When trying to decide between two camshafts for an engine that will spend a majority of its life operating at low-to-moderate rpm on the street, it’s better to err on the side of caution. That means opting for the one with specs that are slightly tamer.
“What if it compromises full throttle performance?” you might ask. Well, you’ll probably be much more willing to spend an afternoon swapping rocker arms than spending a weekend installing a new camshaft just to find out!
A camshaft swap is rather involved. It includes partially disassembling an otherwise good-running engine to replace the cam, and if it’s a flat-tappet, there’s always the risk of lobe and/or lifter failure. To simulate the effects of a slightly larger camshaft, installing a set of high-ratio rocker arms is relatively easy and sometimes enough to noticeably improve performance.
Just how much of a difference is possible? We used our ’74 Super-Duty Trans Am to illustrate the effects. Here’s what we found.
Most Pontiac camshafts contained a maximum lobe lift of 0.271 inch, and the Division’s typical stamped-steel rocker arm featured an opening ratio of 1.5:1. Simply multiplying lobe lift by rocker-arm ratio will yield gross valve lift for a given camshaft. In the instance of many four-barrel and Tri-Power–equipped Pontiacs, it equates to roughly 0.407-inch valve lift—an amount that most Pontiac hobbyists are familiar with when speaking of stock grinds.
Considering that the intake airflow of a typical cast-iron D-port cylinder head peaks around 210 cfm at 28 inches of pressure, and it does so by about 0.450-inch valve lift, the near 0.400-inch valve lift that Pontiac engineers chose was close to the head’s potential. When selecting an aftermarket camshaft for an engine that uses unmodified D-port heads, it makes little sense to push valve lift much beyond 0.450 inch or so.
Most modern high-performance engine rebuilds include high-flow aftermarket cast-aluminum cylinder heads or modified cast-iron originals. And many aftermarket camshafts offer a reasonable amount of valve lift to accommodate the increased airflow capacity. When Pontiac needed more valve lift to take advantage of the new round-port cylinder heads it developed for the ’69-’70 R/A-IV, a stamped-steel rocker arm with a 1.65:1 ratio was employed, effectively increasing valve lift from the No. 041 to nearly 0.520 inch.
According to the late Mac McKellar, the famed engineer responsible for developing a number of Pontiac’s cutting-edge cam designs—including the No. 041—increasing rocker-arm ratio is an easy and effective way to increase gross valve lift from any camshaft. It’s less stressful on the block since the lifter doesn’t have to travel quite as far in the lifter bore, and it reduces the side loading that occurs when running an aggressive flat-tappet grind.
Rocker-arm ratio is determined by the pushrod cup’s position in relation to the fulcrum (or rocker stud). Moving the cup closer to the rocker stud increases ratio, but it also changes the pushrod’s path angle during normal operation. Most often, the drilled pushrod opening in a typical Pontiac cylinder head must be relieved using a grinder to prevent rubbing when using rocker arms with a ratio greater than about 1.6:1. And that’s a task that must be performed with the cylinder heads removed from the engine, and preferably completely disassembled.
There may be instances where the pushrods do not contact the drilled opening when using high-ratio rockers. That doesn’t suggest swapping rocker arms on any engine, however. It’s very important to know the condition and coil-bind limit of the valvesprings being used. (That’s not just limited to high-ratio rocker arms—it includes camshaft swaps, too.) It’s also wise to replace the original 3⁄8-inch bottleneck studs with quality 7⁄16-inch units. Not only does it allow for infinite valve-lash adjustment, bottleneck studs can and do break when valve lift exceeds about 0.450 inch or so.
[1] Rocker-arm ratio is increased...
[1] Rocker-arm ratio is increased by moving the pushrod cup closer to the pivot point. Comparing these two original Pontiac rocker arms, note the pushrod-cup positions on the typical 1.5-ratio unit (left) and the 1.65:1-ratio unit (right).
[2] Melling’s Rocker Arm Kit...
[2] Melling’s Rocker Arm Kit (PN MRK-532) includes a stamped-steel, 1.65:1-ratio rocker arm; a hardened rocker ball; and a 3⁄8-inch rocker nut for the original bottleneck studs. The complete set retails for around $225.
[3] We converted the Super-Duty...
[3] We converted the Super-Duty to a fully adjustable valvetrain with 7⁄16-inch ARP rocker studs (PN 135-7101), which retail for less than $45, and corresponding Melling lock nuts (PN MRM-1793), which sell for about $15.
[4] The rocker-arm swap begins...
[4] The rocker-arm swap begins by removing the valve cover from one cylinder bank. Aged cork valve cover gaskets can break. (Be sure to scrape the cylinder head’s gasket surface clean before reassembly.)
[5] To get the lifters for...
[5] To get the lifters for each rocker arm pair on the base lobe of the camshaft, the coil wire was pulled and the starter was bumped until the intake valve just closed. The rocker nut was removed using a deep 11⁄16-inch socket and the rocker arm was lifted out.
[6] The new ARP rocker studs...
[6] The new ARP rocker studs install without any modifications. The threads were lightly coated with 30W engine oil and screwed into place. A deep 11⁄16-inch socket was used to torque the studs to 60 ft-lb.