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, which is referred to as the Stoichiometric Condition. This ratio provides the ideal combustion to produce the most power. Also see Stoichiometric Condition.
- 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.
- Balance Shaft
- A shaft designed to reduce engine vibration by rotating in the opposite to the crankshaft. This complicates crankshaft and conecting rod modifictions, such as lightening and stroking, slightly as the balance shaft needs to be modified to counter the subsequent change in rotational momentum.
- 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.
- Bottom Dead Center (BDC)
- The point in the rotation of the crankshaft at which the piston is at its lowest point in the cylinder. This lasts for a few degrees of crankshaft rotation. The kinetic energy of the fly wheel helps to overcome dead center.
- 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.
- 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.
- 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.
- Compression Ratio
- The ratio of the volume of the combustion chamber with the piston at top dead center (TDC) to the volume of the combustion chamber and the cylinder with the piston at bottom dead center (BDC). Higher compression ratios tend to increase engine efficiency and performance but also increases the possibility of detonation. Also see Top dead center and Top dead center.
- Crossflow Cylinder Head
- A cylinder head that has the inlet valves and manifolds on one side and the exhaust valves and manifold on the other side.
- 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".
- An electric conductor through which an electric current passes. In a spark plug the electrode is the center core passing through the ceramic insulator.
- 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.
- Helical gear
- A gear with teeth cut at an angle to the center line through the gear. This illuminates intermittent tooth-to-tooth operation of straight-cut gears as at least two teeth are engaged at any time. As a result, helical gears operate quieter than straight-cut gears, but they also absorb a small amount of power due to side thrust.
- An engine with a cylinder head that has a hemispherical combustion chamber that is favored for high performance and racing engines. Also see Hemispherical combustion chamber.
- Hemispherical combustion chamber
- A half round, dome-shaped combustion chamber. Cylinder heads with a hemispherical combustion chamber design has improved engine breathing as the chamber design permits the use of larger valves and straighter intake and exhaust ports.
- See Detonation.
- Lean Fuel Mixture
- An air/fuel mixture that contains less than the required amount of fuel to too much air. Also see Air/Fuel Ratio and Rich Fuel Mixture.
- 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.
- Octane Rating
- A unit of measurement used to indicate the extent to which gasoline fuel can be compressed before it ignites spontaneously and uncontrollably, causing detonation. The higher the octane rating, the more the fuel can be compressed before igniting spontaneously, and greater its the resistance to detonation. Also see Detonation
- 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.
- 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
- 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.
- 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. Also see Air/Fuel Ratio and Lean Fuel Mixture.
- 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.
- Stoichiomertic Condition
- The chemically ideal air/fuel ratio of 14,7 mass of air to 1 mass of fuel. Also see Air/Fuel Ratio.
- The swept movement of the piston caused by a full rotation of the crankshaft, from top dead center (TDC) to bottom dead center (BDC). In other words, the distance the piston travels from TDC to BCD. Also see Top dead center and Top dead center.
- Increasing the stoke of the engine by increasing the distance between the main bearing journal and the big end bearing journal. Also see Stroke.
- 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. Also see Blower.
- 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.
- 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.
- 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-In.
- Top Dead Center (TDC)
- The point in the rotation of the crankshaft at which the piston is at its highest point in the cylinder. This lasts for a few degrees of crankshaft rotation. The kinetic energy of the fly wheel helps to overcome dead center.
- 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".
- True Top Dead Center (True TDC)
- The position at which the big end journal of the crankshaft is at its highest point in the rotation of the crankshaft. It is measured by finding the exact midpoint in the duration of top dead center (TDC) and is used as a datum point for ignition timing and camshaft timing. Also see Top dead center.
- 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.
- 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.
- 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.
- 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. Also see Yaw.