When it comes to the camshaft on a small-block Chevy, enthusiasts tend to overlook the importance of each component in the valvetrain and its contribution to how well the entire system works. The small-block Chevy began a revolution of sorts for production engines in 1955 with its 16 individual rocker studs and spindly looking stamped-steel rocker arms. Conventional wisdom at the time subscribed to the more typical heavy shaft rocker system. At the time, Chevy’s ball stud arrangement was considered radical, and many thought it would never work. But the small-block Chevy valvetrain’s utter simplicity, light weight, and simple adjustability soon won over the industry. And perhaps as much as any other design on the engine, it helped to make the small-block Chevy the most prolific and popular automotive production engine from Detroit.
The best way to understand how this simple system works, and find ways to make it work even better in today’s demanding performance environment, is to take each individual component and look at it in much finer detail. In this chapter we introduce the valves, springs, retainers, and keepers that not only help determine airflow, but also ensure that the valve religiously follows the cam profile as RPM increases. This is often not the case and is a big reason why some engines don’t make the power that they should. If the devil is in the details, then we have to find a way to make sure he stays away from your small-block Chevy so it can make all the power that it should.
This Tech Tip is From the Full Book, HIGH PERFORMANCE CHEVY SMALL BLOCK CAMS & VALVETRAINS. For a comprehensive guide on this entire subject you can visit this link:
SHARE THIS ARTICLE: Please feel free to share this article on Facebook, in Forums, or with any Clubs you participate in. You can copy and paste this link to share: http://www.chevydiy.com/valvetrain-guide-for-small-block-chevys-how-to-tips-techniques/
Let’s begin with a review of the small-block’s valvetrain action. The lifter takes the rotary motion of the camshaft lobe and converts the eccentric profile into vertical lift. The pushrod merely extends that lift up to the top of the cylinder head where the rocker arm operates like a simple fulcrum. The lift generated on the pushrod side of the rocker arm is multiplied by the stock small-block ratio of 1.5:1 to the valve. So, if the cam lobe generates 0.300-inch lobe lift, then, in theory, the valve sees a maximum lift of 0.450 inch. The reality of valvetrain operation rarely achieves all the goals of theoretical operation.
In the very early days of internal combustion engines, valvesprings were a major concern to engine designers. Even at low engine speeds of 3,000 rpm, it was difficult to create a valvespring that would control the valve. Some engine designers back in the 1920s even went so far as to design a desmodronic valvetrain where the rocker mechanically opened and closed the valve without the use of a spring. While innovative, these systems were clumsy, heavy, and unreliable. The obvious solution was, and still is, the valvespring. While somewhat innocuous, the valvespring is one of the hardest working and most heavily abused components in any internal combustion engine. Chapters 5 and 12 spend a significant portion of their length on these wire-wound components and their contribution to valvetrain success.
As simple and efficient as the smallblock Chevy’s original valvetrain was, it encouraged enthusiasts and racers to immediately push the engine right past its limits. The short-stroke 283 and 327 engines of the late 1950s and early 1960s allowed the small-block to spin to RPM levels that had previously been reserved for purebred race engines. The limitation then, and today, continues to be the valvetrain and specifically controlling the valve at these stratospheric engine speeds. The classic horsepower equation tells us that if you can produce the same torque at a higher engine speed, the engine makes more power. That simple formula demands that the engine be capable of not just spinning reliably, but also that the valves are doing their job of opening and closing at the precise time, every time the engine spins over. This, as engine builders and races have discovered, is a vexing proposition.
Overhead cam engine proponents have derided pushrod engines as limited by the monkey motion of all those pushrods and rocker arms flailing about inside the engine. But realistically, the only real difference between an overhead cam engine and a typical smallblock Chevy comes down to the addition of the pushrod. Perhaps more important is the question of how much the pushrod and other valvetrain components contribute to a loss of valvetrain control, since most enthusiasts acknowledge that overhead cam engines do tend to offer a certain RPM advantage.
GM engineers working on the new GEN III and now GEN IV design engines have since returned to the rocker shaft concept in an attempt to stabi lize the valvetrain and improve stiffness. One interesting design test for any valvetrain is to assemble a complete system for one valve and lift the valve off the seat. The system is literally struck with a hammer and measured for its structural frequency with an oscilloscope. Much like a tuning fork when struck, any valvetrain (with the valve off its seat) generates a given frequency based on its structural design. Improve the valvetrain’s strength and its inherent frequency increases. While this is obviously an impractical test for the average backyard engine builder, it is an interesting test for valvetrain stability and strength.
A small design change can make a big difference in many seemingly insignificant areas. As an example, when GM Performance Parts (GMPP) designed the roller rocker arms for its street performance Hot hydraulic roller cam for the LT4 engine, GMPP specified a much tighter clearance in the upper slot in the rocker arm. By reducing the open area of the top of the rocker, this helps determine the amount of maximum valve lift the rocker can accommodate. This smaller slot limits the rocker to a smaller net valve lift figure, but the advantage is that this change alone increases valvetrain stiffness by an amazing 30 percent.
Ironically, while the small-block originally benefited from the individual rocker stud system, most current-design high-performance valvetrains are returning to the shaft design system. The GEN III small-block employs a rocker shaft system in an effort to increase valvetrain stiffness, but that doesn’t mean that the individual rocker arm should be trashed. For a typical street engine, the smallblock’s lightweight and inherent stability still works extremely well. Continuous improvements in rocker arm, pushrod, and certainly valvespring design are a big reason for the incredible durability that the small-block Chevy enjoys. Much of this is the trickle-down concept of components originally built for pure competition now finding their way to the street.
As an example, a mere 10 years ago, only the highest-revving drag race NHRA Competition Eliminator smallblock engine builder would consider using something as exotic as a shaft rocker system conversion from companies like Jesel or T&D. Today, both companies offer shaft rocker systems for street engines that are no more expensive than a well-executed individual roller rocker set up. Add to this the belt-drive systems finding their way onto more and more street engines and the trickle-down approach to street engines is very obvious.
This is also true with respect to valvesprings. Once considered an exotic material and spring design that were the sole dominion of the race community, valvesprings eventually found their way to the mainstream performance market. Spring design is and has always been a challenge for competition engines, but the good news is that advances in these areas in terms of design, material, and application eventually end up on the street. Integrated valvetrain performance is a major reason why it has become so easy to build 500 to 600 hp normally aspirated, big-inch small-blocks. It really comes down to a design competition between the cam designers and the valvespring engineers. As the valvespring guys come up with better springs and ways to control the violent nature of the cam profile, the cam designers merely push the envelope even further by masterfully creating increasingly aggressive lobes.
One of the biggest lessons available to the street enthusiast in terms of creating a high-performance small-block Chevy valvetrain is to create an integrated system that can manage the test you intend to put it through. As we’ve already covered in the camshaft chapters, an ultra-aggressive drag racing cam profile valvetrain does not have the durability to survive the miles required of a streetdriven engine. Conversely, to be competitive in today’s professional racing environment, you must be willing to push the development of each of the components just to remain competitive. NASCAR racing is now spinning those 358-ci small blocks into the 9,500-rpm zone for some of its short-track engine combinations. Those engines are now called upon to do this for practice, qualifying, and then muscle their way through a 500-mile, fender-banging race. The fact that these engines survive and still make power is nothing short of phenomenal.
The key to these successful engine combinations is integrated components designed to work together. Professional engine builders spend lifetimes perfecting these combinations. While the average suburban garage engine builder does not have the luxury of their resources, you can still benefit by carefully researching each component and relying on the information available through the cam manufacturers like COMP Cams, Crane, Lunati, and several others. The trick is to use these companies’ resources to ensure that you carefully match the valvetrain to the camshaft lobe design. This means that every component from the valve, retainers, keepers, rocker arm, pushrod, and lifter is carefully chosen to work together. If properly matched, this makes the job of accurately controlling the valve at higher engine speeds much easier. Ultimately, if the valvetrain is under control, valvetrain breakage becomes a rare occurrence rather than an unfortunate part of basic maintenance, and the engine makes more power. What could be better than that?
Part of the plan around carefully matching components has to do with a better understanding of the cam lobe designs you are employing. For example, COMP Cams has several grinds that produce such violent acceleration rates that these cams should not be used with rocker ratios beyond 1.6:1, and really should be used only with the stock 1.5:1 ratio. This is covered in more detail in the following chapter on rocker arms, but as rocker ratio increases, this radically jacks up the valve acceleration rate. As engine speed increases, these radical acceleration rates can quickly produce uncontrolled valve motion, commonly known as valve float. Merely increasing the rocker ratio from 1.5:1 to 1.6:1 could easily contribute to a loss of 200 to 300 rpm in useable engine speed. Just as important is the fact that these aggressive acceleration rates also take their toll on valvesprings. A spring that might last thousands of very hard miles with one rocker ratio may last mere minutes with a higher rocker ratio. This leads to a gradual yet inevitable loss of top-end power and engine-speed capability.
Another useful concept in valvetrain applications is reducing the weight of the valvetrain pieces, especially in the area of the valve, retainer, and even the valvespring. Reducing weight of the spring means the top portion of the spring offers less mass to accelerate and decelerate with every valve event. This is not only worth RPM potential, but also greater “head room” between peak HP and engine valve float. What you don’t want is an engine that has a valve float redline with a mere 200 or 300 rpm above peak horsepower. For most acceleration runs, shift points should be generally 500 rpm above peak horsepower. This allows the driver to shift at a slightly higher RPM so the engine speed does not drop below peak torque on the shift recovery point. This is more important in automatics with greater RPM drops between gears (especially with the 2speed Powerglide), but is also essential, perhaps even more so, for the narrowly spaced gears of manual 5-speed cars. Most engine builders prefer to create a power curve where the curve drops off as gradually as possible, even past peakhorsepower. Engines that drop off pre cipitously after peak HP are generally engines that could use further cam/intake/exhaust tuning or are experiencing valve float.
Valvetrain weight is always an issue with a strong performance engine. While it’s easy to merely crank in more spring pressure, this is frankly the “bigger hammer” approach, and it usually just creates more problems. The more elegant solution is to lighten the components, including the valvespring itself, in an effort to reduce the weight the spring must control. Notice that we called out the pieces on the rocker side of the valvetrain. While lighter weight is important on the pushrod side, the valve side is dramatically impacted by the effect of the rocker ratio. Therefore, with a given acceleration rate created by the cam lobe, the rocker arm ratio multiplies that rate at the valve. This is then amplified by engine speed. We deal more closely with this phenomenon in Chapter 12 “Valvetrain Dynamics.”
We’ve merely hit the high spots with this overview chapter. The real meat of the material is in the following chapters as we get into the details with each individual component. As you go through these chapters, you begin to see how each piece contributes to the entire valvetrain; just like a football or baseball team relies on each individual member to become a winning team. Think of yourself or your engine builder as the team manager, carefully choosing the right player for each position, weighing each member’s strengths and weaknesses to come up with a winning formula. That’s how the game is played. Do it well, and perhaps the press will write stories about you!
Written by Graham Hansen and Posted with Permission of CarTechBooks