Useful car terms

We have created a list of common car terms and abbreviations that not everyone knows.

They are not very technical and hopefully they will be of some use to you.
So next time the garage mechanic tries to baffle you with rubbish or you are looking at car adverts, you should have a little better understanding and a little less chance of being miss-led.

Some of the car terms and abreviations below have links to our CarBasics articles, these include step-by-step guides, photos and video clips.

ABS – Anti-lock Braking System. Uses sensors at each wheel to sense when the wheels are about to lock, and releases the brakes to prevent locking. This process occurs many times per second, and allows the driver to maintain steering control under hard braking. See article – What are anti-lock brakes.
AC – Air cooling, the cooling of the cabin area of the vehicle with a cool air stream. Not to be confused with Air Conditioning (air-con) or Climate Control.
Air-Con – Air conditioning
Air box – The plastic box in the engine bay that houses the air filter.
Air Mass Meter – Also referred to as an airflow meter. Will be located next to the airbox in the engine bay. It tells the cars computer (ECU) the amount of airflow that is being delivered to the engine so that the correct level of fuelling. See article – what is an air mass meter and see article – how to change an air mass meter.
Alternator – A device that converts rotating mechanical energy into electrical energy. As you are driving along, the alternator is turned by a belt from the engine and sends an electrical charge back into the battery. If you did not have an alternator then your battery would go flat. See article – what is an alternator.
Anti-roll bar – A metal bar mounted across the underside of car, it connects the two sides of the suspension, which counteracts the natural tendency for the car to lean when cornering.
A-pillar – The vertical roof support between the windscreen and front edge of the front side window.
Airflow meter – see air mass meter
Aquaplaning – A word to describe the cars tyres skating across the surface of water. When there is a build up of water on the road, the tyres will sit on the surface of the water and there will be no grip on the road. This is a dangerous occurrence due to loss of braking and steering and should be avoided by reducing your driving speed in bad weather.
Aspect Ratio – The ratio between the width and height of a tyre. Lower aspect ratios are usually found on sports models provide better handling and a firmer ride. e.g – written on the side of your tyres might be 185 x 65 x 15 (185 is the width, 65 is the height and 15 is the wheel diameter).
ATF – Automatic Transmission Fluid.
Automatic transmission – A gearbox that selects the correct gear automatically according to engine speed. These vehicles have no clutch pedal, only a brake and accelerator. In the UK, if you pass your test in an Automatic car, your license does not allow you to drive a manual gear driven vehicle.

Back Pressure – The pressure produced by restrictions in an exhaust system. Back pressure affects the rate at which the exhaust gases are extracted from the engine cylinders. This is an area where increases in engine performance can be achieved. Removing the back pressure from the engine by fitting a more free flowing exhaust will allow the engine to use its efforts to send power to the wheels instead of wasting it pushing exhaust gases out down a restrictive exhaust.
BHP – Brake Horse Power, see Horsepower.
B-pillar – The vertical metal roof support between front and rear side windows
Bio Diesel – It is an alternative to conventional diesel. It can be produced from vegetable oil, animal oil/fats, and waste cooking oil. These oils are converted Biodiesel using a process called transesterification. It costs less than normal diesel and does not require any modifications to your diesel engine. See our article on ‘what is bio-diesel’.
Booster Seat (booster cushion) – A child-safety seat that is designed for children too large for a baby seat, but not big enough to sit safely in the vehicle’s seats. If a booster seat is not used, then it is highly likely that the vehicles seat belts will not do their proper job and also potentially harm the child. In the UK, there are changes being implemented regarding children under a certain height. These children must be seated onto a booster cushion in order that the seat belts operate correctly and and restrain the child in the correct places as they would if worn by an adult.
Booster cables – See also, Jump Leads and Jump Start. Heavy duty electric cables fitted with clips to enable a car’s battery to be connected to another battery for emergency jump starting. See article – How to jump start your car.
Brake caliper – The part of a disc brake system, that houses the brake pads and the hydraulically operated pistons.
Brake disc – A rotating metal disc which is clamped between hydraulically operated brake pads in a disc brake system.
Brake fade – A temporary loss of braking efficiency due to overheating of the brake friction material. This is usually overcome by fitting high performance brake discs and pads. Performance pads will operate at higher temperatures, and performance discs typically have grooves and are cross-drilled to remove gases and heat from the friction surfaces.
Brake pad – Part of the brake system which consists of friction material on a metal back-plate.
Brake shoe – Part of a drum brake system (found at the rear of the vehicle), that consists of friction material and a curved metal former.
Breather – A device that allows air into a system or allows, contaminated air out e.g. oil breather, or crank case breather.
Bump stop – Hard rubber piece used in many suspension systems to prevent moving parts from contacting the body during violent suspension movements. Also, lowered vehicles may choose to have bump-stops fitted around the shaft of the shock absorbers (shocks) so that the shocks do not bottom-out or get damaged when under load.

Camber angle – The angle at which the front wheels are set from the vertical (upright), when viewed from the front. Positive camber is the amount that the wheels are tilted out at the top.
Cam follower – A piece of metal used to transfer the rotary movement of the camshaft to the up-and-down movement required for valve operation.
Camshaft – A rotating metal shaft driven from the engine crankshaft with lobes or cams used to operate the engines inlet/exhaust valves.
Carburettor (carb) – A device that mixes air and fuel in the proportions required for burning by the engine under all conditions of engine running. Only found on older vehicles. Carbs have now been replaced by fuel injection and engine management systems,
Catalytic converter – Part of the exhaust system that creates a heat-producing chemical reaction to convert potentially harmful combustion by-products into carbon dioxide and water. All modern cars are now fitted with catalytic converters.
Central Locking (CL) – On a vehicle with powered door locks, the system locks or unlocks all doors at one time e.g. put your key in to open the drivers door and all the other doors will open at the same time. Remote Central Locking is a further enhancement, meaning that you press a button on your key fob instead of having to put your key in the door.
Charge cooler – See intercooler.
Choke – Usually found on older vehicles, it is a pull type lever found around the steering column that is used to give extra fuelling on cold starting. Later vehicles moved from having a manual choke to having it controlled electronically.
Climate Control – The term for the heating, ventilation and air-conditioning system (HVAC). Most current vehicles have all three – heating, defrost, and AC.
Clutch – A friction device that is found between the gearbox and the engine. It acts as a coupling to be engaged or disengaged both parts smoothly during gear changes, without the need for either to stop moving.
Coil – A transformer used in the ignition system for increasing the voltage of the electric current conducted through the spark plugs. This high level of “electrical pressure” is what causes the current to jump the gap at the tip of each spark plug and create the actual spark that ignites the fuel inside the cylinder.
Coil spring – A spiral coil of spring steel used in many suspension systems.
Combustion chamber – Area in the cylinder head into which the fuel/air mixture is compressed by the piston and where the spark from the spark plug ignites the mixture.
Compression ratio (CR) – A term used to describe the amount by which the fuel/air mixture is compressed as a piston moves from the bottom to the top of its travel, and expressed as a number. The higher the ratio, the more compression during combustion and the more powerful the engine.
Condenser (capacitor) – Older vehicles. A device in a contact breaker point distributor, which stores electrical energy and prevents excessive sparking at the contact breaker points.
Connecting rod (‘con-rod’) – Metal rod in the engine connecting a piston to the crankshaft.
CV Joint – see constant velocity joint.
Constant velocity (CV) joint – On front-wheel drive and all-wheel drive vehicles, a coupling that allows the front axle to turn at a constant speed at various angles when the vehicle turns. The CV joint is a shaft that transmits engine power from the transmission to the wheel.
Contact breaker points – Older vehicles. A device in the distributor, which consists of two electrical points (or contacts), and a cam, which opens and closes them to operate the HT electrical circuit, which provides the spark at the spark plugs. Normally found on older vehicles.
Crankcase – The area of the cylinder block below the pistons, which houses the crankshaft.
C-pillar – The vertical metal roof support between the side edge of the rear screen and the rear edge of the rear window.
Crankshaft – A cranked shaft that is driven by the pistons and provides the engine output to the transmission.
Crossflow cylinder head – A cylinder head in which the inlet and exhaust valves and manifolds are on opposite sides.
Cruise Control – A device that, when set by the driver, will hold the car at the chosen speed.
Crumple Zone – Portions of a vehicle’s structure designed to buckle and fold in an impact, absorbing crash force rather than transmitting it to vehicle occupants.
Cubic capacity (cc) – The total volume within the cylinders of an engine which is swept by the movement of the pistons, example Seat Leon 1.4cc.
CVH – A term applied by the Ford Motor Company to their overhead camshaft engines which incorporate a hemispherical combustion chamber. CVH means Compound Valve angle, Hemispherical combustion chamber.
Cylinder – Close fitting metal tube in which a piston slides. In the case of an engine, the cylinders may be bored directly into the cylinder block, or on some engines, cylinder liners are used which rest in the cylinder block and can be replaced when worn with matching pistons to avoid the requirement for reboring the cylinder block.
Cylinder block -The main engine casting which contains the cylinders, crankshaft and pistons.
Cylinder head – The casting at the top of the engine, which contains the valves and associated operating components.

Dashpot – An oil-filled cylinder and piston used as a damping device in SU and Zenith/Stromberg CD type carburettors.
Diesel engine – An engine that relies on the heat generated when compressing air to ignite the fuel, and which therefore doesn’t need an ignition system. Diesel engines have much higher compression ratios than petrol engines, normally around 20:1. They do not require spark plugs to ignite the air/fuel mixture, only the compression from the engine cylinders.
Diesel Particulate Filter – See also (DPF), is a part that sits in the exhaust system and is designed to catch and then burn off soot/particles.  This burning off is also known as ‘regeneration’.
Differential – A gearbox or fluid coupling that allows the wheels to rotate at different speeds. They are usually located on an axle, allowing the outside wheels to turn faster than the inside wheels during cornering. Four-wheel-drive and all-wheel drive vehicles have two differentials, one for the rear axle and one for the front.
Disc brakes – A brake assembly where a rotating disc is clamped between hydraulically operated friction pads.
Distributor – Part of the ignition (electrical) system, used to distribute the HT current from the ignition coil to the individual spark plugs via the HT leads.
Distributor cap – Plastic cap which fits on top of the distributor and contains electrodes, in which the rotor arm rotates to distribute the HT spark voltage to the correct spark plug.
DPF  – See Diesel Particulate Filter.
– Abbreviation for Double Overhead Camshaft (see ‘Twin-cam’).
Downpipe – The pipe that joins the entire exhaust system to the exhaust manifold.
Driveshaft – Term usually used to describe the shaft (normally incorporating universal or constant velocity joints), that transmits drive from a differential to one wheel. More commonly found in front-wheel-drive cars.
Drum brakes – A brake assembly with friction linings on ‘shoes’ running inside a cylindrical drum attached to the wheel.
Dual circuit brakes – A hydraulic braking system consisting of two separate fluid circuits, so that if one circuit becomes inoperative, braking power is still available from the other circuit.
Dwell angle – A measurement that corresponds to the number of degrees of distributor shaft rotation during which the contact breaker points are closed during the ignition cycle of one cylinder. The angle is altered by adjusting the contact breaker points gap.

Earth strap – A flexible electrical connection between the battery and a car earth point, or between the engine/gearbox and the car body to provide a return current path flow to the battery.
EBD – Electronic Brake Distribution is a component used with ABS and usually a brake assist mechanism, for small powerful cars, like the new Mini of 1998.
ECU – Electronic Control Unit.
EFI – Abbreviation for Electronic Fuel Injection.
EGR – Exhaust Gas Recirculation. Part of the emissions system that recirculates exhaust gases into the intake manifold, cooling the combustion chamber.
Electronic ignition – A system which uses an electronic unit as opposed to an older mechanical style distributor with points (contacts) to control the timing and firing of spark plugs.
Engine management system – Computerised control of the ignition an fuel systems, making driving more economical, quieter, and power-effective.
EW – Electric windows.
Excess – (car insurance) The amount you would have to pay in the event of an insurance claim. See article – Car Insurance Tips
Exhaust Manifold – A set of cast pipes that attach directly to the engine and connect to the exhaust system via the downpipe and remainder of the system. The exhaust gases exit the engine cylinders and are pushed out into the exhaust. The exhaust manifolds are are made from cast metal materials because they need to withstand extreme temperatures of the expelled gases
Expansion tank – A container used in many cooling systems to collect the overflow from the car’s cooling system as the coolant heats up and expands.

Float chamber – Older vehicles. The part of a carburettor, which contains a float and needle valve for controlling the fuel level in the reservoir.
Flywheel – A heavy rotating metal disc attached to the crankshaft and used to smooth out the pulsing from the pistons. It can also be encircled by a ring gear designed to mesh with the gear in a starter motor. The starter motor, using power from the battery will turn the engine (via the flywheel) and to help initiate the the ignition sequence (starting).
Forced Induction – When a gas is blown into the engine to increase speed, by a turbo or supercharger.
Fuel injection – Injectors are used on fuel injection engines to inject fuel directly or indirectly into the combustion chamber. Some fuel injection Systems use a single fuel injector, while some systems use one fuel injector for each cylinder of the engine. The fuel injection system has electronic control to time and meter the fuel flow.

Gasket – Compressible material used between two surfaces to provide a leakproof joint.
Gearbox – A group of gears and shafts installed in a housing, positioned between the clutch and the differential, and used to keep the engine within its safe operating speed range as the speed of the car changes.

Horsepower – A measurement of power. Brake Horsepower (BHP) is a measure of the power required to stop a moving body.
HPI check – A HPI check is a check on the history of a vehicle. HPI checks should be purchased before buying a vehicle. It will tell you whether it has been stolen, written off, ringed, cloned, clocked etc. See Article – What is a HPI check.
HT – High Tension (meaning high voltage) used to describe the spark plug voltage in an ignition system.
Hydroplaning – see aquaplaning.

Idle Speed – The speed of the engine at minimum throttle and the engine in neutral.
Independent suspension – A suspension system where movement of one wheel has no effect on the movement of the other, e.g. independent front suspension.
Ignition coil – An electrical coil, which forms part of the ignition system and which, generates the HT voltage.
Ignition system – The electrical system which provides the spark to ignite the air/fuel mixture in the engine. Normally the system consists of the battery, ignition coil, distributor, ignition switch, spark plugs and wiring.
Ignition timing – The time in the cylinder firing cycle at which the ignition spark (provided by the spark plug) occurs. The spark timing is normally a few degrees of crankshaft rotation before the piston reaches the top of its stroke, and is expressed as a number of degrees before top-dead-centre (BTDC).
Independent Suspension – A suspension design that lets each wheel move up and down independently of the others. A vehicle can have two-wheel or four-wheel independent suspension; sportier models have four-wheel independent suspension.
Inlet Manifold – A set of cast metal pipes through which fuel or air is directed into the engine cylinders.
Inlet Valves – Devices that open passageways within the engine for fuel vapor to enter the cylinders but which also close them to maintain cylinder pressure during compression and combustion.
In-Line Engine – Cylinders are arranged side by side in a single row. Most four-cylinder and some six-cylinder engines are in-line engines. In V-6, V-8 or V-12 engines, the cylinders are divided into two rows angled away from each other in a ‘V’ pattern (Hence the ‘V’ in V8, V12).
Immobiliser – An electronic security device that prevents a car from being stolen. It usually disables the ignition system so that the engine may not be started.
Induction kit – An upgrade to your manufacturers air filter. Usually replacing the airbox and paper airfilter with a cone performance filter – See article – What is an Induction Kit.
Intercooler – A device much like a radiator that cools air as it leaves a turbocharger or supercharger before the air is blown into the engine air intake. Cooling makes the air denser and richer in oxygen, which lets the engine produce more power. See article – What is an intercooler.

Jet – Older vehicles. A calibrated nozzle or orifice in a carburettor through which fuel is drawn for mixing with air.
Jump leads – See also, Booster Cables and Jump Start. Heavy duty electric cables fitted with clips to enable a car’s battery to be connected to another battery for emergency jump starting. See article – How to jump start your car.
Jump start – To transfer electrical power from one car battery to another to enable the cars ignition system to startup, using jump leads. See article – How to jump start your car.

Kerb weight – The weight of a car, unladen but ready to be driven, i.e. with enough fuel, oil, etc, to travel an arbitrary distance.
KPH – Kilometers Per Hour, used as a measure of speed especially in europe. Multiply by 0.621 to convert to miles per hour.

Lag – see turbo lag
Leaf spring – A spring commonly used on cars with a live axle, consisting of several thin, curved steel plates clamped together at the ends to the underside of the vehicle. Usually found on older cars such as the Ford Escort mk2.
Limited slip differential (LSD) – A device that helps prevent the drive wheels from skidding or losing traction by diverting power from the slipping wheel to the opposite wheel on the same axle. Often used on high-performance cars.
Live axle – A solid axle allowing movement of the wheel on one end to affect the opposite wheel. Found on older rear-drive cars and trucks. Also called a rigid axle.
Lock-to-lock – The amount of turns needed to move the steering wheel from one full lock position to the other. Full lock being the point at which you cannot turn the steering wheel any further.
Loom – A complete car wiring system or section of a wiring system consisting of all the wires of correct length, etc, to wire up the various circuits.
LPG – liquid petroleum gas. A natural hydrocarbon fuel made up of propane and butane, cheaper than petrol to run your car on.
LT – Abbreviation of Low Tension (meaning low voltage) used to describe battery voltage in the ignition system.

Manifold – A device used for ducting the air/fuel mixture to the engine (inlet manifold), or the exhaust gases from the engine (exhaust manifold).
MAP Sensor – Manifold Absolute Pressure Sensor. It detects engine load by measuring air pressure or vacuum in the inlet manifold.
Mass Airflow Sensor – Also known as an air-mass meter. A device that measures the flow of air entering the throttle housing.
Master cylinder – A cylinder containing a piston and hydraulic fluid, directly coupled to a foot pedal (e.g. brake or clutch master cylinder). Used for transmitting pressure to the brake or clutch operating mechanism.
MOT – Ministry of Transport annual test of vehicle road safety see article – what is an MOT.
MPG – Miles Per Gallon. Fuel economy measurement. Generally, a vehicle maker may offer mpg ratings for city (urban) driving, motorway driving, and combined driving.
MPH – Miles Per Hours
MPV – Multi-person vehicle. e.g. Renault espace, Ford Galaxy. VW Sharan (7 seater) Toyota Previa etc.
Multi-Link Suspension – Independent suspension controlled with several link arms that restrict undesired motion of the suspension for a smoother ride and more precise handling.

Needle valve – Older vehicles. A component of the carburettor, which restricts the flow of fuel or fuel/air mixture according to the position of the valve in an orifice or jet.
Negative earth – Electrical system (almost universally adopted) in which the negative terminal of the car battery is connected to the car body. The polarity of all the electrical equipment is determined by this.

OBD – On-Board Diagnostics.  A vehicle’s self-diagnostic and reporting capability. OBD systems allow access to the status of the various vehicle subsystems.
Earlier vehicles operated OBD I and OBD 1.5 up to 1996. Vehicles after 1996 may have OBD II (definitely US) and definitely fully supported in UK/Europe after 2000.
– The hydrocarbon substance in fuel that reduces engine knock or pinking, which is a noise caused by premature ignition of fuel in the cylinder combustion chamber. The higher the octane number, the less chance of premature ignition. Typically, higher octane fuels will give improved engine power/performance. Petrol in the UK has a higher octane rating that that in the US.
Octane rating – A scale rating for grading petrol.
OHC (overhead cam) – Describes an engine in which the camshaft is situated above the Cylinder head, and operates the valve gear directly. Single overhead-cam (SOHC) engines have a single cam above the cylinder head. Double overhead-cam (DOHC) engines have two cams above the cylinder head.
OHV (overhead valve) – Describes an engine, which has its valves in the cylinder head, but with the valve gear, operated by pushrods from a camshaft situated lower in the engine.
Oil cooler – Small radiator fitted in the oil circuit and positioned in a cooling airflow to cool the oil. Used mainly on high-performance engines.
Oversteer – A tendency for a car to turn more tightly into a corner than intended. Occurs when the rear tyres lose adhesion, and can lead to a spin if the driver doesn’t reduce acceleration. See also Understeer.
Oxygen Sensor – An emissions related device which senses the presence of oxygen in the exhaust. The voltage it puts out is interpreted by the main computer (ECU) along with other sensor input to determine automatic adjustment of the air/fuel mixture.

Piston – A cylindrically shaped part which compresses fuel vapour within a cylinder (compression stroke) and is thrust downward by the force of the explosion that results when the vapour is ignited.
Piston ring – Hardened metal rings that fit in a groove around the outside of the piston to ensure a gas-tight seal between the piston and cylinder wall.
Positive earth – The opposite of negative earth.
Power steering – A steering system that uses hydraulic fluid pressure (provided by an engine-driven pump) to reduce the effort required to steer the car.
Power-to-Weight Ratio – The power output of a vehicle in comparison to its weight, usually described in number of bhp per ton. For example, if a car has a powerful engine but is also very heavy it may not be as fast as a lighter car with less power, there is less weight for the engine to move.
Projector-Beam Headlights – A headlight that uses a spherical reflector to control the light beam. The bulb directs the light inward, toward the reflector which then projects it forward. These lights are more powerful, accurate and expensive than standard sealed-beam and halogen headlights.
Pressure Plate – Holds the clutch disc against the flywheel.
Propeller shaft (prop-shaft)- The shaft, which transmits the drive from the gearbox to the rear axle in a front-engine rear-wheel-drive car (i.e. BMW)
PSI – Pounds per square inch. A measurement of air pressure, i.e. when inflating a tyre.
Pushrod – A rod that is moved up and down by the rotary motion of the camshaft and operates the rocker arms in an OHV engine.

Quarter light – A triangular window mounted in front or behind the main front or rear windows, usually in the front door, or behind the rear door.

Radiator – Cooling device through which the engine coolant is passed, situated in an airflow and consisting of a system of fine tubes and fins for rapid heat dissipation. Usually has a fan situated behind it that will start when the coolant reaches a certain temperature. This will pull more air across the fins of the radiator to aid cooling.
Redline – The point on the engine rev counter that indicates the maximum RPM the engine can safely withstand.
Rev counter – See Tachometer.
Rolling Radius – This is the radius of your wheels and tyres. If you change your wheels for bigger or smaller diameter items, it is important to try and keep the same rolling radius as the wheels originally fitted to the car. The reason is that if the rolling radius changes greatly, the gearing, 0-60 speed and top end speed can be affected.
Rocker arm – A lever that rocks on a central pivot, with one end moved up and down by the camshaft, and the other end operating an inlet or exhaust valve.
Rotary engine – See ‘Wankel engine’.
Rotor arm – A rotating arm in the distributor, which distributes the HT spark voltage to the correct spark plug.
RPM – Revolutions per Minute. This shows the speed at which the engine is turning.
Running on – A tendency for an engine to keep on running after the ignition has been switched off. Often caused by a badly maintained engine or the use of an incorrect grade of fuel.

Sat-nav – Satelite Navigation. A System that shows you on a small screen where your car is and normally gives you directions to where you want to go.
Sealed beam – A sealed headlamp unit where the filament is an integral part and cannot be renewed separately.
Semi-trailing arm – A common form of independent rear suspension.
Servo – A device for increasing the normal effort applied to a control, i.e. servo assisted braking.
Shock absorber – A device for damping (smoothing) the up-and-down movement of the suspension when the car hits a bump in the road.
SOHC – Single Overhead Cam. An engine with a single cam generally has one intake and one exhaust valve per cylinder; the single cam opens and closes both valves. See also Overhead Cam and Double Overhead Cam.
Spark plug – A device with two electrodes insulated from each other by a ceramic material, which screws into an engine combustion chamber. When the HT voltage is applied to the plug terminal, a spark jumps across the electrodes and ignites the fuel/air mixture.
Starter motor – A powerful electric motor used to start the movement of engine so that combustion can begin. Activating the starter by turning the ignition key, causes the solenoid to thrust the pinion gear in the starter against the engine flywheel ring gear and begin turning it.
Steering rack – See Rack and pinion.
Stroke – The total distance travelled by a single piston in its cylinder.
Stub axle – A short axle that carries one wheel.
Strut – A single, self contained pivoting suspension unit that integrates a coil spring with a shock absorber. Struts are used on front wheel drive automobiles. A suspension element in which a reinforced shock absorber is used as one of the wheel’s locating members, typically by solidly bolting the wheel hub to the bottom end of the strut.
Strut brace – An aftermarket item (commonly used in rallying) that fits inside the engine bay with each end bolted to the top of the left and right suspension tops or turrets. Its purpose is to increase the rigidity of the car by stiffening the front end suspension set-up.
Subframe – A small frame that is mounted on the car’s body, and carries the suspension and/or the drivetrain assemblies.
Sump – The main reservoir for the engine oil, found at the bottom of the engine. Usually has a drain plug built into it for draining oil during and oil change or engine work.
Supercharger – Has the same function as a turbo but has no lag time because it runs off an engine-driven pump. The supercharger usually driven by a belt or gears from the crankshaft, drives a compressor which forces air into the engine, providing increased fuel/air mixture flow, and therefore increased engine performance and efficiency.
Suppressor – A device that is used to reduce or eliminate electrical interference caused by the ignition system or other electrical components. i.e. you would fit a supressor onto your radio if you were getting radio interference when the car was turned on.
Suspension – A general term used to describe the components which suspend the car body on its wheels.
Swing axle – A suspension arm, which is pivoted near the front-to-rear centreline of the car, and which, allows the wheel to swing vertically about that pivot point.
Synchromesh – A device in a gearbox, which synchronises the speed of one gear, shaft with another to produce smooth, noiseless engagement of the gears.
Synthetic oil – Engine lubricant not derived from raw petroleum. It has superior engine-protection properties but costs as much as five times more than petroleum oil.

Tachometer – Also known as a rev counter, indicates engine speed in revolutions per minute (rpm).
Tappet – A term often used to refer to the component which transmits the rotary camshaft movement to the up-and-down movement required for valve operation.
TC – Traction control
TDC – See top dead centre.
Thermostat – A device which is sensitive to changes in engine coolant temperature, and opens up an additional path for coolant to flow through the radiator (to increase the cooling) when the engine has warmed up.
Tie-rod – A rod which connects the steering arms to the steering gear.
Timing belt – On overhead cam equipped engines, a fabric or rubber belt engaged on sprocket wheels and driving the camshaft from the crankshaft. It synchronises the operation of intake/exhaust valves with the compression/ignition process occurring in the cylinder head and engine block.
Timing chain – Metal flexible link chain engaging on sprocket wheels and driving the camshaft from the crankshaft.
Timing marks – Marks normally found on the crankshaft pulley or the flywheel and used for setting the ignition firing point with respect to a particular piston.
Toe-in/toe-out – The amount by which the leading edge of the front wheels point inwards or outwards from the straight-ahead position. Front-drive cars are often aligned with slight toe-in to compensate for the effects of torque steer, or the tendency of the front wheels to pull to the side under hard acceleration.
Top Dead Centre (TDC) – The point at which a piston is at the top of its stroke.
Torque – The measure of turning force generated by a rotating component, given in foot-pounds (lb.-ft.). In vehicle terms it is the twisting force the engine exerts on the crankshaft. Vehicle specifications often include the maximum torque an engine produces at a specific RPM. An engine producing 200 lb.-ft. of torque at 3,000 RPM or 200 lb.-ft.@ 3,000 RPM accelerates better at low speeds than an engine that provides 200 lb.-ft.@5,000 RPM.
Torque steer – The tendency of the front wheels on a front-drive vehicle to pull to the side under hard acceleration. Normally compensated for with toe-in.
Torsion bar – A metal bar, which twists as it is compressed or stretched. Peugeots and Citroens typically have torsion bars as part of their rear suspension set-up.
Traction Control – A system that limits wheel spin under acceleration. It maintains each wheel’s contact with the road surface. Traction-control systems generally use the anti-lock braking system to stop wheel spin and reduce power from one or more engine cylinders when an electronic sensor detects wheel spin.
Trailing arm – A form of independent suspension where the wheel is attached to a swinging arm, and is mounted to the rear of the arm pivot.
Turbocharger – A device which uses a turbine driven by the engine exhaust gases to drive a compressor which forces air into the engine, providing increased fuel/air mixture flow, and therefore increased engine efficiency. Commonly used on high-performance engines.
Turbo lag – The time taken for the turbocharger to kick in upon acceleration. The lag occurs because a turbocharger compressor is spun by exhaust gases in the exhaust manifold and it takes time for it to spin up to speed. Larger turbos have more turbo lag but give more more further up the rev range, whereas smaller turbos spin up faster but are less powerful. Many performance cars now incorporate 2 turbos, a smaller one for low down response and then a larger one that takes over as the engine speed increases.
Twin-cam – Abbreviation for twin overhead camshafts (see ‘OHC’). Used on engines with a crossflow cylinder head, usually with one camshaft operating the inlet valves and the other operating the exhaust valves. Gives improved engine efficiency due to improved fuel/air mixture and exhaust gas flow in the combustion chambers.

Understeer – A tendency for a car to go straight on when turned into a corner. It happens when the front wheels have lost adhesion or the driver is turning the steering wheel too sharply for the vehicle’s speed. In understeer, the front wheels do not follow the steering wheel angle, and the car refuses to turn and goes straight ahead.
Universal joint – A joint that can swivel in any direction whilst at the same time transmitting torque. This type of joint is commonly used in propeller shafts and some driveshafts, but is not suitable for some applications because the input and output shaft speeds are not the same at all positions of angular rotation. The type in common use is known as a HardySpicer, Hooke’s or Cardan joint.
Unsprung weight – The part of the car, which is not supported by the springs.

Vacuum advance – System of ignition advance and retard used in some distributors where the vacuum in the engine inlet manifold is used to act or a diaphragm which alters the ignition timing as the vacuum changes due to the throttle position.
VIN – Vehicle Identification Number A number that is unique to each vehicle. This number is stamped onto the car so that it can be identified. Different manufacturers put VIN numbers in different places.
Vented Discs – A brake disc that has cooling passages between the friction surfaces. When looked at, the disc looks like it is made up of 2 sections sandwiched together with vanes in between. These vane pull in air to keep the friction surfaces cooler and therefore aid braking.

Wankel engine – A rotary engine which has a triangular shaped rotor which performs the function of the pistons in a conventional engine, and rotates in a housing shaped approximately like a broad-waisted figure of eight. Very few cars use this type of engine.
Waste gate – A device used to limit the boost developed in a turbocharger. A waste gate operates by allowing some of the engine’s exhaust flow to bypass the turbocharger’s turbine section under certain conditions. Normally controlled by the ‘waste-gate actuator’.
Wheel balancing – Adding small weights to the rim of a wheel so that there are no out-ofbalance forces when the wheel rotates.
Wishbone – An ‘A’-shaped suspension component, pivoted at the base of the ‘A’ and carrying a wheel at the apex. Normally mounted close to the horizontal.

VGC – Very good condition

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