Engine Assembly Basics -- Cam, Valvetrain, Top-End
In a shaft rocker system,...
In a shaft rocker system, the pivot point and valve tip are in a fixed position, so the geometry is also fixed by the design of the system. Changes can be made to raise and lower the pivot point, or move the rockers forward and back relative to the valves, but these kinds of changes involve precision machining and modification normally in the realm of the most advanced engine builders.
In a stud-mounted system, two of these variables depend on pushrod length, since the actual pivot point of the rocker will change with the pushrod length. With stud-mounted rockers, the pushrod length will affect the position of the rocker pad or roller across the valve tip. In a shaft or fixed-pedestal system, the valve tip position, as well as the rocker's pivot point, is fixed. With these systems, the geometry is essentially set by the rocker design and the valve length. Pushrod length in these installations comes down to achieving the required range or position in the rocker's adjuster. In custom installations, the pushrod length will often need to be determined by mocking up the valvetrain with checking pushrods, available from Powerhouse Tools and others.
With a fixed fulcrum rocker...
With a fixed fulcrum rocker arrangement, the only variable is the pushrod length, and it should be set to position the rocker adjuster in the range of travel recommended by the rocker manufacturer, typically with about one thread of the adjuster showing below the rocker body. With some rocker designs, oiling is dependent on the adjuster falling within a narrow specified range.
The rocker's sweep across...
The rocker's sweep across the valve tip can be found by marking the tip with layout dye, and rotating engine through several lift cycles. A centered pattern with minimal sweep is ideal. With stud-mounted rockers, the sweep and position of the roller depends upon the pushrod length. Longer pushrods generally move the pattern out, while shorter ones move it in.
Rocker clearance is an important...
Rocker clearance is an important consideration to be aware of. The rocker body needs to physically clear the spring and retainer throughout the range of travel. This is something that should not be taken for granted, especially with large springs and deeply dished or large diameter retainers. Springs such as COMP's Beehive design seen on this LS1 offer the greatest clearance.
With brutal race cam profiles,...
With brutal race cam profiles, the valve tips and rocker rollers take a severe beating. Lash caps top the valves with a very hard precision-ground flat surface, which provides a broader surface of contact between the rocker and valve, helping deal with the severe loads involved. Lash caps are typically .080-inch thick, and can also be used to correct minor rocker geometry problems by effectively lengthening the valve.
When ordering custom pushrods,...
When ordering custom pushrods, the best method is to use a checking pushrod and physically measure the required length with a 12-inch caliper. Note whether the pushrod tip has an oil hole, which introduces error in the effective length. If the pushrod features a cup end configuration, measure the overall length minus the cup depth to determine the effective length. Inform the pushrod supplier of the measurement, and the method of measuring. Choose a pushrod wall material suitable for the spring loads, and make certain to specify the correct tip size to match the rockers and lifters being used.
The amount of load a spring...
The amount of load a spring delivers is as much a function of its designed spring rate as its installed height. When ordering springs, a measurement of the available installed height is a key piece of information in selecting the proper springs. These Mopar big-block heads had 2.060-inch installed height. With the required spring cup, the installed height will be 2.00 inches. Simply ordering and installing a spring designed for a Chrysler big-block, which typically has an installed height of 1.880 inches will result in way too little seat load. A COMP PN 924 spring for a big-block Mopar delivers 125 lbs load at 1.880 inches, while at 2.000 inches, the load drops to 87 lbs, too weak for even a stock cam. Measuring and noting the installed height is the only way to determine if a spring will deliver the load required. Powerhouse has these handy height micrometers (PN POW101200), which make measuring installed height a breeze.
Some Things About Springs
As the seriousness of the performance effort increases, the more critical valvesprings become. In lower rpm performance engines, which may never see over 5,500 rpm, the most basic level of care in valvespring selection and installation will generally get you by. As rpms increase, springs become one of the most critical factors in wringing the performance potential from a combination. Things to become familiar with are the seat load, installed height, open load, spring rate, and coil bind clearance. Let's take them one at a time.The seat load is the amount of force the spring exerts with the valve closed. This affects the valve's potential to bounce on closing, which can cost serious power at high rpms. Springs are rated for load at a given height, referred to as the installed height. The more a spring is compressed, the greater the force, so the installed height directly affects the seat load; it's critical to know the installed height to have known amount of seat load.
Open load is the amount of force exerted by the spring when the cam is at peak lift, which is the point when the spring is compressed the furthest. The open load has to be enough to keep the lifter in contact with the cam as it goes "over the nose," yet not so high that it is beyond what the cam, lifters, and valvetrain can withstand for acceptable wear.
Spring rate is a rating on how "stiff" the spring is, or how much load is gained as the spring compresses, normally given in pounds per inch. A spring rated at 350 lbs/inch will gain 350 lbs for a theoretical inch of compression. How much the spring will gain for any amount of valve lift can be calculated by multiplying the lift by the spring rate. A 0.700-inch lift cam with a 600-lbs/inch rate spring will gain 0.700-in x 600 lbs/inch for a gain of 420 lbs. Add the amount gained to the seat load, and the open load is the result. For instance, if in our example the seat load was 200 lbs, gaining 420 lbs in opening will give 620 lbs open. Most spring suppliers publish spring rate specs. The coil bind height is the height of the spring when it's compressed down solid. Most spring manufacturers give specs on coil bind height. By measuring the actual installed height of a spring on a cylinder head, and subtracting the amount of valve lift, the compressed height of the spring at full lift can be calculated. Comparing this figure to the published coil bind height will give the clearance to coil bind. As an example, let's say we have a spring with an installed height of 1.800 inches. If the valve lift is .600 inch, the spring is compressed to 1.200 inches at full lift (1.800 in. - 0.600 in. = 1.200 in.). If the manufacturer's catalog lists the coil bind height at 1.100 inches, it's safe to say there is 0.100-inch coil bind clearance left (1.200 in. - 1.100 in. = 0.100 in.).
How much spring load is the right amount? It depends upon the radicalness of the cam profile; the type of cam - solid, hydraulic, or roller; the valvetrain weight; and the engine's rpm range. Typically, top experienced engine builders spec their own springs, but for the average enthusiast, it's best left to the cam manufacturer.
Although spring loads can...
Although spring loads can usually be found via the manufacturer's catalog, a spring tester such as the digital Moroso unit we are using here will provide all of the information needed. The spring can be checked directly for closed and open load just by compressing to the installed height and then compressing to that number minus the amount of valve lift.
The installed height can be...
The installed height can be adjusted downward by adding shims, typically available in 0.015, 0.030, and 0.060-inch thickness, and a wide range of inside and outside diameters. Spring cups and locators are usually .060-inch thick, and take away installed height.
There are a variety of retainers...
There are a variety of retainers and locks available having different degrees of strength and lightness. Selecting retainers begins with getting one that fits the spring. Retainer choice can also significantly affect the installed height, since retainers vary in the amount of "dish" in their profile, which alters their height. If more installed height is needed, specialty locks can be had which raise the retainer's position. Locks also vary from cheap stamped units to heavy-duty machined pieces heat-treated to carry extreme spring loads. Though most standard retainers and locks mate at a 7-degree angle, some aftermarket manufactures like COMP offer extreme-duty 10-degree pieces, drastically reducing the possibility of a lock pulling through a retainer. The 7- or 10-degree locks and retainers must always be used as a set.
Just as the retainer must...
Just as the retainer must match the spring at the top to properly secure and locate it, a spring cup should be used at the bottom to keep the valvespring from dancing around on the cylinder head side. Several styles of cups and locators are available, which register either to the outside of the spring, or on the inside of a dual spring assembly. We used COMP PN 4700 cups for this iron Mopar head, which locates a 1.550-inch spring by its outside diameter. As is often the case, spring seat machining is required on these production heads to fit the locator.