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.
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.
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.
This same friction-fighting design savvy evolved beyond the mounting joint and ultimately was applied to the rocker-arm tip. The slider-tip design is quite simple and fairly durable, but far from friction friendly and not especially suited for high lift and high RPM. The factory slider-type rocker-arm tip (standard on all GEN I-IV SBC engines with the exception of the LT4) simply rocks-and slides-across the top of the valve stem as it's actuated. The slider tip is not only a friction source, it's also known to push on the valve stem during the actuation process, potentially unsettling the valvetrain. By replacing the slider tip with a roller tip, friction is reduced, and the valve is again stabilized. The roller tip may also help facilitate higher lift, faster actuation, and high-rpm usage.
Reductions in valvetrain friction offer a nice side effect as well. As friction drops, so does the demand for oil. With the demand for oil reduced, the delivery of oil to the valvetrain can be safely restricted. This also means less windage of oil as it returns to the sump. When properly engineered, oil-restriction techniques can unlock a few extra ponies.
Beyond high ratios and friction-abatement technologies, performance-rocker-swap talk must include discussions of materials, strength, and stability. The most common rocker material is steel. Stamped steel was the OEM standard for Gen I and II, while cast steel was-and is-the standard for Gen III and IV. While these are suitable for OEM and basic performance, the aftermarket and racing communities demand more exotic options. Nothing screams high performance more than a set of anodized-aluminum roller rockers, regardless of their true positive effect. Nonetheless, high-strength alloy aluminum rocker arms are good, lightweight performers. Basic aluminum rocker arms are available with cast-alloy or extruded bodies, and high-end aluminum rocker arms are available machined from billet alloys.
Chrome-moly steel is a common material for high-performance parts, and rocker arms are no exception. The strength and rigidity of this mater-ial is hard to beat. Chrome-moly is used for some performance rocker bodies, but it's the material of choice for performance rocker-arm trunnions and roller axles. Other high-strength alloy steels are used in high-end, precision rocker arms, with rock-like rigidity for high-rpm race applications. For the ultimate in valvetrain performance and stability, shaft-mount rocker-arm systems are the way to go.
Expounding on the material engineering of rocker arms, Scooter Brothers, cofounder of Comp Cams, explains some interesting facts about steel-bodied rocker arms. Scooter states that chrome-moly steel, although heavier than other materials, can offer some design advantages and have much thinner sections than aluminum due to its superior strength density. Generally speaking, it takes at least two times the aluminum to approach the strength of steel. The moment of inertia, or performance mass, of properly engineered steel parts can actually be close to that of aluminum. In other words, before jumping for lightweight aluminum rockers, it's important to realize that the effective weight of a quality steel unit may be comparable.
Now that you've passed "Rocker Arms 101," it's time to pick the right set of rocker arms for your engine. Selecting the right rockers can be a daunting task, but following a few basic guidelines can help. First, not all rockers fit all engines. Narrow the search to those rockers applicable for your specific application. For instance, Gen I rockers won't fit a Gen III engine, and so forth. Some high-end aftermarket rockers only work on certain head types, like shaft rockers for SBC Gen I. Confirm compatibility before moving forward.
Next, ask yourself if you have the guts for modifications. In other words, are you ready to clearance valve covers or heads or convert to screw-in studs to make the rockers fit? Narrow your search for parts within your modification tolerance, and find out what else is needed. Some rocker swaps require adding a set of pushrods, locking nuts, guideplates, or other accoutrements. Get a complete understanding of what is needed before making a selection.
While most factory springs can handle a small increase in valve lift from the addition of higher-ratio rocker arms, it's usually recommended to swap in a set of matching performance valvesprings to go with the new rockers. Springs are often overlooked, which can prove costly if a stock spring breaks. Don't go cheap on springs; it's good insurance and usually quite affordable.
And, in the end, the all-important budget will probably help with your decision.
A straightforward valvetrain upgrade enlivens our low-buck '96
To help you wrap your brain around all the heady technical esoterica touched on in the foregoing piece, we thought we'd also provide you with a nice, simple, liberal-arts-level installation story demonstrating the real-world effects of a well-planned, properly executed rocker upgrade.
As a test subject, we selected this author's '96 Polo Green coupe, which appeared in these pages previously as the beneficiary of a dollars-down dyno flog, "What Price Performance?" (Nov. '06). In keeping with the budget-conscious nature of the buildup, we asked the valvetrain experts at Comp to recommend a reasonably priced 1.6-ratio roller rocker that would fit the car's factory LT1 heads without modifications. Comp's Chris Douglas suggested we try a set of the company's narrow-body, self-aligning, aluminum units (PN 1016-16), which are designed to fit all '88-up Chevy small-blocks equipped with center-bolt valve covers.
While we would have preferred to simply bolt on the rockers and go, our research indicated the LT1's stock valvesprings would be uncomfortably close to coil bind at the lift levels yielded by the larger 1.6 ratio. (A '96 LT1 equipped with 1.6-ratio rockers will generate lift measurements of approximately 0.477/0.490 inches at the valve.) With this in mind, we decided to augment the rocker upgrade with a set of Comp's PN 26918-16 beehive springs, which are rated for lift levels of up to 0.600 inch. This would be more than enough for our stock-cammed engine and would give us plenty of room to "grow into" the higher-lift cam upgrade we have planned for the near future. As an added benefit, the stiffer springs would vastly improve the LT1's valvetrain stability at elevated rpm levels, likely unleashing a few extra horsepower in the process.
Comp steel retainers (PN 787-16) and locks (PN 601-16) formed the rest of our valvetrain package, giving the new rockers and springs a bulletproof foundation upon which to work their performance-enhancing magic. Although Comp offers these parts in titanium, we felt the basic steel jobs better suited our pricing criterion. They should also provide more-than-adequate valvetrain support for our lightly modified LT1.
Finally, since we'd be tearing things apart anyway, we picked up a fresh set of Fel-Pro valve-stem seals from a local auto-parts outlet. For around $20, they seemed like a no-brainer maintenance item for our 10-year-old engine.
For the installation, we once again relied on Seffner, Florida-based Corvette-tuner Anti-Venom. A-V's owner, Greg Lovell, has years of experience wrenching on Vettes of all vintages, so he didn't treat us like a bad smell when we rolled up in our far-from-pristine C4. Follow along as we attempt to wring LT4-topping valvetrain performance and stability from a simple, affordable package.