a shorter, wider tire. When used to describe a wing it is the span of the airfoil (the long dimension perpendicular to the airflow) divided by its chord (the dimension parallel to the airflow).  
  • Axle Tramp: A form of wheel hop that occurs on cars with live axles, caused by the axle repeatedly rotating slightly with the wheels and then springing back.