Splitting the valvetrain into two sections allows us to go into more detail for each of the individual components. This chapter deals with the cam side of the valvetrain, including the rocker arm. Since this is perhaps the most important component on this side of the system, we start with the rocker.
All cam-in-block pushrod engines use a rocker arm to convert the reciprocating motion of the lifter and pushrod into motion that controls the valve. In simplest terms, the rocker arm operates like a child’s teeter-totter in which the upward motion of the lifter is converted to downward motion of the valve. Along the way, simple leverage is employed to multiply the characteristics of the lobe by a ratio. In the case of the original small-block Chevy, the stock rocker-arm ratio is 1.5:1. This means that the lift generated by the cam lobe is multiplied by 1.5 times when delivered to the valve. Therefore, a cam with a 0.400-inch lobe lift would theoretically create 0.600 inches of lift at the valve. We emphasize the word “theoretical” because most rocker systems are less than 100 percent efficient, meaning that ratios are not always what they are published and deflection in the valvetrain also costs a certain amount of valve lift, especially when using high-rate valvesprings.
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It’s important to understand how rocker arms work in order to fully appreciate their abilities and limitations. If you look at a straight-side view of a rocker arm, whether it’s a stud mount or rocker shaft version, rocker arms pivot around a central axis and the rocker tip that contacts the valve travels through an arc. The greater the valve lift motion, the greater distance the rocker tip must travel and therefore the wider the arc it transcribes. Since this is an arc, or radius, the rocker arm tip traverses a given distance. If the distance from the rocker pivot point to the valve tip is the same, the ratio remains constant throughout its travel. But because rocker tip creates an arc, the rocker ratio changes as it traverses this arc. When setting up a valvetrain, the ideal plan is to minimize the distance the rocker travels across the valve tip. This limits the change in ratio, and generally enhances performance. We get into the details of how to do that in Chapter 9.
a properly designed valvetrain system, the rocker tip begins at a point just inboard of the valve tip center. As the rocker creates valve lift, the contact point moves outboard (toward the exhaust side of the engine). At roughly half of total valve lift, the contact point has now moved to the outboard of the valve tip centerline. At maximum valve lift, the rocker contact point moves back to its starting point. As the valve lift curve begins its way back down, this reverses the pattern until the valve returns back to the inboard side of the valve tip centerline. If you were to label these points A (inboard) and B (outboard), the rocker tip travel could be described as A-B-A-B-A (see illustration).
A considerable body of evidence suggests an alternative to this configuration, especially when building an engine with extremely high valve lift curves. Some drag-race engine builders prefer to design their valvetrain to place the rocker tip on the valve centerline at max lift. Their idea is that they do not want side loads placed on the valve at maximum lift, which could create excessive valve guide wear. This is achieved by placing the rocker arm tip at the valve centerline just as the valve begins to open. At midvalve lift, the rocker has moved toward the outboard or exhaust side of the valve stem tip. Then at maximum valve lift, the rocker tip actually travels back toward the intake side of the engine, arriving at the valve centerline at maximum valve lift.
With street or endurance engines, the key is to minimize the total travel of the rocker arm across the tip of the valve. The main reason for this is to min imize valve guide wear by minimizing the side-load thrust created by the rocker being off center of the valve tip. Generally, 0.040 inches is considered a decent number. Values exceeding 0.050 inches should be addressed with pushrod length to reduce the distance and improve the geometry. As we’ve noted, the greater the distance the rocker tip travels away from the rocker pivot point, the more this reduces the rocker arm ratio, resulting in reduced total valve lift.
One of the distinct disadvantages inherent with stamped-steel rocker arms is the friction created by the ball-stud design. With increased valvespring pressures, the load and heat created is almost impossible to control, especially for the exhaust rockers that are also subject to increased heat transmitted through the valve stem. High-performance stamped-rockers often come with balls machined with grooves to retain oil to improve lubrication in the ball contact area. Stamped rockers also do not operate exactly like roller rockers. This is because the stamped rockers offer a slot through which the stud protrudes to locate the rocker. The higher the valve lift, the longer this slot needs to be in order to accommodate the increased lift. Because the rocker centerline is constantly changing, this limits the amount of movement of the stamped rocker tip across the valve tip face. In addition, stamped rockers offer an exceptionally wide tip contact area.
Roller rockers use a shaft with roller bearings to reduce the friction, which also reduces oil temperature. The cam companies also build a rollertipped rocker that still uses the stock ball-stud arrangement. While these rockers offer an increased ratio, the fallacy is that the area between the roller tip and the valve tip face experiences some friction reduction. In reality, with a hydraulic tappet camshaft that employs a preload instead of a clearance value, the roller tip slides across the valve stem tip and does not roll. Mechanical cams with lash allow the tip to rotate, but only until the lash has been taken up, then the roller tip also slides across the valve stem tip. So purchasing a set of rollertipped rockers in search of reduced friction does not deliver value based on that assumption. You’re far better off investing in a set of true roller rocker arms.
The two basic types of true roller rockers are made of either aluminum or steel. Originally, all roller rockers were made of aluminum in search of reduced weight. Unfortunately, since aluminum is not nearly as strong as steel, aluminum rockers must employ a larger body to prevent deflection. Steel roller rockers are heavier, but the portion of the rocker that actually moves the greatest distance is still roughly the same weight as an aluminum rocker arm, so there is minimal loss of performance. Plus, many steel rocker arms, such as the COMP Cams HiTech stainless rockers, are rebuildable—something that few aluminum roller rockers can offer.
It’s All in the Ratio
After you’ve decided on the basic rocker arm design, the most important feature you need to decide on is the rocker ratio. From 1955 until the GEN III engine came on the scene, the stock rocker ratio for a small-block Chevy has always been 1.5:1. The GEN III engine pushed this ratio up to 1.7:1 and just recently, the GEN IV LS7 bumps this to 1.8:1! This ratio is achieved by simply moving the rocker’s pushrod cup closer to the rocker’s pivot point. Increasing the ratio requires moving the pushrod cup closer to the pivot point since the distance from the rocker fulcrum to the valve tip must remain constant. Calculating the ratio is relatively easy by simply dividing the two distances into each other.
From this description, increasing rocker ratio also increases the lift. For example, let’s say we have a camshaft offering 0.333 inches of lobe lift working with a 1.5:1 rocker ratio. Multiplying lobe lift time by 1.5 gives us a theoretical valve lift of 0.500. By increasing the ratio to 1.6:1, this bumps the valve lift up to 0.533 inches. Generally speaking, a 0.1:1 ratio jump kicks the valve lift up by around 0.030 inches. Especially if the camshaft is already installed in the engine, increasing rocker ratio is a quick and easy way to add additional valve lift. Several other things worth investigating also occur when you increase rocker ratio.
Increasing the rocker ratio does not change the opening and closing point of the cam lobe. However, once the rocker arm begins its valve lift curve, the increased ratio opens the valve sooner in relationship to piston position. If you remember our discussion in Chapter 2 on cam basics, a more aggressive cam lobe (like a big roller cam versus a mild flat tappet for example) creates more duration. This is evidenced by an increase in duration at 0.200-inch valve lift. An increased rocker ratio also creates a smaller version of this same effect with its more aggressive lift curve. Again, the opening and closing points don’t change, but the valve lift curve does change.
Common sense also dictates that the valve must be subjected to a much quicker opening (and closing) rate any time the rocker ratio is increased. In our previous chapter discussion on valvesprings, it’s obvious that notching up the rocker ratio means that the entire valve assembly accelerates up and back down from maximum lift much more quickly than it did with its lower rocker ratio. This means that the valvespring must be able to now control these additional acceleration rates. This is especially important on the closing side, since the idea is to prevent the valve from bouncing off the seat. As we mentioned before, this is often where the onset of valve “float” occurs.
Increasing rocker ratio means you have more to pay attention to than just measuring for coil bind, retainer-to-seal, and valve-to-piston clearance. All of these checks are important, but changing ratio also means that a valvespring fully capable of controlling the valve at 6,500 rpm with a 1.5:1 rocker ratio may succumb to valve float with a 1.6:1 ratio long before 6,500 rpm. This depends upon the quality of the valvespring, as we discussed in Chapter 5. The final word here is that bumping the rocker ratio may not require any significant changes, or in cases where you’re jumping to a much larger ratio like 1.7:1, the valvetrain may need a dramatic overhaul.
Studs and Guideplates
For the first 25 or 30 years, stock small-block Chevy heads used simple, 3/8-inch pressed-in studs that did an adequate job of offering a rigid pivot point for the rocker arm. But as performance applications, engine speed, and valvespring pressures continued to increase, the aftermarket, and later the factory, converted to screw-in studs. If you really wanted to increase stud strength, many opted to go to 7/16-inch studs, which automatically required a different rocker arm to accommodate the larger stud. For many enthusiasts, this is as far as they’ve ever taken the thought process concerning rocker studs. But it turns out that the story goes far deeper.
As engine builders continue to push the small-block Chevy envelope, larger valvesprings and heavy-duty roller rockers have begun to squeeze the available real estate, which makes paying attention to details like the clearance between the valvespring retainer and the rocker arm a critical dimension. In one particular case, we noticed that we had a clearance problem between two rocker arms and their respective valvespring retainers.
It seemed odd that only two valves would have this problem. Subsequent investigation revealed that several rocker studs, including the two with the clearance problem, exhibited significant lateral runout, what machinists call total indicated runout (TIR). When we meas ured these two particular studs, we discovered a total TIR of more than 0.025 inches! At first, we suspected the studs were bent, but when almost all studs showed significant runout, we realized that even new studs from this budget source were machined that way from the factory! We next measured a set of ARP small-block studs and found all were within 0.003 inches.
What we learned is that ARP machines their studs using a centerless grinding technique in which the studs are machined between two counterrotating grinding wheels. This ensures a much more accurate stud as opposed to lathe cutting, which is based on an assumed centerline with a tolerance stack-up that can create eccentricity. In our case, the two rocker studs that had a clearance problem just happened to place their eccentricity directly in line with the valve, reducing the distance between the valve and stud. If you think about this, any eccentricity causes all kinds of problems for accurate valvetrain operation. This can move the rocker arm tip so that it is no longer directly in line with the valve, or move the rocker tip in or out relative to the valve. This changes the effective rocker ratio. So with a cheap rocker stud, you could easily end up with an engine running with multiple rocker ratios! This is hardly conducive to making power!
This is not necessarily an exclusive endorsement of ARP products, although we’ve found very few examples of problems with any fasteners from that company. While other reputable fastener companies are certainly out there, we’ve found that we don’t have to worry about the quality of ARP’s products. In addition, ARP backs their pieces with significant engineering, sometimes difficult to obtain from other fastener companies.
Along with using a quality stud, we’re also big proponents of using some type of poly-lock type stud-nut as opposed to a factory-style lockingnut. This is a requirement with roller rockers, but this discussion is aimed more at stock-stamped rocker use. All new stamped rockers come with those factory-type pinch nuts. Take our advice and throw them away and invest in a set of poly locks. If you know an engine builder, he probably has a pile of poly locks in his valvetrain bin. The problem with pinch nuts is that they tear up the threads on the stud very quickly and also only work the best if you never adjust them once installed. Every time you move them, they lose a little of their locking force. Have you ever wondered why the older 1960s mechanical cams seemed like they needed constant adjustment? It’s our opinion that these cheap locking nuts are the main reason for the necessity of constant valve lash maintenance.
Guideplates were a performance aftermarket invention designed to accurately locate the pushrod in high-RPM applications. Originally, stock smallblock iron heads employed slots cut into the head to guide the pushrod. This is important because the stamped rocker/ball stud arrangement would allow the rocker to slide sideways unless the designers employed some type of guide system. With the advent of aluminum cylinder heads, the soft alloy could be easily eroded by a steel pushrod, so guideplates became required.
The most important point to drive home about aftermarket pushrod guideplates is that these are hardened steel plates to prevent them from undue wear when performing their intended function. This demands that the pushrods also be hardened. We get into pushrods later in this chapter, but do not make the mistake of using stock, non-hardened pushrods with hardened guideplates, or you will end up with a pile of metals shavings that will find its way throughout your engine and into the bearings. That can ruin your whole day!
But just adding hardened pushrods and guideplates is not the end of the story. Unfortunately, not all guideplates position the pushrod where you want it. Most guideplates offer slightly larger holes that allow the engine builder to adjust their position relative to the valves, but often the engine builder may need more. In the past, builders often resorted to cutting these hardened plates and then re-welding them in the proper position. Isky took notice of this practice and now sells adjustable pushrod guideplates for the small-block Chevy that allow you to accurately position the rocker tip over the valve stem tip with the guideplate. Once the position is set, then you can permanently weld the two halves of the guideplate together.
On the surface, it appears that as an internal combustion engine component, life doesn’t get much simpler for pushrods. The part itself moves, but it has no moving parts. It merely connects the lifter to the rocker arm while also transporting oil to the rocker through its hollow center. But when it comes to performance small-block Chevys, it seems that nothing is very simple. As it turns out, the pushrod is a critical component in the small-block valvetrain. It all comes down to durability and strength.
If you’ve ever watched a track and field event like at the Olympics, the pole vault is an interesting event from an engineering perspective. The vaulter uses a fiberglass pole designed to bend in the center, which lofts the athlete over the horizontal bar. While this works well for Olympic decathlon contestants, that same situation with a pushrod engine can be counter-productive to performance. As the small-block has continued to increase in engine speed in search of greater horsepower levels, the loads on pushrods have steadily risen with higher spring loads and more aggressive rocker ratios. Add in more RPM, and that stock 5/16-inch pushrod is stressed beyond its limits.
When that happens, the pushrod bends. More often, this is a temporary bending effort. Acceleration rates decrease as the lifter approaches the nose of the lobe, and the pushrod then straightens, just like a vaulter’s pole. This imparts additional acceleration and lift on the rocker arm and valve at a position where acceleration is not desirable. This can result in coil bind, or possibly valveto-piston contact, which can often result in a grenaded engine—not good.
This is especially prevalent in engines that spin well in excess of 7,000 or 8,000 rpm, but in certain situations this can occur even in engines around 6,000 rpm. The best way to counteract this situation is to employ much stronger pushrods that minimize this deflection. All the major cam companies offer high-strength pushrods, generally made of seamless, 0.080-inch wall thickness tubing with pressed-in ends. While these stronger pushrods do weigh more than stock units, this is one place where the additional weight is considered acceptable because of the positive effects of reduced bending. Several cam companies make adjustable-length checking pushrods to allow you to properly set the pushrod length. This procedure is covered in Chapter 9.
As we’ve reviewed in the cam basics chapter, four types of lifters are used in a small-block Chevy. For flat tappet lifters, both hydraulic and mechanical styles are used. For roller lifters, the same hydraulic and mechanical styles are also used. Let’s begin with the flat tappet design.
The small-block Chevy has retained the stock 0.842-inch tappet diameter since the very first small-block Chevy engine made noise in 1955. Amazingly, you could take a stock small-block lifter out of an original 265-ci small-block and use it in a late-model small-block even today. While referred to as a flat tappet, the name belies its actual construction. Each flat tappet lifter is actually ground with a slight crown to its face. This gives the lifter face a curvature that helps create a spin motion required of all lifters. This spin motion helps the lifter survive its first few minutes of life in a new engine while it establishes its wear pattern. Once properly broken in, the lifter continues to spin slightly during engine operation to help maintain a uniform contact area with the cam lobe.
A “flat” cam lobe is most often blamed on poor material quality or heat treat, but usually a flat cam lobe occurs due to improper cam break-in procedures. The details of this procedure are covered in Chapter 9. It’s also worth mentioning here that flat tappets should always be kept paired with their respective cam lobes if the cam is removed from the engine, and used lifters should never be used on a new cam.
Mechanical flat tappets are the simplest of all the lifter designs. The lifter is literally solid with machining accomplished to transfer oil from the lifter galley up through the pushrod. The solid lifter requires a certain amount of clearance, or lash, in the entire valvetrain system for each lifter in order to compensate for physical growth in the engine as it transitions from cold to normal engine operating temperature. This lash is generally between 0.010 and 0.030 inches and specified by the cam manufacturer. Lash is most often spec’d as “hot,” meaning that it should be checked only after the engine has achieved normal operating temperature. Generally, this lash should be checked periodically. The last production smallblock Chevy to use solid lifters was the 1970 Z/28 LT1 engine. A quick and easy way to set lash is covered in Chapter 9.
Hydraulic flat tappets are designed to eliminate the need for periodic lash adjustment by using a small piston inside the lifter body supported by a small volume of oil in a chamber underneath the piston. Engine oil under pressure is supplied in this chamber and automatically compensates for growth in the valvetrain due to temperature changes. This is accomplished by depressing (or preloading) the piston in the lifter chamber a specified amount.
Hydraulic flat tappets are by far the most popular for street performance applications because of their inexpensive price and low maintenance requirements. The only limitation to the hydraulic flat tappet design is that the engine builder can only increase valvespring pressure up to the point at which the spring pressure eventually pushes the oil out of the lifter, eliminating much of the cam lift and killing power. Increasing engine oil pressure can compensate slightly for this situation, but the best fix at that point is to swap to a mechanical cam instead. High spring pressures are intended for high engine speeds, which means little need for the low-maintenance advantages of a hydraulic tappet anyway.
Mechanical roller tappets were first designed for diesel engines used in the very early 1900s. Later, they were used mainly in ultra-high-end race engines and were very expensive. Like many race-only parts, these roller tappets eventually found their way to the street, but they were still very pricey. It wasn’t until Chevrolet began to use hydraulic roller tappets in production engines that the trickle-down effect became more of a torrent. By the mid 1990s, performance hydraulic roller cams had become commonplace and accepted by an increasing number of enthusiasts. While the lifter diameters remained the same at 0.842 inches, production hydraulic roller engines use a specific lifter retaining system that was difficult to retrofit back to earlier non-hydraulic roller tappet small-blocks.
This created a demand for retrofit hydraulic roller tappets that now all the major cam companies offer. These lifters use the more traditional aftermarket tiebar design to connect two roller tappets together so that the roller remains aligned with the cam lobe. The factory design uses cast bars that slip over the taller lifter body. These tie bars are retained by a large, spring-loaded device referred to as a spider because of its eight legs bolted to the lifter valley with three bolts. This means that factory hydraulic roller tappets cannot be used in an earlier, non-roller cam block. Most cam com panies offer a complete retrofit kit to accompany an aftermarket roller cam for these earlier engine blocks.
The top of the line for roller lifters has to be the mechanical roller lifter. While completely streetable, these applications do become expensive, although the price difference between a complete mechanical roller lifter cam and valvetrain and a hydraulic roller cam assembly is not that great. The biggest hurdle for mechanical roller cams seems to be that they have a reputation for requiring almost constant lash adjustment. This couldn’t be further from the truth. The reality is that if the valvetrain is set up properly and the components experience minimal wear, the lash really shouldn’t change unless problems arise.
Where there have been cases of failures, this most often occurs in those cases where overly-enthusiastic street engine builders spec a race-only mechanical roller cam for a street engine in search of ultimate power. Generally, drag-race-oriented camshafts are designed with ultraaggressive profiles that tend to be extremely hard on the valvetrain. This requires the racer to perform lash adjustments much more often. Eventually, these race cams, if used on the street, cause valvetrain problems (usually spring failures) because of the aggressive nature of the lobe profiles. This may be part of the reason for the street set believing that all mechanical cams require constant lash adjustment.
We’ve now covered most of the cam and valvetrain components on an individual basis to give you an overview of how these components inter-relate. While each piece should be taken on its own merits, hopefully, you can begin to see how all these parts rely on the rest of the system to operate properly. Like the links of the proverbial chain, the valvetrain is only as effective as its weakest component. But before we wrap up this discussion, let’s move on to the next chapter, which deals with cam drives.
Written by Graham Hansen and Posted with Permission of CarTechBooks