Valvetrain Variables
Setting up the valvetrain can be one of the most complex and time-consuming aspects of a high-end engine build. javascript:preview_story_session()Often, professional engine builders will need to extensively blueprint the valvetrain geometry in the course of a custom buildup. Within the scope of this story, we are addressing the more basic considerations, dealing with traditional valvetrains. OEM valvetrain layouts came in a variety of styles, including stud-mounted, shaft-mounted, and pedestal valve-gear. A variety of aftermarket systems cross over from the OEM configuration to an entirely different style of valvetrain. With such a wide variety of systems the permutations are nearly endless. The key rules are to use the correct length pushrods, ensure proper motion and clearance, and maintain as close to central contact on the valve tip as possible with a minimum of lateral sweep. There are three relevant geometric points in a valvetrain, the position of the rotational center of the rocker, and the contact points where the rocker picks up the pushrod and valve tip.

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.

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.