It’s in every gearheads nature to try and find the cheapest ways to improve the performance of any engine they come into contact with. One of the most efficient ways of doing this is to increase the lift and duration that the valvetrain experiences. One of the tried and true ways of achieving this effect has been the addition of a higher ratio rocker arm.
Making this simplistic change can increase the lift and duration of the intake and exhaust valves, without pulling all the accessory drives, water pump, and timing cover to swap out the camshaft. Whereas, a mere rocker ratio change will yield an improvement in horsepower by just removing the valve covers.
Another technique that surfaced in the early days of modifying small-block engines was the idea that you would better control the flow characteristics of a cylinder head with a set of split ratio rocker arms, back before the aftermarket world flourished with different dual pattern camshaft combinations.
Where this type of ingenuity shines is when you’re dealing with a single pattern camshaft, which is almost entirely the norm when you’re talking about pushrod factory camshafts. The proper way to determine how much change ratio rocker yields is to divide the lift of the cam by the original rocker ratio (this number is referred to as the lobe lift of the valvetrain), then multiply the lobe lift by the new rockers ratio.
For example, an F-303 Ford Racing small-block cam has a 0.512-inch lift at the valve. In order to find how a ratio change in the rocker will affect the lift, you must divide this number by the stock ratio of 1.6 to get the lobe lift of .320 of an inch. Multiplying this .320-inch lobe lift by the new rocker ratio 1.7 results in a new lift of .544 of an inch in valve lift.
Going back to the split ratio rocker discussion, choosing to increase the lift on the intake side or exhaust side of a combination will result in changes in the flow characteristics of the heads. This allows you to break away from the single pattern camshaft design by modulating the placement of different ratio rocker arms in the valvetrain and counteract the drawbacks your particular head design may have.
Factory small-block head casting tends to have a poor exhaust flow port characteristic; this might be an instance where you would want to increase the lift and duration to overcome this drawback. In retrospect, you may want to increase the duration of your intake slightly, and again this type of modification will result in improved flow numbers. Utilizing split ratio rockers can also be used as an effective tuning tool to dial in a combination, but yet again, the same effect can be achieved with a custom camshaft.