One of the things that made the small-block Chevy so revolutionary in 1955 was its valvetrain. Up until this point, shaft rocker systems were the accepted standard for most production engines of the day. This design was cumbersome, and the small-block dumped that idea in favor of individual rocker studs and an innovative stamped-steel rocker arm that was lighter and much cheaper to build. The idea soon became a production standard for production V8 engines. For performance engines, the stamped rocker soon gave way to the roller rocker while still retaining the stud mount concept.
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While individual rockers have many advantages, they also harbor many negative traits that become increasingly apparent as engine speed and valvespring pressures increase as part of the search for more horsepower and torque. Racers and manufacturers have developed several band-aids to cover up these stud-mounted rocker limitations such as rev kits and stud girdles, but the real fix for a solid, reliable, high output, high-RPM performance engine is, ironically, to take a giant leap back with a shaft rocker system.
A shaft rocker system is inherently superior to a rocker stud design, and only a couple major items make that decision difficult. The biggest hurdle to overcome is the price. Shaft rocker systems always cost more than studmounted rocker arms. The price differential has softened in the past several years with budget-based “street” rocker shaft systems introduced by Jesel
that now cost around $800 for a smallblock Chevy. Other systems can run $1,000 or more depending upon several variables. The other minor factor has more to do with a slightly more complex installation procedure, but that’s a small point when compared to the many advantages that shaft rocker systems offer. In order to understand why shaft rockers are superior, we first have to look at the basic design limitations of the stud-mounted rocker arm.
The small-block Chevy studmounted rocker system employs a set distance between the centerline of the stock rocker stud and the valves. Since the small-block valves are in line, this is a spec for both the intake and exhaust valves. On a given roller rocker, for example, this spec also dictates the distance from the centerline of the roller fulcrum to the centerline of the roller tip. Jesel calls this distance 1.425 inches while T&D references 1.450. As we’ve seen on rocker arms in Chapter 6, this distance establishes an arc created by the rocker arm tip as it traverses through its lift curve. If you could establish a longer fulcrum arm, either by moving the rocker stud away from the valves or moving the valves to increase the distance to the rocker stud, this would increase the distance of the fulcrum arm and therefore reduce the radius of the arc. Increasing this length reduces the distance the rocker arm tip travels across the face of the valve tip. While this may not seem like much, when we get into extremely high valvespring pressures and tall valve lifts, this distance becomes important.
The original small-block Chevy engineers probably never envisioned that racers would take their basic design and cram valve lifts approaching 0.800 inches into their original design. These stratospheric valve lifts create increased rocker arm tip travel across the face of the valve tip. In the days before rocker shafts, or if class rules did not allow shaft systems, racers resorted to moving the rocker stud away from the valve center line and using big-block Chevy rocker arms that employed a longer 1.650 inches of fulcrum length. The increased length is the real key to understanding why rocker shaft systems are inherently superior to stud-mounted rockers for the small-block Chevy.
Relocating the rocker stud on any engine is a tremendous amount of work, which only dedicated racers would undertake in search of more power and durability. Back in 1979, Dan and his brother Wayne Jesel realized the disadvantage of the rather short small-block Chevy fulcrum length. They decided the easiest way to overcome this limitation was to increase the length by creating a rocker shaft system. By first building a base that bolts into the stock rocker stud holes and using that base to move the centerline of a set of paired rockers on shafts, they could increase the fulcrum length and gain an advantage over the stock stud-mounted rocker system. For example, increasing the stock fulcrum length from 1.425 to 1.545 inches is only 0.120 inches, but this small increase is worth a significant reduction in roller tip movement across the face of the valve tip. While only incremental improvements, multiplied by 16 valves and working against valvespring pressures upwards of 800 pounds at maximum lift, the improvements become readily apparent. Jesel’s technical information claims a 1.545 fulcrum length shaft rocker system creates 10 percent reduction in arc sweep.
Jesel, T&D, COMP Cams, Crane, Crower, and others all build shaft rocker systems that would make an excellent high-end street valvetrain, but not all of them build their systems the same way. Jesel and T&D change the fulcrum length, depending on the rocker ratio. For a stock 23-degree small-block Chevy head using stock a 1.5:1 rocker ratio, Jesel, for example, creates a shorter ful crum length of 1.515 compared to higher ratios of 1.6 to 1.7:1 ratios using a 1.545inch fulcrum. For T&D shaft rockers, the relationships are the same, but with fulcrum lengths of 1.53, 1.60, and 1.650. So far, this discussion has been limited to stock type 23-degree valve angle heads. When you get into the 18-degree, 15degree, and SB2.2 heads, these relationships also change since both the valve angle and the distance between the rocker stud and the valve centerline also change—for the better. Since this book it targeted at the performance street market, we stick with the 23-degree cylinder heads and leave the race head valvetrain discussion for another time.
But shaft rockers are about much more than just fulcrum length. It should be apparent that placing a pair of rocker arms on a large-diameter, common shaft would also create a much more stable platform from which to operate a pair of valves at high RPM. When a rocker arm begins to open the valve, it multiplies the lift from the pushrod by the rocker ratio, which also multiplies the force from the valvespring that works against the fulcrum point. By design, an individual rocker pinpoints this force on the stud, which creates a bending force. This is why rocker studs have become so important to a performance engine. With a shaft rocker system, the bending force from the valve is distributed over a much broader area of the entire shaft with the result being far less valvetrain deflection with a given valvespring pressure.
As horsepower levels continue to escalate, cylinder head ports continue to grow, especially in width. This encroaches on the area reserved for the pushrod, requiring the engine builder and valvetrain designer to come up with offset lifters and rocker arms. Crane, for example, offers both 0.150- and 0.225-inch left and right offset studmounted roller rockers, but these rockers virtually demand some kind of stud girdle to help control the additional bending moment created by the offset pushrod cup in the rocker. This motion is also concentrated on the stud centerline, subjecting the stud to now two different forces.
The better solution is a rocker shaft system that, by design, can more easily accommodate these offset forces. With an offset pushrod cup, the bending force tends to be spread over a much larger area with increased bearing surface compared to the rocker stud design. Both Jesel and T&D offer various offsets for both left- and right-hand rockers. T&D, for example offers eight offsets from 0.080 to a massive 0.700 inches. Often an offset rocker can be combined with an offset lifter to move the pushrod out of the way of a wide intake port. Many engine builders would rather include as much offset in the rocker as possible since they feel that the tradeoff of increased lifter bore loading from the offset lifter can be excessive, while others are willing to accept increased lifter bore wear in favor of less angle on the pushrod. Lifter offset also reduces the amount of offset required from the rocker arm. It’s always a good idea to operate a pushrod as close to vertical as possible, since pushrod angle also imparts a bending moment into the pushrod that is not an issue in pure vertical motion.
Another reason for offset rockers is to accommodate aftermarket cylinder heads with revised valve placement. The “60/40” arrangement for the smallblock Chevy is becoming increasingly popular with cylinder head manufacturers where the intake and exhaust valves are relocated in relation to the cylinder. The intake valve is moved closer to the cylinder bore centerline to unshroud the valve, while the exhaust valve is moved away from the intake to leave room for larger valves. With stud-mounted rocker systems, these slight variations in valve placement can be easily accommodated with pushrod guideplate changes. But with shaft rocker systems, moving the valves in relation to the cylinder bore requires changes to the entire system since the rockers are fixed. This is the reason for the multiple part numbers required for shaft rocker systems and another reason for their added cost.
Stud-mounted rockers have an additional disadvantage when used with mechanical lifter camshafts in regard to lash. With a stud-mounted rocker, when the cam reaches the base circle, the rocker arm immediately drops down on the stud by the amount of the lash divided by the rocker ratio. So if you have a roller cam using 0.020 inches of lash, the rocker slides down the stud by roughly 0.012 inches, assuming a 1.6:1 rocker ratio. While this isn’t much, at high RPM when the cam comes up on the opening flank of the lobe, the entire rocker arm slams up against the poly lock until this clearance is eliminated. This imparts a vibration that Spintron studies have identified as a spike in the valvetrain. While this lash is still present in a rocker shaft system, that clearance does not accelerate the rocker arm vertically. It reduces mass moving to the length of the fulcrum of the rocker arm only as opposed to the mass of the entire rocker arm body.
Another small factor has to do with the clearance between the hole in the stud-mounted rocker and the size of the stud. The ideal situation is a snug fit between these two components, but production variables can make this somewhat sloppy. Any clearance here tends to allow the rocker to move around on the stud, which is also not desirable. This has more to do with lower-quality rocker studs than variations with rocker arms, but it’s something else that should not be ignored.
As we’ve seen in the chapter on rocker arms, pushrod length is critical to improving valvetrain efficiency. With a rocker shaft system, pushrod length is especially critical for several reasons. Since the rocker arm pivot point is fixed on a shaft system, the lash adjuster location moves to the pushrod end of the rocker arm. Reducing mass in a rocker arm is especially important at high engine speeds, so adding an adjuster on this end must be done with an eye toward reducing weight. If you add a large adjuster with its required larger area in the rocker, this additional mass must be accelerated up and down with each valve motion curve. Every gram of mass the rocker shaft companies can trim from the rocker sys tem (especially that mass located farther from the pivot point) means less mass for the valvespring to control at high engine speeds
To reduce this mass, the shaft rocker companies limit the range of adjustment in the pushrod cup to around 0.050 to 0.080 inches. The reason for this is to ensure sufficient thread engagement with the adjuster to the rocker arm body. In addition, T&D creates internal oil passages in the rocker arm that direct oil from the pushrod. Oil is fed by an internal passage in the pushrod cup adjuster that flows directly to the bearings in the fulcrum and also out to the roller tip bearings as well. Plus, T&D offers an optional spring oiling passage that directs some of this oil to spray directly on the coil spring for cooling.
Because the rocker arm scribes an arc through its lift curve, several opportunities are available to establish what the true rocker ratio is. Most companies choose to establish the rocker ratio at the point of maximum valve lift, where most engine builders check it. With stud-mounted rocker systems, the cam may in fact be generating the proper valve lift, but because of significant rocker stud and rocker arm deflection, this may end up creating 0.010 to 0.020 inches less maximum valve lift, even with the proper rocker ratio. Shaft rocker systems are subject to much less valvetrain deflection and as a result can recoup much of this lost valve lift, like gaining valve lift without going to a bigger cam or more rocker ratio.
This longer fulcrum length also generates much more clearance for larger diameter valvesprings. Packaging is already a tight issue with the small-block’s rather cramped real estate under the valve cover, so anytime you can gain an advantage of more clearance along with better performance, this is an additional plus.
Crane has recently introduced a new shaft rocker system for the small-block Chevy they claim is worth an additional 20 hp over other shaft type systems. It’s based on an innovative bushing design capable of withstanding extreme valvespring pressure abuse. Crane’s point is that roller bearings require power to accelerate and decelerate with each valve lift curve. By using this new bushing material (actually, Crane prefers to call this a bearing), Crane claims that this power can be recouped as “lost” horsepower along with reduced heat with lower oil temperatures.
While we have not tested this valvetrain system as yet, discussions with Crane revealed that these horsepower improvements would only be seen on a high-RPM, competition engine where these gains can truly be measured. In other words, your 6,000rpm street engine would probably not see much, if any, improvement over a competitive shaft system. This brings up an interesting question of how much power resides in a shaft rocker system over a quality stud-mounted roller rocker system. The answer to this question is always predicated with multiple layers of qualifying statements like the standard “it depends upon the application” response. However, it would be safe to say that engines spending more time above 6,000 rpm would be better served than engines that rarely achieve that higher speed. The engine’s actual horsepower level also plays into this equation. Ten hp from a 500-hp engine is significant since it’s a two percent gain, but 10 horsepower from a 750-hp engine represents barely more than one percent. But to hazard a rule of thumb, most of the rocker shaft companies offer a rocker shaft system with the same rocker ratio that should be capable of roughly 10 hp on a 500-plus hp engine.
When you combine the obvious geometry advantages of a shaft rocker system over stud-mounted roller rockers, this alone is probably worth the investment, especially if you’re building a stout small-block that plans on seeing some serious RPM. Add the increased durability and possible power increases that can come from these improvements and the only reason you shouldn’t invest in a shaft rocker system is if your budget can’t stand another hit. In that case, we’d suggest finding the money because these shaft systems are certainly the way to go.
Written by Graham Hansen and Posted with Permission of CarTechBooks