Technical Motorsports Jargon - Engine Terms

Technical terms are freely bandied around by those of us who have been involved in motorsports for a while. But every year there are thousands of newcomers to the sport who don't have a handle on even the basic terms. So if you're sometimes overwhelmed by the technical jargon used to describe engine building and performance, here's the lowdown

Bore/stroke ratio Bore diameter divided by stroke (e.g., small-block Chevy: 4-inch bore and 3.48 stroke. 4.00/3.48 = 1.149). Any number that is bigger than 1.0 is considered "over square" and any number smaller is "under square."

Cubic inches of displacement (cid) This is the "swept volume" of a cylinder multiplied by the number of cylinders. The swept volume is the amount of air displaced or inhaled as the piston moves from the top of the stroke to the bottom.

Deck height Distance from the centerline of the main bearings to the block face on which the cylinder head mounts.

Major thrust face The side of the bore that the rod is pushed into on the power stroke.

Minor thrust face The opposite side of the bore to the major thrust side.

Top Dead Center (TDC) The position of the piston at exactly the top of the stroke.

Bottom dead center (BDC) This is the position of the piston in the bore when at the bottom of its stroke.

Pin height The distance from the center of the wristpin to the part of the piston crown that comes flush (or nearly so) to the top of the block at TDC.

Piston Speed The average speed of the piston (usually in ft-min) throughout its stroke. This is equal to: (rpm x 2 x stroke)/12.

Rod/stroke ratio This term describes the geometric proportions of the rod and crank. The number is obtained by dividing the center-to-center distance on the rod by the stroke of the crank. For a small-block Chevy, this would be 5.7/3.48 = 1.638. Anything below 1.6 can be considered a short rod ratio while anything above about 1.9 is long.

Lobe centerline angle (LCA) This angle describes the number of degrees there are between the intake lobe centerline and the exhaust lobe centerline. It is the only cam term that is described in terms of cam rotation instead of crank rotation.

Cam advance/retard A cam is said to be "straight up" when it is timed into the engine such that the intake lifter/valve reaches full lift at the same angle after top dead center as the cam's lobe centerline angle (i.e., 110 LCA timed in to 110 degrees after TDC to full lift). If the cam is advanced, all the events happen sooner, so 4 degrees advanced would mean the cam was timed in at 106. "Retarded" is when everything happens later, so 2 degrees retarded would mean our 110 LCA cam reaches full lift 112 degrees after TDC.

Cam timing The physical act of timing the cam into the engine so that the valve events occur at the right angle in relation to crank rotation. This job requires a degree wheel and dial indicator.

Duration The angle the crank turns during the time the lifter is above a certain specified point over the profile's base circle. Duration is normally quoted at a lifter rise of 0.006 and 0.050 for a hydraulic cam and 0.020 (most often, but not always) and 0.050 for a solid. The lower lifter rise figures are often referred to as the "advertised" duration.

Hydraulic lifter This is a type of lifter that uses oil pressure to adjust the plunger in the lifter so as to take up all the clearance in the valvetrain, minimizing noise and eliminating the need to make periodic valve lash adjustments.

Intake centerline An imaginary line drawn through the midpoint of the intake lobe between the point at which it opens the valve and the point at which it closes the valve.

Lobe lift The difference in height between the peak on a cam lobe as compared to the lobe's base circle.

Overlap The number of degrees the crank turns at the end of the exhaust stroke and the beginning of the intake, when both the exhaust and intake valves are open. This phase is very important in a race engine, as it allows a negative pressure pulse in the exhaust to scavenge the combustion chamber of exhaust and start the intake flow sooner.

Solid lifter This type of lifter, unlike its hydraulic counterpart, has no moving internal parts. This means it has no self-adjusting capability and therefore needs to have the rockers adjusted to give a certain amount of working clearance.

Valve lash This refers to the working clearance between the tip of the rocker and the tip of the valve. This clearance is necessary for a solid lifter cam.