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Symbol(s) | Event | Definition, description, limits |
TDC | top dead center | 0°, 360° & 720° crankshaft position; the highest point of piston travel, but not necessarily the top of the cylinder. This is alternately the top on the ignition stroke, and the top on the overlap stroke. |
BDC | bottom dead center | 180° & 540° crankshaft position; the lowest point of piston travel, but never the bottom of the cylinder. This is alternately the bottom on the intake stroke, and the bottom on the exhaust stroke. |
BTDC | before top dead center | The intake valve’s opening position is located in this range (but not always, when given @ .050” lift, especially with very mild cams). The spark advance position is always located in this range. |
ABDC | after bottom dead center | The intake valve’s closing position is always located in this range. |
BBDC | before bottom dead center | The exhaust valve’s opening position is always located in this range. |
ATDC | after top dead center | The exhaust valve’s closing position is located in this range (but not always, when given @ .050” lift, especially with very mild cams). |
IVO, IO | intake valve opens (at) | It’s always BTDC; in crankshaft degrees. It’s adjustable, but only by rotating the entire camshaft. |
IVC, IC | intake valve closes (at) | It’s always ABDC; in crankshaft degrees. It’s adjustable, but only by rotating the entire camshaft. |
In-Dur | intake duration | The total of IVO + 180° + IVC; in crankshaft degrees. |
EVO | exhaust valve opens (at) | It’s always BBDC; in crankshaft degrees. It’s adjustable, but only by rotating the entire camshaft. |
EVC | exhaust valve closes (at) | It’s always ATDC; in crankshaft degrees. It’s adjustable, but only by rotating the entire camshaft. |
Ex-Dur | exhaust duration | The total of EVO + 180° + EVC; in crankshaft degrees. |
ICL | intake lobe centerline | The half-way point as to duration in degrees, frequently (but not always) the maximum lift point as well. Most frequently located between 100-120° ATDC (intake) and 100-120° BTDC (exhaust). |
ECL | exhaust lobe centerline |
LCA, LSA | lobe center angle, lobe separation angle | The separation in crankshaft degrees between the intake and exhaust lobe centerlines (ICL, ECL), calculated as (ICL + ECL) ÷ 2. Most frequently between 100-120°. Smaller LSA/LCA increases overlap. Lower numbers are hotter. Also called the cam lobe “phasing”. This is not an adjustment. |
OL, O/L | overlap | The range around TDC, 360° away from the power stroke, when both valves are partially open, called the “cross-over point” 50 years ago. OL duration = IVO + EVC; in crankshaft degrees. |
DCR | dynamic compression ratio | Static (nominal) compression ratio, measured from the IVC point rather than full stroke. |
Event | Definition, description, limits |
Camshaft | A long iron or steel cylinder, incorporating a “lobe” or eccentric for each intake or exhaust valve. |
Lobe height | The eccentric highest point of an individual cam lobe; the difference between its top-to-bottom and left-to-right dimensions. |
Lift | The distance the valve opens. It’s the lobe height × the rocker arm ratio. |
Nominal, official,
theoretical, or
“paper” duration |
The total time in camshaft degrees that the valve is open, typically given at a very low lift such as the lash point or .006” lift, always larger than the .050” duration (below). |
.050” duration | The time in camshaft degrees that a valve is open .050” or more, always less than nominal duration. |
Intensity | The difference in degrees between the nominal duration and the .050” duration, a smaller number is hotter. |
Tappet, lifter | This rides directly on the cam lobe; the pushrod seats in a recess at the top. |
Valve face | The bottom of the valve opposite the stem; this part of the valve head is inside the combustion chamber. |
Valve margin | The small area between the valve seat and the face; it’s parallel to the stem, almost always between .040” and .100” wide. |
Valve seat | The angled part of the valve or head that forms the mating & sealing surfaces. The two seats align, generally at the same angle (but not always the same intake angle and exhaust angle) and contact each other when the valve is closed. |
Valve bowl, throat | The circular part of the port area just above the seat, roughly parallel to the valve stem. |
Valve stem | The rod-shaped part of the valve that rides inside the valve guide; its OD is slightly smaller than the valve guide’s ID. |
Valve head | The disc-shaped part of the valve that, when seated, closes off the port from the combustion chamber. |
Valve guide | A cast-iron insert in the head; supports and aligns the valve stem; its ID is slightly larger than the valve stem’s OD. |
Valve guide boss | The raised area in the port (part of the head casting) which surrounds & supports the valve guide. |
Valve curtain area | The product of the valve’s head diameter and its lift, calculated as: D × Pi × L; in square inches. |
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