The camshaft doesn't move the valve directly in a pushrod engine — the rocker arm stands between them, acting as a lever that trades force for distance. The rocker arm ratio is the mechanical advantage of that lever: pivot-to-valve length divided by pivot-to-pushrod length. A 1.6:1 rocker means the valve tip moves 1.6 inches for every 1.0 inch the pushrod pushes up.

Why the ratio matters: the cam grinds a fixed lobe lift — say 0.350" — but the valve doesn't have to open that far. Multiply lobe lift by rocker ratio and you get gross valve lift:

• 0.350" lobe lift × 1.5 rocker = 0.525" valve lift
• 0.350" lobe lift × 1.6 rocker = 0.560" valve lift
• 0.350" lobe lift × 1.7 rocker = 0.595" valve lift

Swapping from stock 1.5 to aftermarket 1.6 rockers adds 0.035" of lift and roughly 4-6 degrees of duration above 0.050" — a real cam change without pulling the cam. Chevy small-block builders have done this trick for 60 years: a $200 set of 1.6 roller rockers on an LS1 wakes the top end up noticeably.

The tradeoffs are brutal:

• Spring load skyrockets. Higher lift compresses the spring further, and the rocker also multiplies the force the pushrod sees. A 1.7 rocker with 400 lb open pressure at the valve puts 680 lb through the pushrod. Weak pushrods bend, tips mushroom, and lifters collapse.
• Geometry gets ugly. Rocker geometry is optimized so the roller tip sweeps the valve at mid-lift. Changing ratio moves the sweep pattern — the tip walks off the valve, side-loads the guide, and eats guide clearance in 20,000 miles.
• Piston-to-valve clearance shrinks. Extra lift means the valve dips deeper into the chamber. Always re-check P-to-V clearance after a rocker swap — 0.100" intake, 0.120" exhaust minimum.

Rule of thumb: if you increase rocker ratio, add roughly 10% more open spring pressure to control the higher velocity, and verify pushrod length with an adjustable checker — the geometry moves and the old pushrod is almost never correct. On a variable-ratio rocker (Jesel, T&D), the effective ratio grows through the lift curve, opening the valve faster off the seat where flow matters most.

Overhead-cam engines usually use finger followers or direct-acting buckets where the "ratio" is close to 1:1 — which is why OHC cams need physically taller lobes to make the same valve lift a pushrod engine gets from a small lobe and a big rocker.