 |  | | | Rocker arms can cost power, lots of it in some cases. Typically, a factory rocker arm's ratio is less than advertised. For example, many stock Chevy small-block rockers "check" at between 1.4:1 and 1.47:1—few attain the advertised number of 1.5:1. No big deal? Not quite. If the cam has a lobe lift of .300", multiplying by the advertised rocker ratio of 1.5:1 gives the engine a theoretical valve lift of .450". If the stock rockers only have a ratio of 1.4:1, then the real valve lift is actually .420".
In order to correct the problem, you can either rummage through boxes of new rockers to find a "perfect" set or install aftermarket rockers. With aftermarket rockers, you not only have correct ratios, but you also can specify larger-than-stock ratios. Changing to a larger-ratio rocker has a number of benefits. For one, rockers are easier to swap than a camshaft, so you can pick up a good amount of power with a simple rocker-arm swap.
Aftermarket Benefits
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Aftermarket rocker arms offer several improvements over stock. Stamped-steel models such as those from Crane (see Tip 2) might look identical to stock components, but they aren't. The ratios are correct, and typical features include grooved rocker balls along with jam nuts, longer-than-stock slots and higher-than-stock ratios. Constructed from high-strength steel, aftermarket rockers usually feature more "beef" in the pushrod cup location, and they don't have metal flash around the pushrod oil hole. In addition, the valve-tip contact surface is often smoother than the corresponding stock pieces'.
Other benefits of rockers such as Competition Cams' Magnum series (see Tip 3) include a stronger-than-stock pushrod cup area. Also, many of these rocker arms have grooves that direct oil from the pushrod to the rocker ball. With a conventional stamped-steel rocker, the tip (the area that contacts the top of the valve) sweeps or glides across the tip of the valve as the rocker nose is forced down by the camshaft. Instead of dragging a steel rocker across the valve tip, the Magnum rocker rolls over the valve tip. Wear to both the valve guide and tip's face are reduced. Competition Cams advises that the outer spring limit for these rockers should be 350 pounds (open).
Roller rockers offer yet another advantage in their pivot or fulcrum area. As engine speed increases, the rockers are cycled at a higher rate. Near the upper reaches of the engine RPM range, the rockers are almost always stressed. Under these conditions, lubrication between the rocker arm body and the rocker ball can be lacking—oil squirts out the pushrod hole but isn't specifically directed to the rocker-ball area.
State-of-the-art roller rocker arms solve this problem by using roller bearings instead of the rocker-ball pivot. A roller bearing produces far less friction and heat than the stock rocker's sliding action. Because of this, oil flow to the topside of the engine can be restricted. Not only does this reduce the overall engine oil temperature, it can help produce more usable horsepower.
Rocker Ratios
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Check out this example: A small-block Chevy has a stock theoretical ratio of 1.5:1. If that ratio is changed to 1.6:1, then the gross valve-lift numbers increase without affecting the valve-seat timing. In short, the advertised duration stays the same, but the lift is larger.
In addition to lifting the valves higher, a larger-than-stock-ratio rocker also opens the valve quicker and closes the valve slightly later. Naturally, there are some tradeoffs. For one, springs must be checked for coil bind. In addition, the quicker acceleration and deceleration rates produced by high-ratio rockers can produce a certain amount of instability in the valvetrain, which in turn can result in valve float. Fortunately, this instability problem is almost always restricted to maximum-effort, on-the-ragged-edge race engines.
How is the ratio changed in the rocker? When the rocker is designed and constructed, the ratio is increased by moving the pushrod cup closer to the fulcrum or pivot area. The physical differences in the bases are visible in the two Crower big-block Chevy rockers shown in Tip 1.
So which rocker is right for you? There's a rocker arm for every application. Some are low-buck, others are exotic. But in any case, there could be extra horsepower lurking under the valve covers.
Resources
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Competition Cams, 3406 Democrat Rd., Memphis, TN 38118, (800) 999-0853, (901) 795-2400, www.competitioncams.com
Crane Cams, 530 Fentress Blvd., Daytona Beach, FL 32114, (904) 258-6174, www.cranecams.com
Crower Cams, 3333 Main St., Chula Vista, CA 91911-5899, (619) 422-1191, www.crower.com
Proform, Specialty Auto Parts USA, P.O. Box 306, Roseville, MI 48066, (810) 774-2500, www.proformparts.com
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