User Guide

Dyno2000 Advanced Engine Simulation—67
Street engines driven hundreds-of-thousands of miles operate their valvetrain com-
ponents billions of cycles . If the overall camshaft and valvetrain design is good, a
precision micrometer will detect only negligible wear.
The camshaft controls the valve opening and closing points by the shape and
rotational location of the lobes. Most cams are ground to a precision well within one
crankshaft degree, ensuring that the valves actuate exactly when intended. Timing
variations of several degrees can develop in the cam drive, especially in chain-drive
systems, but racing gear drives reduce variations to within one or two crank degrees
of indicated timing. Camshaft lobes also determine how far the valves will lift off of
the valve seats by the height of the lobes (heal to toe height) and the multiplying
ratio of the rockerarms (if used). The rates at which the valves are accelerated open
and then returned to their seats are also “ground into” cam lobe profiles. Only a
limited range of contours will maintain stable valve motion, particularly with high-lift,
racing profiles. Unstable profiles or excessive engine speed will force the valvetrain
into “valve float,” leading to rapid component failure.
Valve Events
There are six basic cam timing events ground into the lobes of every camshaft.
These timing points are:
1—Intake Valve Opening (IVO) 2—Intake Valve Closing (IVC)
3—Exhaust Valve Opening (EVO) 4—Exhaust Valve Closing (EVC)
5—Intake Valve Lift 6—Exhaust Valve Lift
These six points can be “adjusted” somewhat (we’ll discuss which and how cam
timing events can be altered in the next section), but for the most part they are fixed
by the design of the cam. Other timing numbers are often discussed, but they are
always derived from the basic six events. Derivative events are:
7—Intake Duration 8—Exhaust Duration
9—Lobe Center Angle (LCA) 10—Valve Overlap
11—Int. Center Angle (ICA) 12—Exh. Center Angle (ECA)
Camshaft Modeling
Common “Cam Card” Timing
Before engine simula-
tions were widely used,
cam manufacturers
established a method-
ology for identifying
and classifying cam-
shafts. Unfortunately,
these “catalog” specs
place the emphasis on
the span between the
valve events rather
than on the events
themselves.