INSTALLED CENTERLINE ANGLE (ICA)
Sometimes, there is confusion between lobe separation angle (LSA), the topic above, and the cams' installed centerline angle (ICA). Both terms have to do with angles, lobes, and reference the peak lift point, and are somewhat related, but are two entirely different measurements. The LSA is a measurement of how far apart the peak lift points of the intake and exhaust lobes are ground into the camshaft. The cam has a LSA as soon as it's ground, or just sitting on the workbench. The installed centerline angle simply describes the installed position in the engine. It is a measurement the cam's phasing; the relative position of the cam vs. the engine's crank. This measurement is usually referenced by the crank degrees from TDC at which the cam's intake lobe reaches max lift.
Probably why these terms are so often confused, is that the numbers turn up in the same range of values. A cam ground with 110-degree lobe separation will read an installed centerline angle of 110 if installed "straight-up," or with no advance. Since the cam is connected to the crank via the timing chain, the cam's ICA, or phasing to the crank, can be adjusted forward or back, changing the installed centerline angle. This is called advancing or retarding the cam. The installed centerline is what is checked when a camshaft is degreed in.Most aftermarket cams are ground with some advance built in, typically about 4 degrees. Advancing the cam makes all the valve events happen earlier, and generally favors low rpm operation, helping idle quality, cylinder pressure, vacuum, and lower speed torque. Retarding the cam deteriorates these characteristics, though in some cases high rpm power may be enhanced. The real score here is that advance will almost always help improve performance as listed above, but most of the time retarding the cam will gain little if anything, even up top.
VALVE LASH AND LIFTER PRELOAD
What separates a hydraulic lifter from a solid tappet is the addition of an internal hydraulically operated plunger within the hydraulic lifter's body. The way the valvetrain is set up will depend upon whether the lifters are hydraulic or solid. Hydraulics generally run with pre-load, while solids must be set up with lash. Let's first consider a hydraulic. It's helpful to understand how the hydraulic mechanism works and what it does.
Oil pressure enters the lifter through an orifice in the lifter body, and flows through another orifice into the hollow body of the lifter plunger. A one-way check valve at the bottom of the plunger allows oil to fill the cavity below until all the valvetrain clearance gone, effectuating the hydraulic self-adjustment to zero lash. When the cam rotates into the lift cycle, the check valve at the base of the plunger closes under the pressure imparted by the valvespring, preventing the oil from being squeezed back out as the valve opens. At the top of the plunger of some hydraulic lifters is a metering valve or plate, which supplies oil to the pushrods for valvetrain oiling.
With the valvetrain installed (or adjusted), the pushrod compresses the plunger within its range of travel, and the hydraulic mechanism automatically zeros the lash. How far down the lifter plunger has been displaced at its base setting is called the lifter pre-load. The recommended pre-load with hydraulic lifters is usually in the range of 0.020-0.040 inch. Many stock engines came with non-adjustable valvetrains. Usually, this is not a problem when going with another hydraulic cam, but many things during engine building can alter the intended factory pre-load in these fixed systems. Things such as milling or decking the block or heads, the design of replacement lifters, gasket thickness, or the cam's base circle diameter can alter the pre-load. In these cases, the solution is custom-length pushrods, or making the change to adjustable rockers.With adjustable rockers, setting the pre-load is simply a matter of setting the lobe being adjusted to the base circle, and tightening the adjuster until the clearance in the valvetrain is just taken out (zero clearance). Then turn the adjuster in 1/2 - 3/4 of a turn and lock the adjuster down.
Solid lifters have no self-adjusting hydraulic mechanism, and need to run with clearance in the valvetrain. The lash specification is given on the cam card for a solid cam. The adjustment is made with the lobe being adjusted set on the base circle. A feeler gauge of a thickness matching the lash spec is inserted between the rocker and the valve tip, and then the adjuster is taken to zero lash and locked down. When the feeler gauge is removed, the lash will be set at the thickness of the feeler gauge. Lash is usually set cold when the engine is built, and then re-adjusted once the cam is run-in, with the engine hot. The hot setting will be more true to the engine conditions in operation.