Modified Mustangs & Fords
Rocker Arms and Adjustable Valvetrain Geometry
Rocker Arm Reaction
The typical four-stroke engine is made up of hundreds of parts. These parts are all designed and engineered to work together to make a prescribed amount of power, torque, engine longevity, and other parameters for the engine's design (or more accurately what management has told the engineers it needs to do for a specific cost). As long as these parameters are kept in check, the engine will provide the horsepower, torque, longevity, et al, it was designed for. However, if you begin to make changes to that internal combustion engine, be it a camshaft upgrade, new high-flow cylinder heads, or any number of hard part upgrades, you have effectively changed that engine's engineered design and steps need to be taken to ensure that the new parts are engineered to work with the other new or existing parts of the engine.
One of the internal combustion engines' greatest mysteries centers on its valvetrain components. The inner workings of an overhead-valve engine's valvetrain is often a head scratcher to the most died-in-the-wool car person. You can talk stroker engines, torque converters, or rear gear ratios, but start talking about pushrod length, rocker arm ratios, and spring seat pressures and most of your buddies with automotive knowledge will start looking at their watches and mumbling something about having to go home. Fear not, the mystery behind all those moving parts isn't black magic, but instead is simple math.
When it comes to your engine's rocker arms, their job is to transfer the camshaft's rotational movement into an up and down movement via the rocker arm's pivot point, which in turn opens the engine's valves. The rocker arm's size is expressed in a ratio, as the pivot is not in the center of the rocker (like a see-saw), but instead, offset to one side. For example, the stock small-block Ford rocker arm has a 1.6:1 ratio, and the parts are more commonly referred to as "1.6 rockers." This mechanical advantage means that the rocker arm tip moves 1.6 times the camshaft's lobe lift. For example, let's say your camshaft is a single pattern cam with 0.310-inch of lobe lift. With a 1.6 rocker mounted the cam's lift becomes 0.496-inches of lift (0.310x1.6). This is the typical measurement you see on a cam card or in a catalog with the footnote "with 1.6:1 rocker arm ratio" or sometimes stated as "with stock rocker arm ratio." This is a very effective way to increase valve lift (and duration slightly) without ever touching the camshaft in the engine. For example, the same cam specs, but with a popular aftermarket 1.7:1 ratio rocker arm would be 0.527-inch of lift (0.310x1.7). You've just increased your cam's lift 0.031-inch by simply bolting a different rocker arm on. Comp Cams tells us that the newest cam profiles are more aggressive and designed for the 1.6 rocker ratio, where as older cams and Ford Racing "alphabet" cams work well with the 1.7 rocker. You have to consider though that when you go up in lift, it's harder on the valvespring, it speeds up the cam profile (makes it more aggressive, and can be noisier), and makes the valve come off the seat faster. You also have to pay more attention to piston-to-valve clearance and coil bind.