Hi Tony . . .

What you're measuring is rocker arm inefficency . . . and is yet another reason why we have over head cam designs today.

Essentially the problem is that rocker arm motion is an arc - not a straight line. This means that the rocker arm ratio changes as lift increases.

As I understand it, on these old engines the 'advertised' rocker arm ratio is measured at rest. So you start off at approximately 1.5:1 and as you move through the the lift the ratio degrades to 1.4 or worse. Roller rockers do a somewhat better job of maintaining a more constant ratio - but the motion is still an arc so the tip of the rocker is moving away from the valve stem toward the fulcrum:

and since only the valve side distance gets shorter - because the pushrod tip follows the arc on the lifter side by moving off perpendicular - the ratio fades the higher you lift.

Here's a link to a tech article on the Chevy Performance site that describes the problem - Rocker Arm Geometry. It inlcudes kudos for Crane stamped rockers for v-thingies. These are advertised at 1.6:1 but actually start off at a higher ratio and stay above 1.6:1 throughout the arc.

stock49

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