ABDC or After Bottom Dead Center: Any position of the piston in the cylinder bore after its lowest point in the stroke (BDC). ABDC is measured in degrees of crankshaft rotation after BDC. For example, the point at which the intake valve closes (IVC) may be indicated as 60-degrees ABDC. In other words, the intake valve would close 60 degrees after the beginning of the compression stroke (the compression stroke begins at BDC).

Air-Fuel Ratio: The proportion of air to fuel: by weight: that is produced by the carburetor or injector.

ATDC or After Top Dead Center: Any position of the piston in the cylinder bore after its highest point in the stroke (TDC). ATDC is measured in degrees of crankshaft rotation after TDC. For example, the point at which the exhaust valve closes (EVC) may be indicated as 30-degrees ATDC. In other words, the exhaust valve would close 30 degrees after the beginning of the intake stroke (the intake stroke begins at TDC).

Atmospheric Pressure: The pressure created by the weight of the gases in the atmosphere. Measured at sea level this pressure is about 14.69psi.

Back Pressure: A pressure developed when a moving liquid or gaseous mass passes through a restriction. "Backpressure" often refers to the pressure generated within the exhaust system from internal restrictions from tubing and tubing bends, mufflers, catalytic converters, tailpipes, or even turbochargers.

BBDC or Before Bottom Dead Center: Any position of the piston in the cylinder bore before its lowest point in the stroke (BDC). BBDC is measured in degrees of crankshaft rotation before BDC. For example, the point at which the exhaust valve opens (EVO) may be indicated as 60-degrees BBDC. In other words, the exhaust valve would open 60 degrees before the exhaust stroke begins (the exhaust stroke begins at BDC).

Big-Block: A generic term that usually refers to a V8 engine with a displacement that is large enough to require a physically "bigger" engine block. Typical big-block engines displace over 400 cubic inches.

Blowdown or Cylinder Blowdown: Blowdown occurs during the period between exhaust valve opening and BDC. It is the period (measured in crank degrees) during which residual exhaust gases are expelled from the engine before the exhaust stroke begins. Residual gasses not discharged during blowdown must be physically "pumped" out of the cylinder during the exhaust stroke, lowering power output from consumed "pumping work."

Bore or Cylinder Bore: The internal surface of a cylindrical volume used to retain and seal a moving piston and ring assembly. "Bore" is commonly used to refer to the cylinder bore diameter, unusually measured in inches or millimeters. Bore surfaces are machined or ground precisely to afford an optimum ring seal and minimum friction with the moving piston and rings.

Brake Horsepower (bhp): Brake horsepower (sometimes referred to as shaft horsepower) is always measured at the flywheel or crankshaft by a "brake" or absorbing unit. Gross brake horsepower describes the power output of an engine in stripped-down, "race-ready" trim. Net brake horsepower measures the power at the flywheel when the engine is tested with all standard accessories attached and functioning. Also see Horsepower, Indicated Horsepower, Friction Horsepower, and Torque.

Brake Mean Effective Pressure (bmep): A theoretical average pressure that would have to be present in each cylinder during the power stroke to reproduce the force on the crankshaft measured by the absorber (brake) on a dynamometer. The bmep present during the power stroke would produce the same power generated by the varying pressures in the cylinder throughout the entire four-cycle process.

BTDC or Before Top Dead Center: Any position of the piston in the cylinder bore before its highest point in the stroke (TDC). BTDC is measured in degrees of crankshaft rotation before TDC. For example, the point at which the intake valve opens (IVO) may be indicated as 30-degrees BTDC. In other words, the intake valve would open 30 degrees before the intake stroke begins (the intake stroke begins at TDC).

Cam Timing, @ 0.050-Lift: This method of determining camshaft valve timing is based on 0.050 inches of tappet rise to pinpoint timing events. The 0.050 inch method was developed to help engine builders accurately install camshafts. Lifter rise is quite rapid at 0.050-inch lift, allowing the cam to be precisely indexed to the crankshaft. Camshaft timing events are always measured in crankshaft degrees, relative to TDC or BDC.

Cam Timing, @ Seat-To-Seat: This method of determining camshaft timing uses a specific valve lift (determined by the cam manufacturer) to define the beginning or ending of valve events. There is no universally accepted valve lift used to define seat-to-seat cam timing, however, the Society of Automotive Engineers (S.A.E) has accepted 0.006-inch valve lift as its standard definition. Camshaft timing events are always measured in crankshaft degrees, relative to TDC.

Camshaft Advance/Retard: This refers to the amount of advance or retard that the cam is installed from the manufacturers recommended setting. Focusing on intake timing, an advanced cam closes the intake valve earlier. This setting typically increases low-end performance. The retarded cam closes the intake valve later which tends to help top end performance.

Camshaft Follower or Lifter: Usually a metal cylinder (closed at one end) that rubs against the cam lobe and converts the rotary motion of the cam to an up/down motion required to open and close valves, operate fuel pumps, etc. Cam followers (lifters) can incorporate rollers, a design that can improve reliability and performance in many applications. Roller lifters are used extensively in racing where valve lift and valve-lift rates are very high, since they can withstand higher dynamic loads. In overhead cam engines, the cam follower is usually incorporated into the rocker arm that directly actuates the valve; in this design push rods are eliminated.

Camshaft Grind: The shape of the cam lobe. Determines when the intake and exhaust valves open and close and how high they lift off of the seats. The shape also determines how fast the valves open and close, i.e., how much acceleration the valves and springs experience. High acceleration rate cams require large-diameter solid, mushroom, or roller lifters.

Camshaft Lift: The maximum height of the cam lobe above the base-circle diameter. A higher lobe opens the valves further, often improving engine performance. Lobe lift must be multiplied by the rocker ratio (for engines using rocker arms) to obtain total valve lift. Lifting the valve more than 1/3 the head diameter generally yields little additional performance. Faster valve opening rates add stress and increase valvetrain wear but can further improve performance. High lift rates usually require specially designed, high-strength components.

Camshaft Lobe: The eccentrically shaped portion of a camshaft on which the camshaft follower or lifter rides. The shape of intake and exhaust cam lobes are important engine design criterion. They directly affect engine efficiency, power output, the rate (how fast) the valves open and close, and control valvetrain life and maximum valvetrain/engine rpm.

Camshaft Timing: The rotational position of the camshaft, relative to the crankshaft, i.e., the point at which the cam lobes open and close the valves relative to piston position. Two common methods are used to indicate the location of valve events: the Seat-To-Seat and 0.050-inch timing methods. For simulation purposes, Seat-To-Seat timing values yield more accurate horsepower and torque predictions. Camshaft timing can be adjusted by using offset keys or offset bushings (or by redesigning the cam profile). Valve-to-piston clearance will vary as cam timing is altered; always ensure that adequate clearance exists after varying cam timing from manufacturer’s specifications. See Cam Timing @ Seat-To-Seat, Cam Timing @ 0.050-Inch Method.

Carburetor: A device that combines fuel with air entering the engine; capable of precision control over the air volume and the ratio of the fuel-to-air mixture.

Centerline: An imaginary line running through the center of a part along its axis, e.g., the centerline of a crankshaft running from front-to-back directly through the center of the main-bearing journals.

Closed Headers or Closed Exhaust System: Refers to an exhaust system that includes mufflers; not open to the atmosphere.

Combustion Chamber or Combustion Chamber Volume: The volume contained within the cavity or space enclosed by the cylinder head, including the "top" surfaces of the intake and exhaust valves and the spark plug. Not the same volume as the combustion space volume.

Combustion Space or Combustion Space Volume: The volume contained within the cylinder head, plus (or minus) the piston dome (or dish) volume, plus any volume displaced by the compressed head gasket, plus (or minus) any additional volume created by the piston not fully rising to the top of the bore (or extending beyond the top of the bore) of the cylinder at TDC. This volume is used to calculate compression ratio.

Compression Pressure: The pressure created in the cylinder when the piston moves toward top dead center (TDC) after the intake valve closes, trapping the induced charge (normally a fuel/air mixture) within the cylinder. Compression pressure can be measured by installing a pressure gauge in the cylinder in place of the spark plug and "cranking" the engine with the starter motor. To improve measurement accuracy, the throttle is usually held wide open and the remaining spark plugs are removed to minimize cranking loads and optimize pressures in the cylinder under test.

Compression Ratio: The ratio of the total volume enclosed in a cylinder when the piston is located at BDC compared to the volume enclosed when the piston is at TDC (volume at TDC is called the combustion space volume). The formula to calculate compression ratio is: (Swept Cylinder Volume + Combustion Space Volume)/Combustion Space Volume = Compression Ratio.

Compression Stroke: One of the four 180-degree full "sweeps" of the piston moving in the cylinder of a four-stroke, internal-combustion engine (originally devised by Nikolaus Otto in 1876). During the compression stroke, the piston moves from BDC to TDC and compresses the air/fuel mixture. Note: The 180-degree duration of the compression stroke is commonly longer than the duration between the intake valve-closing point and top dead center or ignition, sometimes referred to as the true "Compression Cycle." The compression stroke is followed by the power stroke.

Cubic Inch Displacement or CID: The swept volume of all the pistons in the cylinders in an engine expressed in cubic inches. The cylinder displacement is calculated with this formula: (Bore x Bore x Pi x Stroke x No.Cyl.)/4. When the bore and stroke are measured in inches, the engine displacement calculated in cubic inches.

Cylinder and Cylinder Bore: The cylinder serves three important functions in an internal-combustion (IC) engine: 1) retains the piston and rings, and for this job must be precisely round and have a uniform diameter (for performance applications 0.0005-inch tolerance is considered the maximum allowable); 2) must have a surface finish that ensures both optimum ring seal (smooth and true) and yet provides adequate lubrication retention to ensure long life for both the piston and rings; and 3) the cylinder bore acts as a major structural element of the cylinder block, retaining the cylinder heads and the bottom end components. The cylinder bore design, finish, and its preparation techniques are extremely important aspects of performance engine design.

Cylinder Block: The casting that comprises the main structure of an IC engine. The cylinder block is the connecting unit for the cylinder heads, crankshaft, and external assemblies, plus it houses the pistons, camshaft and all other internal engine components. The stability, strength, and precision of the block casting and machining are extremely important in obtaining optimum power and engine life. Cylinder blocks are usually made from a high grade of cast iron.

Cylinderhead: A component (usually made of cast iron or cast aluminum) that forms the combustion chambers, intake and exhaust ports: including water cooling passages: and provides support for valvetrain components, spark plugs, intake and exhaust manifolds, etc. The cylinderhead attaches to the engine block with several large bolts that squeeze a head gasket between the block deck and head surfaces; and when attached, the head becomes a load-carrying member, adding strength and rigidity to the cylinder block assembly. Modern cylinderhead designs fall into three major categories: 1) overhead-valve with wedge, canted-valve, or hemispherical combustion chambers; 2) single-overhead cam with wedge or hemispherical cambers; 3) double-overhead cam with hemispherical chambers.

Degree: 1) An angular measurement. A complete circle is divided into 360 degrees; equal to one crankshaft rotation; 180 degrees is one-half rotation. 2) A temperature measurement. The temperatures of boiling and freezing water are: in the Fahrenheit system 212 and 32 degrees; in the Celsius system 100 and 0 (zero) degrees.

Density: A measurement of the amount of matter within a known space or volume. Air density is the measurement of the amount of air per unit volume at a fixed temperature, barometric pressure, altitude, etc.

Detonation: The secondary ignition of the air/fuel mixture in the combustion space causing extreme pressures. Detonation is caused by low gasoline octane ratings, high combustion temperatures, improper combustion chamber shape, too-lean mixtures, etc. Detonation produces dangerously high loads on the engine, and if allowed to continue, will lead to engine failure. Detonation, unlike preignition, requires two simultaneous combustion fronts (fuel burning in two or more places in the combustion chamber at once); whereas preignition occurs when the fuel-air mix ignites (with single burning front) before the spark plug fires. Both preignition and detonation produce an audible "knock" or "ping," but detonation does not produce the rapid "wild pinging" noise that is typically associated with preignition. The extreme pressures of detonation can lead to preignition, but even worse the high temperatures of preignition can cause detonation.

Duration or Valve Duration: The number of crankshaft degrees (or much more rarely, camshaft degrees) of rotation that the valve lifter or cam follower is lifted above a specified height; either seat-to-seat valve duration measured at 0.006-, 0.010-inch or other valve rises (even 0.020-inch lifter rise), or duration measured at 0.050-inch lifter rise called 0.050-inch duration. Intake duration is a measure of all the intake lobes and exhaust duration indicates the exhaust timing for all exhaust lobes. Longer cam durations hold the valves open longer, often allowing increased cylinder filling or scavenging at higher engine speeds.

Dynamometer: A device used to measure the power output of rotating machinery. In it’s simplest terms, a dynamometer is a power-absorbing brake, incorporating an accurate method of measuring how much torque (and horsepower) is being absorbed. Braking is accomplished through friction (usually a hydraulic absorber) or by an electric dynamo (converts energy to electricity). Modern computer-controlled dynamometers for high-performance automotive use have sophisticated speed controls that allow the operator to select the rpm point or range of speeds through which the torque is to be measured. Then the operator opens the throttle and the dynamometer applies the precise amount of load to maintain the chosen rpm points; horsepower is read out directly on a gauge and/or computer screen.

Dynomation: A engine simulation program developed by V.P. Engineering that uses full wave-action analysis, currently using the Method Of Characteristics to provide solutions to the complex equations of wave dynamics.

Empirical or Empirical Testing: Meaning "after the fact" or "experimental," empirical testing involves actual "real-world" experiments to determine the outcome of component changes.

Exhaust Center Angle/Centerline or ECA: The distance in crank degrees from the point of maximum exhaust valve lift (on symmetric cam profiles) to TDC during the valve overlap period.

Exhaust Manifold: An assembly (usually an iron casting) that connects the exhaust ports to the remainder of the exhaust system. The exhaust manifold may include a heat-riser valve or port that heats the intake manifold to improve fuel vaporization.

Exhaust Ports: Cavities within the cylinder head that form the initial flow paths for the spent gases of combustion. One end of the exhaust port forms the exhaust valve seat and the other end forms a connecting flange to the exhaust manifold or header.

Exhaust Stroke: One of the four 180-degree full "sweeps" of the piston moving in the cylinder of a four-stroke, internal-combustion engine (originally devised by Nikolaus Otto in 1876). During the exhaust stroke, the piston moves from BDC to TDC and forces exhaust gases from the cylinder into the exhaust system. Note: The 180-degree duration of the exhaust stroke is commonly shorter than the period during which the exhaust valve is open, sometimes referred to as the true "Exhaust Cycle." The exhaust stroke is followed by the intake stroke.

Exhaust Valve Closing or EVC: The point at which the exhaust valve returns to its seat, or closes. This valve timing point usually occurs early in the intake stroke. Although EVC does not have substantial effects on engine performance, it contributes to valve overlap (the termination point of overlap) that can have a significant effect on engine output.

Exhaust Valve Duration: See Duration

Exhaust Valve Lift: See Valve Lift

Exhaust Valve Opening or EVO: The point at which the exhaust valve lifts off of its seat, or opens. This valve timing point usually occurs late in the power stroke. EVO usually precedes BDC on the power stroke to assist exhaust-gas blowdown. This EVO timing point can be considered the second most important cam timing event.

Exhaust Valve: The valve located within the cylinder head that control the flow of spent gases from the cylinder. The exhaust valves are precisely actuated (opened and closed) by the camshaft, usually through lifters, pushrods, and rockerarms. Exhaust valves must withstand extremely high temperatures (1500 degrees-F or higher) and are made from special steels, e.g., SAE J775 that has excellent strength at high temperatures and good resistance to corrosion and wear.

Filling & Emptying Multidimensional Simulation: This engine simulation technique includes multiple models (e.g., thermodynamic, kinetic, etc.), and by dividing the intake and exhaust passages into a finite series of sections it describes mass flow into and out of each section at each degree of crank rotation. The Filling And Emptying method can accurately predict average pressures within sections of the intake and exhaust system and dynamically determine VE and engine power. However, the basic Filling And Emptying model can not account for variations in pressure within individual sections due to gas dynamic effects. See Gas-Dynamic Multidimensional Simulation.

Finite-Amplitude Waves: Pressure waves of higher energy levels higher than acoustic waves. Finite-amplitude waves exhibit complex motions and interactions when traveling through engine passages. These actions make their mathematical analysis very complex.

Flat-Tappet Lifter: A camshaft follower having a flat surface at the point of contact with the cam lobe. Flat-tappet lifters actually have a shallow convex curvature at their "face" to allow the lifter to rotate during operation, extending the working life.

Flow Bench and Flow-Bench Testing: A flow bench is a testing fixture that develops a precise pressure differential to either "suck" our "blow" air through a cylinder head or other engine component. A flow bench determines the flow capacities (restrictions) of cylinder head ports and valves and assists in the analysis of alterations to port contours.

Four-Cycle Engine: Originally devised by Nikolaus Otto in 1876, the four-cycle engine consists of a piston moving in a closed cylinder with two valves (one for inlet and one for outlet) timed to produce four separate strokes, or functional cycles: Intake, Compression, Power, and Exhaust. Sometimes called the "suck, squeeze, bang, and blow" process, this technique: combined with a properly atomized air/fuel mixture and a precisely timed spark ignition: produced an engine with high efficiency and power potential. The software discussed in this book is designed to simulate the functional processes of a four-cycle engine.

Friction Horsepower (Fhp): The power absorbed by the mechanical components of the engine during normal operation. Most frictional losses are due to piston ring pressure against the cylinder walls. Frictional power losses are not easily measured, however, they can be accurately calculated knowing the brake horsepower (from dyno testing) and the indicated horsepower (from pressure measurements). Also see Indicated Horsepower and Brake Horsepower.

Friction Mean Effective Pressure (Fmep): A theoretical average pressure that would have to be present in each cylinder during the power stroke to overcome the power consumed by friction within the engine. Fmep is usually calculated by first determining the Indicated Mean Effective Pressure (Imep): the maximum horsepower that can be produced from the recorded cylinder pressures. The Brake Mean Effective Pressure (Bmep) is then measured by performing a traditional "dyno" test. The Fmep is calculated by finding the difference between the Imep and the Bmep: Fmep = Imep - Bmep. Fmep can also be directly measured with a motoring (electric) dyno.

Friction: A force that opposes motion. Frictional forces convert mechanical motion into heat.

Full Three-Dimensional (CFD) Simulation: This highly advanced engine simulation technique incorporates multiple models (e.g., thermodynamic, kinetic, etc.), including full three-dimensional modeling that subdivides an area, such as the combustion chamber or port junction, into a series of volumes (or cells) through which the model solves the differential equations of thermodynamics and fluid flow (using Computational Fluid Dynamics: CFD). The interaction of these cells can reveal very subtle design features within the induction and exhaust systems. It can thoroughly evaluate their effect on horsepower, fuel efficiency, and emissions throughout the rpm range. This simulation is not available as a commercial program and it currently remains a "laboratory-only" tool.

Gas-Dynamic Multidimensional Simulation: This engine simulation technique includes multiple models (e.g., thermodynamic, kinetic, etc.), plus powerful finite-wave analysis techniques that account for variations in pressures within individual sections of the ports due to gas dynamic effects. This detailed, highly math-intensive technique can predict engine power with remarkably high accuracy. The Dynomation program from V.P. Engineering is an example of a program using this simulation method.

Helmholtz Resonator: A device that increases the amplitude of pressure wave through a resonance phenomenon (an effect similar to the deep "whir" produced when air is blown over the neck of a jug). In some cases, the induction system in an IC engine can be modeled by employing Helmholtz resonance equations.

Horsepower: Torque measures how much work (an engine) can do, power is the rate-based measurement of how fast the work is being done. Starting with the static force applied at the end of a torque arm (torque), then multiplying this force by the swept distance through which the same force would rotate one full revolution finds the power per revolution: Power Per Revolution = Force or Weight x Swept Distance. James Watt (1736-1819) established the current value for one horsepower: 33,000 pound-feet per minute or 550 pound-feet per second. So horsepower is currently calculated as: Horsepower = Power Per Revolution/33,000, which is the same as Horsepower = (Torque x 2 x Pi x RPM)/33,000, or simply: Horsepower = (Torque x RPM)/5,252. The horsepower being calculated by these equations is just one of several ways to rate engine power output. Various additional methods for calculating or measuring engine horsepower are commonly used (to derive friction horsepower, indicated horsepower, etc.), and each technique provides additional information about the engine under test.

Hydraulic Lifter: See Lifters, Hydraulic

IC Engine: See Internal Combustion engine.

Inches of Mercury and Inches of Water: A standard method of pressure measurement, where pressures are compared to atmospheric or ambient pressure. Inches of displacement are recorded for a water or mercury column measured in a "U" shaped tube with one end open to the air and the other end connected to the test pressure. Commonly called a manometer, this pressure comparison device is quite sensitive and accurate. When mercury is used in the manometer tube, one psi differential from atmospheric pressure will displace 2.04-inches of mercury. However, when water is the liquid in the "U-tube," a substantial increase in pressure sensitivity is obtained: one psi will displace 27.72 inches of water. A water manometer is used to measure small vacuum and pressure signals.

Indicated Horsepower (Ihp): is the maximum power that a particular engine can theoretically produce. It is calculated from an analysis of the gas pressures measured by installing pressure transducers in the cylinders throughout the entire four-cycle process (with no losses due to friction). Also see Brake Horsepower and Friction Horsepower.

Indicated Mean Effective Pressure (Imep): A theoretical average pressure that would have to be present in each cylinder during the power stroke to generate the maximum horsepower possible from the pressures recorded within the cylinder of an engine during an actual dyno test. The Imep pressure assumes that the recorded pressures within the test engine will be entirely converted into motive force (with no losses due to friction).

Induction Airflow: The airflow rating (a measurement of restriction) of a carburetor or fuel injection system. Four-barrel carburetors are rated by the measured airflow when the device is subjected to a pressure drop equal to 1.5-inches of Mercury. Two-barrel carburetors are tested at 3.0-inches of Mercury.

Induction System: Consists of the carburetor or injection system and the intake manifold. The intake manifold can be of many designs such as dual plane, single plane, tunnel ram, etc.

Intake Centerline Angle: The distance in crank degrees from the point of maximum intake valve lift (on symmetric cam profiles) to TDC during the valve overlap period.

Intake Stroke: One of the four 180-degree full "sweeps" of the piston moving in the cylinder of a four-stroke, internal-combustion engine (originally devised by Nikolaus Otto in 1876). During the intake stroke, the piston moves from TDC to BDC and inducts (draws in by lowering the pressure in the cylinder) air/fuel mixture through the induction system. Note: The 180-degree duration of the intake stroke is commonly shorter than the period during which the intake valve is open, sometimes referred to as the true "Intake Cycle." The intake stroke is followed by the compression stroke.

Intake Valve Closing or IVC: Considered the most important cam timing event. The point at which the intake valve returns to its seat, or closes. This valve timing point usually occurs early in the compression stroke. Early IVC helps low-end power by retaining air/fuel mixture in the cylinder and reducing charge reversion at lower engine speeds. Late IVC increases high-speed performance (at the expense of low speed power) by allow additional charge to fill the cylinder from the ram-tuning effects of the induction system at higher engine speeds.

Intake Valve Duration: See Duration

Intake Valve Lift: See Valve Lift

Intake Valve Opening or IVO: The point at which the intake valve lifts off of its seat, or opens. This valve timing point usually occurs late in the exhaust stroke. Although IVO does not have a substantial effect on engine performance, it contributes to valve overlap (the beginning point of overlap) that can have a significant effect on engine output.

Internal Combustion Engine: An engine that produces power from the combustion and expansion of a fuel-and-air mixture within a closed cylinder. Internal-combustion engines are based on two methods of operation: two cycle and four cycle. In each method, a mixture of fuel and air enters the engine through the induction system. A piston compresses the mixture within a closed cylinder. A precisely timed spark ignites the charge after it is compressed. The explosive burning produces very high temperatures and pressures that push the piston down and rotate the crankshaft, generating a motive force. Also see combustion space, compression ratio, compression stroke, power stroke, exhaust stroke, and intake stroke.

Lifters, Hydraulic Flat-Tappet: A camshaft follower having a flat surface at the point of contact with the cam lobe. Flat-tappet lifters actually have a shallow convex curvature at their "face" to allow the lifter to rotate during operation, extending the working life. A hydraulic lifter incorporates a mechanism that automatically adjusts for small changes in component dimensions, and usually maintains zero lash in the valvetrain. Hydraulic lifters also offer a slight "cushioning" effect and reduce valvetrain noise.

Lifters, Roller Solid Or Hydraulic: A camshaft follower having a round, rolling element used at the contact point with the cam lobe. A hydraulic lifter incorporates a mechanism that automatically adjusts for small changes in component dimensions, and usually maintains zero lash in the valvetrain. Hydraulic lifters also offer a slight "cushioning" effect and reduce valvetrain noise. Solid lifters lack this hydraulic adjusting mechanism and require a running clearance in the valvetrain, usually adjusted by a screw or nut on the rockerarm.

Lifters, Solid Flat-Tappet: A camshaft follower having a flat surface at the point of contact with the cam lobe. Flat-tappet lifters actually have a shallow convex curvature at their "face" to allow the lifter to rotate during operation, extending the working life. Solid lifters lack an automatic hydraulic adjusting mechanism and require a running clearance in the valvetrain, usually adjusted by a screw or nut on the rockerarm. Solid lifter cams usually generate more valvetrain noise than hydraulic-tappet cam.

Lobe-Center Angle or LCA: The angle in cam degrees from maximum intake lift to maximum exhaust lift. Typical LCAs range from 100 to 116 camshaft degrees (or 200 to 232 crank degrees).

Multi Dimensional: As it refers to engine simulation programs, multi dimensional indicates that the simulation is based on multiple models, such as thermodynamic and kinetic, plus the multidimensional geometric description of inlet and outlet passages and a dynamic model of induction and exhaust flow. Also see Quasi Dimensional and Zero Dimensional.

Negative Pressure: A pressure below atmospheric pressure; below 14.7psi absolute. Very low pressures are usually measured by a manometer. See Inches of Water.

Normally Aspirated: When the air-fuel mix is inducted into the engine solely by the lower pressure produced in the cylinder during the intake stroke; aspiration not aided by a supercharger.

Otto-Cycle Engine: See Four-Cycle Engine

Overlap or Valve Overlap: The period, measured in crank degrees, when both the exhaust valve and the intake valve are open. Valve overlap allows the negative pressure scavenge wave to return from the exhaust and begin the inflow of air/fuel mixture into the cylinder even before the intake stroke begins. The effectiveness of the overlap period is dependent on engine speed and exhaust "tuning."

Pocket Porting: Relatively minor porting work performed below the valve seat and in the "bowl" area under the valve head. These changes, while straightforward, can produce a significant improvement in airflow and performance. Proper contours must be maintained, particularly below the valve seat, to produce the desired results.

Porting or All-Out Porting: Aggressive porting work performed to the passages within the cylinderhead with intention of optimizing high-speed airflow. Often characterized by large cross-sectional port areas, these ports generate sufficient flow velocities only at higher engine speeds; low speeds produce weak ram-tuning effects and exhaust scavenging waves. This porting technique is a poor choice for low-speed power and street applications.

Pounds Per Square Inch: See PSI.

Power Stroke: One of the four 180-degree full "sweeps" of the piston moving in the cylinder of a four-stroke, internal-combustion engine (originally devised by Nikolaus Otto in 1876). During the power stroke, the piston moves from TDC to BDC as the burning air/fuel mixture forces the piston down the cylinder. Note: The 180-degree duration of the power stroke is commonly longer than the duration between top dead center and the exhaust-valve opening point, sometimes referred to as the true "Power Cycle." The power stroke is followed by the exhaust stroke.

Pressure Crank-Angle Diagram: Also called a "Pie-Theta "diagram, the crank-angle diagram is a plot of the indicated cylinder pressures vs. the angular position of the crankshaft during the entire four-cycle process. This diagram provides an easily understood view of the widely varying pressures in the cylinder. Also see Pressure-Volume Diagram.

Pressure: A force applied to a specific amount of surface area. A common unit of pressure is psi, i.e., pounds per square inch. The force that develops the pressure is sum total of all the slight "nudges" on a surface generated by each molecule striking the surface; the greater number of impacts or the more violent each impact, the greater the pressure. Therefore, the pressure increases if the same number of molecules are contained in a smaller space (greater number of impacts per unit area) or if the molecules are heated (each impact is more violent). Also see psi.

Pressure-Volume Diagram: Also called a PV (pronounced Pee-Vee), or "indicator" diagram, the pressure-volume diagram plots indicated pressure against the displaced volume in the cylinder. A PV diagram has the remarkable feature of isolating the work consumed from the work developed by the engine. The area within the lower loop, drawn in a counterclockwise direction, represents the work consumed by the engine "pumping" the charge into the cylinder and forcing the exhaust gasses from the cylinder. The upper loop area, drawn in a clockwise direction, indicates the work produced by the engine from pressures generated by expanding gasses after combustion.

PSI or Pounds Per Square Inch: A measure of force applied on a surface, e.g., the force on the wall of a cylinder that contains a compressed gas. A gas compressed to 100 psi would generate a force of 100 pounds on each square inch of the cylinder wall surface. In other words, psi equals the force in pounds divided by the surface area in square inches. Also see pressure.

Restriction: A measure of the resistance to flow for (usually) a liquid or gas. Exhaust or intake flow restriction can occur within tubing bends, within ports, manifolds, etc. Liquid restriction can occur in needle-and-seat assemblies, fuel pumps, oil filters, etc. Some restriction is always present in a flowing medium.

Roller Lifter: See Lifters, Roller

Roller Tappet Lifter: See Lifters, Roller

RPM: Revolutions Per Minute. A unit of measure for angular speed. As applied to the IC engine, rpm indicates the instantaneous rotational speed of the crankshaft described as the number of crank revolutions that would occur every minute if that instantaneous speed was held constant throughout the measurement period. Typical idle speeds are 300 to 800rpm, while peak engine speeds can reach as high as 10,000rpm or higher in some racing engines.

Seat-To-Seat Cam Timing Method: See Cam Timing, @ Seat to Seat.

Simulation and Engine Simulation: A engine simulation process or program that attempts to predict real-world responses from specific component assemblies by applying fundamental physical laws to "duplicate" or simulate the processes taking place within the components.

Smallblock: A generic term that usually refers to a V8 engine with a displacement small enough to be contained within a "small" size engine block. Typical smallblock engines displace under 400 cubic inches.

Solid Lifter: See Lifter, Solid.

Stroke: The maximum distance the piston travels from the top of the cylinder (at TDC) to the bottom of the cylinder (at BDC), measured in inches or millimeters. The stroke is determined by the design of the crankshaft (the length of the stroke arm).

Top Dead Center or TDC: The position of the piston in the cylinder bore at its uppermost point in the stroke. Occurs twice within the full cycle of a four-stroke engine; at the start of the intake stroke and 360 degrees later at the end of the compression stroke.

Torque: The static twisting force produced by an engine. Torque varies with the length of the "arm" at which the twisting force is measured. Torque is a force times the length of the measurement arm: Torque = Force x Torque Arm, where Force is the applied or the generated force and Torque Arm is the length through which that force is applied. Typical torque values are ounce-inches, pound-feet, etc.

Valve Head and Valve Diameter: The large end of an intake or exhaust valve that determines the diameter. Valve head temperature can exceed 1200 degrees F during engine operation and a great deal of that heat is transferred to the cylinder head through the contact surface between the valve face and valve seat.

Valve Lift Rate: A measurement of how fast (in inches/degree) the camshaft raises the valve off of the valve seat to a specific height. If maximum valve lift is increased but the duration (crankshaft degrees) that the valve is held off of the valve seat is kept the same, then rate at which the valve opens must increase (same time to reach a higher lift). High lift rates can produce more horsepower, however, they also increase stress and valvetrain wear.

Valve Lift: The distance the valve head raises off of the valve seat as it is actuated through the valvetrain by the camshaft. Maximum valve lift is the greatest height the valve head moves off of the valve seat; it is the lift of the cam (lobe height minus base-circle diameter) multiplied by the rockerarm ratio.

Valve Motion Curve or Valve Displacement Curve: The movement (or lift) of the valve relative to the position of the crankshaft. Different cam styles (i.e., flat, mushroom, roller) typically have different displacement curve acceleration rates. Engine simulation programs calculate a valve motion curve from valve event timing, maximum valve lift, and other cam timing specifications.

Volumetric Efficiency: Is calculated by dividing the mass of air inducted into the cylinder between IVO and IVC divided by the mass of air that would fill the cylinder at atmospheric pressure (with the piston at BDC). Typical values range from 0.6 to 1.2, or 60% to 120%. Peak torque always occurs at the engine speed that produced the highest volumetric efficiency.

Work and Net Work: Work is the energy required to move an object over a set distance. Both motion and force must be present for work to occur. In an IC engine, work is developed from pressures within the cylinder acting on the face of the piston (producing a force) times the distance through which the piston travels. In the internal combustion engine some of the work produces power output (such as pressures producing piston movement during the power stroke) and some of the work is negative (like compressing the fresh charge on the compression stroke). The difference between the positive and negative work is the net work produced by the engine.

Copyright (c) 1997-2023 by Motion Software, Inc. All Rights Reserved. No part of this glossary may be copied or reproduced without the explicit written permission of Motion Software, Inc.

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