Engine Questions

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Glossary Terms:

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.
Atmospheric Pressure: The pressure created by the weight of the gases in the atmosphere. Measured at sea level this pressure is about 14.69psi.
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).
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.
Blow down or Cylinder Blow down: Blow down 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 blow down 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 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 valve train wear but can further improve performance. High lift rates usually require specially designed, high-strength components.
 
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 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 valve train life and maximum valve train/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.
Cylinder head: 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 valve train components, spark plugs, intake and exhaust manifolds, etc. The cylinder head 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 cylinder head 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.
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.
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 its 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.
Electronic Fuel Injection (EFI): This type of system uses computer-controlled fuel injectors to spray fuel into the engine rather than mechanically controlled injectors or a carburetor. EFI comes in several varieties: "throttle body injection" (TBI), "multi-port injection" (MFI) or Sequential Fuel Injection (SFI). Electronic fuel injection is considered to be superior to carburetion because it allows more precise fuel metering for easier starting, lower emissions, better fuel economy and performance.
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: 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 rocker arms. 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.
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.
Friction: A force that opposes motion. Frictional forces convert mechanical motion into heat.
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
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.
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 valve train. Hydraulic lifters also offer a slight "cushioning" effect and reduce valve train 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 valve train. Hydraulic lifters also offer a slight "cushioning" effect and reduce valve train noise. Solid lifters lack this hydraulic adjusting mechanism and require a running clearance in the valve train, usually adjusted by a screw or nut on the rocker arm.
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 valve train, usually adjusted by a screw or nut on the rocker arm. Solid lifter cams usually generate more valve train 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.
Multi-port Fuel Injection (MFI): A type of fuel injection system that has one injector for each engine cylinder. Each injector sprays its fuel directly into the intake port in the cylinder head. Multi-port fuel injection is considered to be the "hot" setup because it provides better cylinder-to-cylinder fuel distribution for more power.
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.
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 cylinder head 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.
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.
 
Small block: 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 small block 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).
Throttle Body Injector (TBI) A type of electronic fuel injection system that uses a single injector or pair of injectors mounted in a centrally located throttle body. The throttle unit resembles a carburetor except that there is no fuel bowl, float or metering jets. Fuel is sprayed directly into the throttle bore(s) by the injector(s).
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 Lift: The distance the valve head raises off of the valve seat as it is actuated through the valve train 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 rocker arm ratio.