Rocker Arm Ratios: Is more better?

Introduction

What modification should I do next? It is the proverbial question, pondered by every automotive enthusiast at least once a day. Most of us use the same universal formula for deciding upon the next performance modification -the mod which will provides 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.

In this article Tech Editor Jim Langley replaces the stock sled style rocker arms on a stock 5.0L engine, and tests whether this modification is worth anything at the dragstrip.

Before we get into the installation and results, let's review in detail the theory behind increasing the rocker arm ratio.

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 Fords come with a 1.6:1 ratio rocker arm.

Virtually all the aftermarket camshaft manufacturers offer high-quality aluminum roller rockers in ratios greater than stock. For small block Fords you can easily round up a set of 1.7:1 or 1.72:1 rockers from Crane, Comp Cams, Harland Sharp or Omega. For the Cleveland and 429/460 motors most companies offer a 1.8:1 ratio rocker arm over the 1.73:1 production ratio. Winston Cup engines can be setup with as much as 2.0:1 rocker arm ratios.

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 side bar above shows how we can calculate the valve lift as a result of changing the rocker arm ratio.

Many people incorrectly assume that a change in rocker arm ratio has no effect on the duration the valves are open. In actuality there is some change to the duration at the valve depending on the lobe profile of the camshaft. Comp Cams explains that by lifting the valve an additional 7-8%, but beginning and ending the lift at the same point, the opening and closing rates become steeper. This increases duration, measured at the valve at 0.050" lift, by 1-2 degrees. You can measure the actual duration change by using a degree wheel and dial indicator set on the valve retainer. Coincidentally the principle of a steeper rate of opening and closing is what Comp Cams has engineered into their Extreme energy camshafts to enable more valve duration for a given lobe duration.