A-Z glossary of engine terms.

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The amount before the piston reaches the top of its travel that the spark fires to ignite the mixture. Expressed as degrees of engine rotation.

Als (anti-lag system):

Designed to keep turbos spinning for improved throttle response. Usually involves introducing neat fuel and some air into the turbine housing to keep the turbo spinning when the throttle is closed.

Ambient (pressure or temperature):

The pressure or temperature of overall atmospheric conditions.


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Unit of air pressure. 1 bar = 14.7psi = atmospheric pressure (nominal).

Big end:

The part of the conrod that bolts to the crank .


A solid piece of metal from which a component is machined.


The main structure of the engine.


The pressure a turbocharger or supercharger develops in order to force air into the engine. Measured in bar or psi.


The diameter of the cylinder the pistons slide up and down within.

Bottom end:

The collective term for the block and the components it houses – crankshafts, connecting rods, pistons, oil and cooling systems.


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Rotating shaft that opens and closes the valves, allowing air into and exhaust gases out of the engine.


A process for forming various components, where molten metal is poured into mould to produce the required port.

Charge cooler:

Reduces the temperature of incoming air from a turbo or supercharger, via an air/water heat exchanger, using water as coolant.

Combustion chamber:

The chamber where the explosive combustion takes place, to increase pressure to force the piston down.

Compression ratio:

The ratio that the incoming air is compressed to, in order to provide a volatile mixture, e.g. 11:1 for a normally-aspirated engine or 7:1 for a turbo engine.

Compressor wheel:

The wheel on a turbo which pulls air and forces into the engine.

Connecting rod (conrod):

Connects the piston to the crankshaft.


The crankshaft translates up and down motion of the pistons to rotational motion to drive the gearbox.


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The situation where the inlet charge in the combustion chamber ignites uncontrollably, usually before the spark ignites he mixture. Potentially very damaging to engine components.

Dry sump:

Where the oil reservoir is remote from the engine and oil is forced in and out under the pressure by mechanical pumps.

Dump valve:

A valve to vent off turbo-pressurised air when the throttle is closed on gearchanges, to prevent the turbo stalling.


The amount of time a camshaft causes the valve to operate, from ‘beginning to open’ to ‘fully closing’.


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Ecu (electronic control unit):

Stores the ignition and fuelling maps and determines how much of each of he engine needs.


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A process where a piece of metal is stamped into the required shape. Grain structure results in high inherent strength.


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Hydrodynamic lubrication:

Oil pressure within a bearing builds to very high pressure, when it’s forced into contracting cone as the components spin.


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Another name for a turbo’s compressor wheel.


An air/air heat exchanger designed to reduce the temperature of the incoming inlet charge on the turbo or supercharged engines. Normally, mounted in the air stream at the front of the car.


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The delay between opening the throttle and a turbo producing boost.


The distance a camshaft causes the valve to open.


Cylinder linings made of different metal to the block. They are cast or pressed into the block to give the required different characteristics to the block metal.

Little end:

The part of the conrod that fits to the piston.


The egg-shaped parts of the camshaft that actually act on the valve, to open and close it.


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Maf (mass air flow sensor):

Accurately measures the actual amount of air entering the engine to determine how much fuel is required.

Mains (main bearings):

The bearings that house the crankshaft within the cylinder block.

Map (manifold absolute pressure):

The total pressure of air going into the engine – boost pressure plus atmospheric pressure (1 bar).


A two-dimensional or three-dimensional series of curves storing the information regarding fuel and ignition for the engine, depending on the engine’s requirements. Stored in ECU.


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A rating of how much fuel can be compressed before it ignites spontaneously. Ranges from 95 (road fuel) to 117 (very high quality race fuel).


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The part of the crankshaft that the conrod bolts to.


The components that slide up and down within the cylinder bores, under the action of the combustion pressure.

Piston crown:

The very top of the piston.

Piston rings:

Two or three rings fit around the piston to seal the combustion pressure above and prevent oil passing into the combustion chamber from below.

Piston skirt:

The cylindrical part of the piston which sits within the bore.


The inlet or exhaust ports allow air to flow into or out of the cylinder head. Run from the manifold face to the valves.


A measure of the rate at which an engine does work. Numerically, torque multiplied by engine speed, and expressed in horse brake power (bhp).

Pulse tuning:

Using negative pressure pulses within exhaust systems to create low pressure areas at exhaust valves as they open, helping to get gas out more efficiently.


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A process where powdered metal is forced, under pressure and heat, into a mould to form the required component.

Slipper pistons:

Pistons with skirts with material removed, to reduce friction while maintaining strength.


Action of forcing pockets of air within the combustion chamber back towards the spark plug for better fuel distribution.


When fuel burns completely in available air. Ratio is 14.6:1 (air/fuel).


The distance the piston moves up and down within the cylinder bore.


A mechanically-driven pump to force additional air into engine.


Uneven and unpredictable airflow into the engine, often caused by a poor choice of turbo and/or too much boost. Can cause serious engine damage.


Horizontal turbulent motion of air entering engine. Helps to give good fuel distribution and improve combustion.


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The force (or energy) an engine actually produces. Measured in foot pounds (lb/ft), as well as other units.

Tubular manifold:

An exhaust manifold that has a single pipe per cylinder, to prevent cross-cylinder mixing of gases.


Vertical turbulent motion of air entering engine. Helps to give good fuel distribution.


The wheel on a turbo which is driven by the exhaust gas.


An exhaust gas-driven compressor which forces air into an engine under pressure.


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Opens to allow air into or exhaust out of the engine and close to seal the combustion pressure within the chamber.

Valve bounce:

If the speed of valve is closing too high, the valve can hit the seat so hard it bounces off again. Can lead to valve/piston contact.

Valve float:

If the cam lobe accelerates the valve too quickly, the valve can bounce off the cam itself as it opens. Can result in piston/valve contact.

Valve overlap:

The amount of time that the inlet and exhaust valves are open at the same time, to allow momentum charging of the cylinder or to scavenge additional exhaust gas from the cylinder.

Valve spring:

Provides the closing force for the valve and keeps it in contact with the cam lobe.


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A valve which limits boost pressure by allowing exhaust gas to bypass the turbine when the boost pressure limit is reached.

Water injection:

Water is injected into the inlet charge, to remove heat as the water vaporises.

Wet sump:

Where the oil reservoir for the engine is in the bottom. Circulated oil falls down to the sump, to be picked up and recirculated.