Rocker Arms 101

Everything You Always Wanted To Know About SBC Valvetrain Tech But Were Afraid To Ask

Brian Reese Aug 23, 2007 0 Comment(s)
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An engine's power production is directly proportional to its air-processing capacity. Internal-combustion engines consume both fuel and air, but the air is far more difficult to ingest than the fuel. Additionally, gasoline engines require up to 14.7 times more air than fuel. That is why the vast majority of performance tuning revolves around the task of getting more air to move through the engine.

An engine's ability to consume air is graded by its volumetric efficiency (VE). Volumetric efficiency is the ratio of an engine's actual consumed air to its theoretical air-consumption potential at any given operating point. The higher an engine's VE, the more efficiently and effectively it can produce power based on its displacement. Naturally aspirated engines strive for a VE as close to perfect (100 percent) as possible. Forced-induction engines enjoy volumetric efficiencies over 100 percent because the compressed intake charge is volumetrically greater than its naturally aspirated potential.

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The most basic rocker-arm design is the stamped-steel type. The design originated early in the overhead-valve era and was standard issue on production small-block Chevy engines from '55 through '96. Only a few insignificant design changes occurred during the life of the stamped rocker. Most notably, a self-aligning version replaced pushrod-guideplate-type rockers across the board in '87. The stamped rocker is basic, inexpensive, durable, and gets the job done. The Comp Cams Hi-Energy stamped-steel rocker arms shown here offer a high-quality replacement for budget or stocker-minded builds.

An engine's volumetric efficiency is dependent on a near-limitless number of factors. These factors fall into four main categories: atmosphere, intake, pump, and exhaust. Atmosphere determines the potential density of air to be ingested as defined by the temperature, atmospheric pressure, and humidity of ambient air around the inlet of the engine. The intake comprises hardware from the air filter to the intake valves of a cylinder head. The pump consists of the camshaft and piston-cylinder system. The exhaust encompasses the exhaust valves to the end of the tailpipe(s). Improvements in any of these areas will likely result in engine-performance gains.

At wide-open throttle (WOT), the command for air is directed by the actuation of the intake valves in relation to the descending piston during the engine's intake stroke. Subsequently, the expulsion of spent exhaust from an engine's cylinder is similarly controlled by the actuation of the exhaust valves in relation to the ascending piston during the engine's exhaust stroke. In cam-in-block, overhead-valve (OHV) pushrod engines (used in all our beloved Vettes save for the ZR-1), this actuation of intake valves is controlled by a rocker arm driven by the camshaft-lobe profile through a lifter and pushrod.

The rocker arms themselves are the subject of this review. They are easy to swap and offer high-value gains (gain per dollar spent). A rocker arm is simply a mechanically advantaged lever that translates camshaft data into valve actuation. The mechanical advantage is defined by a rocker's ratio. The standard small-block Chevy (SBC) uses a 1.5:1-ratio rocker arm. In other words, the rocker-arm tip (output) moves 1.5 times the displacement of its pushrod socket (input), or camshaft-lobe lift. The 1.5:1-ratio rocker arm translates 0.350 inches of camshaft-lobe lift into 0.525 inch of valve lift (0.350 inch x 1.5 = 0.525 inch). By increasing the rocker-arm ratio, it's possible to increase valve lift without ever touching the camshaft. A 1.6:1-ratio rocker arm translates the same 0.350 inch of camshaft-lobe lift into 0.560 inch of valve lift (0.350 inch x 1.6 = 0.560 inch). This is a lift increase of about 6.7 percent. Valve lift can typically be increased as much as 10 percent by increasing rocker ratio.

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Comp also offers a basic cast rocker arm with a roller tip. Called the Magnum Roller, this rocker arm uses a ball-type stud mount. The body is cast from alloy steel and heat-treated for strength, rigidity, and longevity. This rocker arm is available in high-ratio versions as well.

Since rocker arms are used to control both the intake and exhaust valves, swapping high-ratio rocker arms onto an engine increases both the intake-air command and the exhaust-scavenging potential. Generally speaking, a bump in rocker-arm ratio results in a noticeable performance gain. The almighty General knows this; GM swapped in a set of high-ratio 1.6 (up from the LT1's 1.5) rockers on the LT4 and later specified the LS7 ratio at a healthy 1.8 (up from the LS2's 1.7).

Although the rocker arm's ratio is arguably its most important attribute, rocker arms can unlock performance by other means as well. With the exception of the LT4 Gen II engines, all Gen I/II SBC engines in Corvettes utilizes a slider-tip, ball-and-socket-mount, stamped-steel rocker arm. (Gen III/IV engines employ a basic roller trunnion but keep the slider tip.) The metal-to-metal contact between the mounting ball and rocker-body socket is a source of friction, heat, and instability. The simplest of roller-rocker designs replace the ball and socket with a set of roller bearings and a transverse mounting axle. This design distributes the mounting, pivot loads, and friction efficiently through the roller bearings. These designs tend to offer greater longevity as well, since friction and wear contact are minimized. The superior rigidity of the roller-bearing mounting joint also helps stabilize the rocker arm at high rpm.




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