Whether you are building a street performance engine, a killer drag motor or a circle track powerplant, the valvetrain requires special attention if you are going to maximize reliability and performance. Most stock valvetrain components are only good up to about 5,500 rpm. Beyond that point, upgrades are necessary to handle the higher speeds and loads.
The most important parts in the valvetrain are the valve springs, rocker arms, pushrods and lifters.
Even though the camshaft turns at only half the speed of the crankshaft (one revolution of the cam for every two revolutions of the crank), engine speed reaches a point where the springs cannot pull the valves shut quickly enough to keep the lifters on the cam. Instead of following the lobes back down to the base circle, the lifters begin to kick off their lobes. And the steeper the profile of the lobes, the worse the problem becomes as rpms go up.
When the springs cannot keep up with the cam, the lifters bang back down on the cam and bounce slightly, causing the valves to also bounce as they seat. In addition to increasing wear and the likelihood of fatigue failure, valve bounce also screws up airflow into and out of the combustion chamber and hurts high rpm performance.
If engine speed continues to increase, the point is soon reached where the springs cannot close the valves fast enough before they start to open again. The valves begin to "float" (stay open), which allows compression to blow right past the open valves. The engine begins to misfire, and if the driver does not back off on the throttle he runs the risk of a valve kissing a piston goodbye.
Lighter valves reduce the risk of valve bounce and float by reducing the mass of the valvetrain. For street engines, valves with smaller stems and thinner heads can reduce unwanted weight. For serious performance engines and customers who can afford the best, titanium valves are the only way to go.
Stiffer springs are a must for any performance engine. In fact, the springs are probably the most crucial part of the entire valvetrain in terms of rpm potential and durability.
Stiffer springs exert more force to keep the lifters in constant contact with their lobes and to overcome the increased momentum of the valves, rocker arms and pushrods at higher RPMs. But, springs that are too stiff for an application can create just as many problems as ones that are too weak. A really stout set of springs will increase cam lobe and lifter wear, especially with a flat tappet cam. Roller lifters are much better able to withstand higher valve spring pressures. For more information about cam and lifter wear in performance engines with flat tappet camshafts, see ZDDP - What is it & Why do you need it?.
Stiffer valve springs may also over stress stock rocker arms, pushrods, spring retainers and keepers. This requires beefing up these parts with stronger aftermarket performance components that can handle higher loads. For high revving engines, a shaft style rocker arm system provides the best support and stiffness.
If stiffer springs are necessary for a performance engine, follow the valve spring recommendations of the cam manufacturer. Better yet, buy a cam kit that also includes new lifters and new valve springs. This way you will know the parts are properly matched.
For small-block street/strip flat tappet cams with up to about a .450 inch lift, you can usually get by with stock springs. Or, you can upgrade to stiffer single springs that give about 100 to 115 lbs. of seat pressure when the valves are closed, and up to 300 lbs. when the valves are fully open. Going with stiffer springs than are necessary only overloads the cam and valvetrain and increases the risk of something failing.
For small-block street/strip flat tappet cams with .450 to about .500 inch lift, stiffer springs are a must (single or double springs depending on the RPM range of the cam).
For a cam with more than .500 inch lift, even stiffer conical springs or double-springs are usually essential to prevent valve float and unwanted harmonic vibrations in the valvetrain. Other modifications that may also be needed include:
A "rev kit" is another add-on that can improve reliability and rpm potential. The kit consists of extra springs that fit over the lifters to assist the valve springs and to keep the lifters in their bores should a rocker arm or pushrod break.
Most NASCAR teams run dual springs with seat pressures of 190 to 200 lbs. and open pressures of 500 to 600 lbs. at .750˝ lift. NASCAR engines typically rev to 9,000 to 9,500 RPM.
Pro Stock drag racers, by comparison, typically run triple springs with seat pressures of 375 to 475 lbs. with the valves closed, and up to 1,000 lbs. open. Many of these engines rev to over 10,000 RPM!
Spring durability is always an issue, and it depends on the quality of the materials used to make the springs and the heat treatment they receive. Some performance valve springs are nitrided to extend their endurance, but it is a tricky process that has to be carefully controlled. Some valve spring suppliers also have their springs cryogenically treated to reduced residual stress. Those who freeze their springs to -300 degree F say it can extend spring life 2X to 3X times.
Most NASCAR teams will only run a set of springs for one race, then replace them with a fresh set that typically costs $500 to $700. But many budget racers run the exact same kind of springs all season long with no breakage or problems at all.
There are several things you need to pay close attention to when installing valve springs. One is installed spring height. This assures the springs have the required pressure to keep the valves shut. Height is checked by measuring the distance between the spring seat in the head and the retainer on the valve stem.
Most performance valve springs are closely matched in terms of height and spring rate. But if adjustments are needed it can be done by shimming the valves to equalize pressures. Shims are made of hardened steel, come in various thicknesses and are usually serrated on one side to prevent rotation (the serrated side faces the head). The thicker the shim, the more it increases spring pressure.
CAUTION: Do not overshim valve springs! Over compressing a spring by shortening its installed height with a several shims may cause coil bind with a high lift cam or rocker arms.
Always check the spacing between the spring coils when the valves are fully open. High lift cams and/or rocker arms push the valves further open, so it is important to make sure there is still some room between the coils at maximum lift. This can be done by inserting a feeler gauge between the coils. A minimum clearance of .060 inch is usually recommended, though some racers run somewhat tighter clearances.
If you are using double-springs that generate more than 350 lbs. of force when the valves are open, the stock stamped steel rocker arms should be replaced with stronger aluminum rockers.
Two alternatives for upgrading valvetrain performance are aftermarket hardened steel retainers or lightweight titanium retainers (aluminum retainers are generally considered too soft). For the street, steel retainers with stock 7 degree keepers work fine. For racing or high rpm roller cams, titanium retainers with 7 or 10 degree keepers are best. Some keepers have an extra step inside that reinforces the bottom of the retainer and reduces the risk of the valve pulling through at high RPM.
When the valve locks are installed around the valve stem, their edges must not touch each other. They should clamp against the valve stem to hold it securely.
Keep in mind that the design of the retainer affects the installed valve height and spring tension. Be sure to check the clearance between the retainer and the top of the valve guide/seal at maximum lift. A minimum clearance of .060 inch is usually recommended. If there is less, the valve guides will have to be machined down to provide the necessary clearance.
The stock lift ratio for a small-block Chevy V8 rocker arm is 1.5:1. For a big block Chevy V8, it is 1.7:1. Bolt-on aftermarket high-lift rocker arms with higher ratios are a simple way to increase the net lift of an existing cam without the expense and hassle of changing the cam. Installing a set of high-lift (1.6:1 to 1.9:1) rockers n a small-block Chevy will add more horsepower with virtually no loss in low speed torque, idle quality or vacuum. But high-lift rockers alone will not provide as much of a gain in performance as a hotter cam/rocker combination.
The trend today is to use high-lift rockers to achieve greater lifts rather than relying so much on the cam lobe to do all of the lifting. High-lift rocker arms allow the valves to open and shut more quickly. This increases effective duration as well as lift for better performance without increasing lifter travel.
Ideally you want a steep ramp on the opening side of the cam lobe so it will open the valves quickly, then hold the valves open at maximum lift. Likewise, , the closing side of the cam lobe should be steep to close the valves quickly, but not so steep that it causes the valves to bounce on their seats when they close.
Increasing the lift ratio of the rocker arms has that same effect as increasing the slope on the cam lobes. It opens and closes the valves in less time, and multiplies how far the valves are help open at peak lift.
If you bolt on a set of 1.95 rocker arms with a stock Corvette cam that has a .295˝ lobe lift, you increase the valve lift to .575 inch, which is a nice improvement for a street engine, The same goes for any other engine with a stock cam. Bolting on a set of high lift rockers gives you the performance of a hotter cam without having to change the cam. It's a lot easier to bolt on a set of rockers than it is to tear the front of the engine apart to swap out a cam.
For street motors, cast or forged aluminum rocker arms with roller tips are a good upgrade over stock stamped steel rockers because they reduce wear on the valve stems and guides. For performance engines, forged or billet aluminum roller rockers are essential because of their stronger design. For really serious high RPM engines, extra strong steel rockers can provide added durability to handle extreme spring pressures and RPMs!
Most aftermarket aluminum rocker arms feature needle bearing fulcrums and roller tips to reduce friction. The less expensive ones are usually die-cast while the strongest ones are extruded or forged from 7075-T6 aluminum billet bar stock. Some rockers are also shot peened to increase durability even more.
Installing a set of rocker arms is a simple bolt-on job, but there are several areas that need to be checked when valve lift is increased. More lift means less valve-to-piston clearance, less spring retainer-to-valve guide/seal clearance, more valve spring compression (and possible coil spring bind), possible interference between the pushrods and cylinder head, and between the rocker arms, cylinder head and valve covers.
Measure for coil bind and retainer-to-valve guide clearance at maximum lift. As a rule, the valvetrain should have clearance to travel an additional .060 inch beyond maximum valve lift.
Also check clearances between the rocker arm and its stud. Stock rockers on Chevy small-block V8s can handle up to about .470 inch lift. More lift requires switching to "long slot" rockers or needle bearing rockers.
Maintaining correct rocker arm geometry is important, too. The tip of the rocker should rest on the valve stem, slightly off-center (toward the intake side). As the rocker pushes the valve down, it should align with the middle of the stem at 50 percent lift. If it is not on dead center, it will push the stem sideways every time the valve opens and closes. This will accelerate guide wear and possibly lead to valve failure.
To correct a rocker arm geometry problem, you will have to use either shorter or longer length pushrods. Some inexpensive aftermarket cams that are made by regrinding used or stock cams require longer pushrods because the diameter of the base circle is smaller than the stock cam.
Two things to keep in mind when choosing a set of replacement pushrods are length and strength. They have to be the right length to maintain proper valvetrain geometry, and they have to be strong enough to handle the valve springs without bending or breaking.
Installing a reground performance cam that has lobes with a smaller base circle than the stock cam will require LONGER pushrods to maintain the same rocker geometry as before.
Installing new lifters (especially roller lifters) that may be longer than the stock lifters will require SHORTER pushrods to maintain the same rocker geometry as before.
Replacing the stock rocker arms with aftermarket roller rockers or an entirely different type of rocker system (such as a shaft system), may also affect the height of the rockers. Longer or shorter pushrods may be needed depending on how things line up.
Even things like milling the surface of the engine block or milling the surface of the heads can change the distance between the cam and rockers. The thickness of the head gasket can also change this dimension. So depending on the changes that are made you may need longer pushrods or not.
Installing a set of aftermarket performance heads or replacing a stock block with an aftermarket "tall" block may also change valvetrain geometry. So again the correct pushrod length will have to be determined so you can have custom pushrods made to the correct length.
Determining the "right" pushrod length may require using an adjustable pushrod. Install the adjustable pushrod in place of an ordinary pushrod, then turn the threaded adjuster to make the pushrod longer or shorter as needed until you have zero valve lash in the valvetrain Turn the engine over twice by hand. When the tip of the rocker rubs on the center of the valve tip as it opens and closes the valve, you have the correct length.
Once the proper rocker geometry has been achieved, measure the end-to-end length of the pushrod and order new custom length pushrods based on those measurements. CAUTION: Different pushrod suppliers measure pushrods differently. Make sure you use the same method they do so you don't end up with pushrods that don't fit. For example, the ball on the end of a pushrod has an oil hole that makes the rod a bit shorter than it would if it has a solid ball at the end. Some pushrod manufacturers want that taken into account when measuring the length end-to-end.
Pushrods must also be strong enough to overcome valve spring pressure without flexing. Stiffer springs, therefore, require stiffer pushrods. Most performance pushrods are 4130 chrome moly steel. The diameter and wall thickness of the pushrod should be chosen to match the springs. Generally speaking, the larger the diameter of the pushrod, the stiffer the rod. Tapered pushrods are also available for added stiffness. The diameter of the pushrod will be limited by the engine design, so you can only go so large before you run out of clearance.
For street engines with single springs, 5/16 inch diameter moly pushrods with a wall thickness of .080 inch should be adequate. For dual springs and/or heavier valvetrains, 3/8 inch diameter moly pushrods would be a better choice.
For a serious high rpm drag motor, 7/16 to 9/16 inch diameter moly pushrods with up to .120 inch wall thickness are recommended.
Pushrods and rocker arms also tend to wobble around quite a bit at high rpm, a condition that is usually made worse by stiffer springs and/or a high lift cam or rockers. Installing "guideplates" that fit under the rocker arm studs can help keep the pushrods properly aligned at high rpm. Just make sure the pushrods are hardened so they won't wear excessively when they rub against the guide plates.
Always check pushrods (even new ones) for straightness before installing them. If you see any wobble when the pushrod is rolled on a flat surface, it is no good and needs to be replaced.
If you are installing a new flat tappet cam, replace the lifters too. Never use worn flat tappet lifters on a new cam unless you want to wipe out the lobes in short order. Use plenty of assembly lube on the cam lobes to protect them when the engine is first started, and run the engine at 2,000 rpm or more for 20 minutes after the initial start up to break-in the new components. Do not let the engine idle during this critical break-in period.
Should you use solid or hydraulic lifters with a performance cam? It all depends on the application. A typical street engine might never see more than 6,200 rpm, so hydraulics would probably be your best choice since they require no periodic adjustments and are much quieter than solid lifters. On the other hand, if you are putting together a serious drag or circle track motor that will turn up to 8,000 to 9,000 rpm or higher., solid lfiters would be your only choice since mot hydraulic lifters are limited to about 7,000 RPM at most.
For street engines, hydraulic lifters are pretty much standard anymore because they automatically maintain zero valve lash and require no maintenance or adjustments. But for all-out performance, many racers prefer solid lifters, and usually roller cams because of their ability to handle higher RPMs and spring loads. Roller cams (roller lifters) not only reduce friction significantly compared to flat tappet cams, they also greatly improve cam and lifter durability. Roller cams can also use much steeper cam lobes so the valves open and close faster than those of a flat tappet cam.