terms to know for modders
Below is a glossary of car modification terms:
A/R RATIO The ratio between the cross-sectional area (A) of the turbine scroll of a turbocharger at any one point and the distance or radius (R) from that point to the center of the turbine-wheel. This ratio is always constant along the turbine scroll.
AIR/FUEL RATIO The ratio of air to fuel in the air/fuel mixture. In the internal combustion engine, the ideal air/fuel ratio is 14,7:1. This ratio provides the ideal combustion to produce the most power. Also see STOICHIOMETRIC CONDITION.
ANTI-REVERSION An attempt to inhibit reversion, i.e., an attempt to restrict the flow of exhaust gasses back into the combustion cylinder. Also see REVERSION.
BACK PRESSURE A restriction in the flow of exhaust gasses caused by the exhaust system or by a turbocharger but is ultimately required of efficient scavenging of the exhaust gasses in the combustion chamber.
BLOWER Used to refer to a supercharger. Technically, a blower is supercharger that does not have internal compression. The term has become interchangeable with the term "supercharger" but refers specifically to positive-displacement superchargers. Also see SUPERCHARGER and POSITIVE-DISPLACEMENT SUPERCHARGER. BRAKE FADE Brake fade occurs when brake effectiveness is lost after repeated hard braking. There are actually three types of brake fade: friction fade, mechanical fade, or fluid fade. Friction fade and mechanical fade occurs due to the effect of temperature on the coefficient of resistance. With high heat coefficient of resistance between the brake pads and the rotor is greatly reduced. Hence the brake pedal feels hard but the brakes are not effective. With fluid fade, heat is transferred to the brake fluid, causing it to boil and produce compressible gasses which makes the brake pedal feel spongy.
CAMBER The degree of inward or outward tilt from the wheel center line when viewed from the front of the vehicle. An outward tilt produces positive camber and tops of the tires are further apart than the bottoms. An inward tilt produces negative camber and tops of the tires are closer together than the bottoms.
CAMSHAFT DURATION The number of degrees at least one valve is open measured in degrees rotation of the crankshaft. NOTE: the crankshaft completes two full rotations for every rotation of the camshaft.
CASTER The angle between the vertical line through the wheel when viewed from the side and the line from the center of the wheel to the upper connecting point on the vehicle. The caster causes the front wheels to straighten out after a corner.
CENTER OF GRAVITY A fixed point in a body through which the resultant force of gravitational attraction acts. The entire weight of a body may be considered as concentrated at this point so that if supported at this point the body would remain in equilibrium in any position. NOTE: the correct scientific term for this phenomenon is "center of mass". CENTRIFUGAL SUPERCHARGER A supercharger that has more of a belt driven turbocharger than a blower. These superchargers have step-up gears that spin an impeller wheel much faster than the rotors or screws in blowers and causes compression. Also see BLOWER, SUPERCHARGER, POSITIVE-DISPLACEMENT SUPERCHARGER and TURBOCHARGER.
COLD AIR INTAKE (CAI)-
CROSSFLOW CYLINDER HEAD A cylinder head that has the inlet valves and manifolds on one sides and the exhaust valves and manifold on the other side.
DETONATION The spontaneous combustion of the air/fuel mixture ahead of the flame front that can and will cause serious engine damage if left unchecked. Also referred to as "knocking" and "pinging" or "pinking".
ELECTRODE An electric conductor through which an electric current passes. In a spark plug the electrode is the center core passing through the ceramic insulator. ENDOTHERMIC A chemical reaction in which heat is absorbed.
FIXED DISPLACEMENT SUPERCHARGER See POSITIVE-DISPLACEMENT SUPERCHARGER.
FORCED INDUCTION The process of using a mechanical or chemical device to increase the amount of air, and consequently fuel, consumed by the engine. This also increases the volumetric efficiency of the engine and can be accomplished by using an air pump, such as a supercharger or a turbocharger, or NOS, Also see SUPERCHARGER and TURBOCHARGER.
GROUND CLEARANCE The vertical distance between level ground and the lowest fixed item on a car.
KNOCK See DETONATION.
LEAN FUEL MIXTURE An air/fuel mixture that contains less than the required amount of fuel to too much air.
NEGATIVE CAMBER See CAMBER.
NEGATIVE OFFSET Refers to wheel offset where the mounting face of the wheel hub is between the center line through the width wheel and the rear end of the wheel. Also see WHEEL OFFSET.
OVERSTEER A phenomenon that occurs when the rear wheels of a car do not track trajectory of the front wheels during cornering but slides out toward the outside of the turn, resulting in a tighter turn than what was intended and can also result in a spin. Also see UNDERSTEER.
PING See DETONATION.
PITCH CIRCLE DIAMETER (PDC) The diameter of a circle drawn through the centre of the wheel's stud holes and measured in millimeters. The PCD also indicates the number of studs the wheel has.
POSITIVE CAMBER See CAMBER.
POSITIVE-DISPLACEMENT SUPERCHARGER Superchargers that consist of two lobe rotors or two screws inside a housing and works by pumping air into the engine at a faster rate that the engine would normally ingest. These superchargers pump air at a fixed rate in relation to engine speed and supercharger size and therefore do not cause over boost. Also see BLOWER.
POSITIVE OFFSET Refers to wheel offset where the mounting face of the wheel hub is between the center line through the width wheel and the front end of the wheel. Also see WHEEL OFFSET
PRE-IGNITION The spontaneous combustion of the air/fuel mixture ahead of spark ignition. NOTE: Pre-ignition is not the same as "detonation", "knocking" or "pinging" and should not be confused with these terms. Also see DETONATION.
PROGRESSIVE SPRING RATE Refers to a spring rate that rises progressively as the spring load increases. Also see SPRING RATE.
REVERSION The flow of exhaust gasses back into the combustion chamber when the downward movement of the piston creates a vacuum in the cylinder during valve overlap, i.e., while the exhaust valve is closing.
RICH FUEL MIXTURE An air/fuel mixture that contains more than the required amount of fuel to too little air.
SHORT RAM INTAKE (SRI)-
SPRING RATE The force required to compress a linear spring one inch and expressed in lbs/in. Linear springs have a constant spring rate. Progressive springs have progressive spring rate that rise has the spring load increase.
STEERING GEOMETRY The related angles made by the front wheels when the vehicle is negotiating a turn. Sometimes used to describe the various angles made by the components of the front wheel turning arrangement, such as camber, caster, and toe-in. Also see CAMBER, CASTER and TOE ANGLE.
STOICHIOMETRIC CONDITION The chemically ideal air/fuel ratio of 14,7 mass of air to 1 mass of fuel. Also see AIR/FUEL RATIO.
SUPERCHARGER A mechanical forced induction device that is usually driven by a belt off the crankshaft. It is also referred to as a blower and used to increase the volumetric efficiency and power output of the engine by pumping air into the engine at a faster rate that the engine would normally ingest.
TOE ANGLE Toe angle is the direction in which the front wheels of the car are pointed. When both front wheels are parallel there is 0 degrees toe. When the front of the wheels is pointed inward, there is a positive degree toe called toe-in. When the front of the wheels is pointed outward, there is negative degree toe called toe-out. Also see TOE-IN and TOE-OUT.
TOE-IN Where the distance between the front of the two front wheels is less than the distance between the back of the two front wheels, causing the wheels to point inward. A slight amount of toe-in is usually required to offset forces that tend to spread the wheels out. Also see TOE-OUT.
TOE-OUT Where the distance between the front of the two front wheels is more than the distance between the back of the two front wheels, causing the wheels to point outward. Also see TOE-OUT.
TRACK WIDTH The distance between the centerlines of the left and right wheels on the same axle. Track width affects cornering as it resists the overturning moment due to the inertia force at the center of gravity (CG) and the lateral force at the tires. Also referred to as "wheel track" or simply as "track".
TURBOCHARGER A mechanical forced induction device that consists of a compressor-wheel and interconnected a turbine-wheel and is driven by the exhaust gasses of the engine. It is used to force compressed air into the engine, increasing the volumetric efficiency and power output of the engine.
TURBO LAG The lapse in time between the point when the accelerator pedal is depressed and the point at which the turbocharger produces boost pressure above atmospheric pressure.
TURBO TIMER An electronic device designed to keep the engine running for a pre-specified period of time in order to allow the turbo to cool-down and to prevent oil coking in the bearing chamber.
UNSPRUNG MASS The part of a car suspension that is not supported by the suspension springs and includes the brake system and the wheel and tires and other components directly connected to them.
UNDERSTEER A phenomenon that occurs when the front wheels of a car do not have the required level of grip to follow the trajectory of a turn and tends to slide out toward the outside of the turn, resulting in a wider turning circle than was intended. Also see OVERSTEER.
VALVE LIFT The distance a valve moves off the face of the valve seat. This can be measured in inches or millimeters.
VALVE OVERLAP The number of degrees of duration during which both the intake and exhaust valves are open. This is what causes roughness at idle or low RPM. VOLUMETRIC EFFICIENCY (VE) The amount of air/fuel mixture that is drawn or forced into a cylinder of the engine during the intake stroke, relative to the volume of that cylinder. The aim of engine modification is to improve Volumetric Efficiency.
WHEELBASE The distance between the centerlines of the front and the rear axles of a car.
WHEEL OFFSET The distance between the mounting face of the wheel hub and the center line through the width wheel. This can be negative if the mounting face is closer to the back of the wheel or positive if the mounting face is closer to the front of the wheel.
YAW The rotation around the vertical axis that passes through the center of gravity of a car.
YAW ANGLE The difference between a car's longitudinal axis and its true direction of motion when negotiating a turn.