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Rocker Arm Ratio


What modification should I do next?
It is the proverbial question, pondered by automotive enthusiasts at least once a day. Most of us use the same universal formula for deciding upon the next performance modification which will provide the most gain for the least amount of money and effort.

One of the modifications that seems to meet this criteria are higher ratio roller rockers. The increased ratio in effect is like installing a slightly larger cam, and the roller bearing fulcrum and tip provide slight gains in power and longevity by reducing frictional losses.

The rocker arm is the link between the camshaft and the valves - functioning as a lever arm to multiply the cams lobe lift to the valve lift. Stock rocker arm ratios vary between manufacturers and engine types, but generally all small block Chevy's come with a 1.5:1 ratio rocker arm.

Virtually all the aftermarket camshaft manufacturers offer high-quality aluminum roller rockers in ratios greater than stock. For small block Chevy's you can easily round up a set of 1.6:1 rockers from Crane, Comp Cams, Harland Sharp or Omega.

Are there any cons to higher ratio rockers?
The answer is yes. The increased leverage can contribute to faster valve guide wear, especially if pushrod length is not properly checked and adjusted. The increased lift may also necessitate a stiffer set of valve springs, or springs with more clearance before bind. Finally be sure to check for sufficient clearance under the valve covers, as the beefier rocker arms can hit baffles or other parts of the cover.

Lift and Duration
We all know that stepping up to a cam with a little more lift and duration can yield a nice gain in performance. Unfortunately cam swaps are time consuming, and when you factor in the cost of the cam, lifters, gaskets, and fluids, it quickly becomes a project not worth tackling, especially on a stock motor.

The idea behind increasing the rocker arm ratio is to essentially give the motor a slightly larger cam, without all the hassle. Naturally, decreasing the rocker arm ratio provides the opposite effect making the cam events slightly smaller.

The Math
The ratio of a rocker arm is determined by the distance between the centerline of the pivot point to the centerline of the roller tip (or area of contact with the valve stem), divided by the centerline of the pushrod to the centerline of the pivot point (X). Most aftermarket roller rockers have the ratio stamped on them. See picture above.

To determine the change in lift when changing rocker arm ratios, divide the lift of the cam by the original rocker ratio (which gives you the lobe lift), then multiply this number by the new rocker ratio. For example, a popular small block chevy cam has .468 lift at the valve. Divide this number by the stock rocker ratio of 1.5 to get the lobe lift of .312. Multiply .312 by the new rocker ratio 1.6, to get the new lift of .499.