Glossary of Terms

Align Boring – A machining process which creates a straight and aligned camshaft housing or more often the crankshaft housing in the engine block.

Bore Size – The diameter, in inches or millimeters, of the cylinders in the engine block. If an engine has its originally designed engine bore size it is referred to as “standard” and, if it has been bored out, it is referred to by the amount it has been bored out from standard. An example is “.30 over”

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.

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.

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.

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.

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 valvetrain. 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.”

Detonation – A condition in which, after the spark plug fires, some of the unburned air-fuel mixture in the combustion chamber explodes spontaneously, set off only by the heat and pressure of the air-fuel mixture that has already been ignited. Detonation, or “knock,” greatly increases the mechanical and thermal stresses on the engine.

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.

Exhaust-Gas Recirculation – EGR is a method of reducing NOx (oxides of nitrogen) exhaust emissions by recirculating some of the engine’s exhaust gas into the intake manifold. The exhaust gas serves as inert filler that absorbs heat during the combustion process and reduces the peak temperature reached during combustion.

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.</p"

Feedback Fuel-Air-Ratio Control – A feature of a computer-controlled fuel system. By using a sensor to measure the oxygen content of the engine’s exhaust, the system keeps the fuel-air ratio very close to the proportion for chemically perfect combustion. Such tight control of the fuel-air ratio is mandatory for the proper operation of three-way catalysts.

Flywheel – A heavy disc attached to an engine’s crankshaft to increase its rotary inertia, thereby smoothing its power flow.

Four Valves Per Cylinder – A valvetrain with a total of four valves in the combustion chamber, typically two intakes and two exhausts. Compared to the more common two-valve-per-cylinder designs, a four-valve layout offers improved breathing and allows the spark plug to be located closer to center of the combustion chamber.

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.

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.

Hydraulic Lifter – A valve lifter that, using simple valving and the engine’s oil pressure, can adjust its length slightly: thereby maintaining zero clearance in the valvetrain. Hydraulic lifters reduce valvetrain noise and are maintenance-free.

Intake Charge – The mixture of fuel and air that flows into the engine.

Intake Manifold – The network of passages that direct air or air-fuel mixture from the throttle body to the intake ports in the cylinder head. The flow typically proceeds from the throttle body into a chamber called the plenum, which in turn feeds individual tubes, called runners, leading to each intake port. Engine breathing is enhanced if the intake manifold is configured to optimize the pressure pulses in the intake system.

Intake Port – The passageway in a cylinder head leading from the intake manifold to the intake valve(s).

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.

Knock Sensor – A sensor mounted on the engine that is designed to detect the high-frequency vibrations caused by detonation. By employing a knock sensor, a computerized engine-control system allows an engine to operate very near its detonation limit: thereby improving power and efficiency.

LPG – Liquefied petroleum gas. Also referred to as “GPL,” “LP Gas” or “autogas.” A blend primarily of two hydrocarbon gases, propane and butane. Propylene and butylenes can also be present in small concentration. Ethanethiol, a powerful odorant, is also added so that leaks can be detected.

Magnaflux Testing – Also known as magnetic particle inspection, Magnaflux Testing uses a magnetic field and an iron oxide powder or solution to identify surface cracks in metals. It is a time-proven, reliable, and cost-effective method of identifying external cracks.

Main Bearings – The bearings in an engine block that support the crankshaft.

NOx – An abbreviation for six chemical compounds produced during high temperature combustion, containing only nitrogen and oxygen atoms, that react with volatile organic compounds in the presence of sunlight to create photochemical smog.

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.

Plenum Chamber – A chamber, located between the throttle body and the runners of an intake manifold, used to distribute the intake charge evenly and to enhance engine breathing.

Port Fuel Injection – A type of fuel injection with at least one injector mounted in the intake port(s) of each cylinder. Usually the injector is mounted on the air intake manifold close to the port. Port fuel injection improves fuel distribution and allows greater flexibility in intake-manifold design, which can contribute to improved engine breathing.

Pound-Feet – The unit of measurement for torque. One pound-foot is equal to the twisting force produced when a one-pound force is applied to the end of a one-foot-long lever.

Power – The rate at which work is performed. Power is proportional to torque and rpm and is measured in horsepower.

Power Band – The subjectively defined rpm range over which an engine delivers a substantial fraction of its peak power. The power band usually extends from slightly below the engine’s torque peak to slightly above its power peak.

Powertrain – An engine and transmission combination.

Pushrod – A general term for any rod that transfers force in compression. In a valvetrain, pushrods are used to transfer reciprocating motion from the cam followers to a more distant part of a valvetrain, typically the rocker arms.

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.

Single-Rate Spring – A spring with a constant spring rate. For example, if a 100-pound force deflects the spring by one inch, an additional 100 pounds will deflect it one more inch, and so on until the spring either bottoms or fails.

SOHC – Single overhead camshaft: SOHC engines use one camshaft in each cylinder head to operate both the exhaust valves and the intake valves.

Stroke – The distance between the extremes of a piston’s travel in a cylinder.

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 turbocharger

Throttle-Body – A housing containing a valve to regulate the airflow through the intake manifold. The throttle-body is usually located between the air cleaner and the intake plenum.

Throttle-Body Fuel Injection – A form of fuel injection in which the injectors are located at the engine’s throttle-body, thereby feeding fuel to more than one cylinder. Such an arrangement saves money by using fewer injectors; but because it routes both fuel and air through the intake manifold, it eliminates some of the tuning possibilities offered by port fuel injection.

Torque – The rotational equivalent of force, measured in pound-feet.

Tuned Intake and Exhaust Systems – Intake and exhaust systems that, by harnessing the pressure pulses and resonances inside the various passages and chambers of the intake and exhaust manifolds, increase the flow of intake charge into and out of the combustion chambers.

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.

Turbo Lag – Within a turbocharger’s operating range, lag is the delay between the instant a car’s accelerator is depressed and the time the turbocharged engine develops a large fraction of the power available at that point in the engine’s power curve.

Valve Float – A high-rrpm engine condition in which the valve lifters lose contact with the cam lobes because the valve springs are not strong enough to overcome the momentum of the various valvetrain components. The onset of valve float prevents higher-rpm operation. Extended periods of valve float will damage the valvetrain.

Valve Lifter – Also called a “valve follower”: the cylindrically shaped component that presses against the lobe of a camshaft and moves up and down as the cam lobe rotates. Most valve lifters have an oil-lubricated hardened face that slides on the cam lobe. So-called “roller lifters,” however, have a small roller in contact with the cam lobe: thereby reducing the friction between the cam lobe and the lifter.

Valvetrain – The collection of parts that make the valves operate. The valvetrain includes the camshaft(s) and all related drive components, the various parts that convert the camshaft’s rotary motion into reciprocating motion at the valves, and the valves and their associated parts.

Waste Gate – A valve 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.

ZEV – Zero emission vehicle. The most restrictive emissions standard.