Cam Quandary

The '70s was a trying time for auto manufacturers as much of the decade was spent trying to increase fuel economy and reduce tailpipe emissions without compromising performance. For Pontiac engineers, this meant working with less compression in 1971 and EGR in 1973. The '75 model saw yet another compression ratio reduction and the addition of an exhaust system catalyst. Combine all this with high rear-axle ratios aimed at reducing engine speed, and actual vehicle performance was lackluster at best.

A new optional performance package emerged for '77. And as we learned in "W72, A Complete Performance Package" in the Sept. '04 issue of HPP, it included the first emissions-conscious performance engine of its time: the T/A 6.6. The new 400-inch mill churned out as much horsepower as the 455 from the previous year. In addition to its higher compression ratio and specifically calibrated carburetor and distributor, the T/A 6.6 incorporated three unique camshafts to achieve its goals. But until now, researchers had only limited knowledge of the camshaft details. So we gathered examples of each for an in-depth comparison. Follow along as we share the results.

Camshaft BasicsThe Pontiac V-8 camshaft is responsible for several key functions of engine operation. Though it drives the mechanical fuel and oil pumps and ignition distributor, its primary function is to open and close the intake and exhaust valves at specific points during the crankshaft's rotation. Lift and duration are the specifications hobbyists are most familiar with.

Lift is the inch amount that the tappet is raised off its base lobe, and gross valve lift is lobe lift multiplied by the rocker arm ratio. Duration is the total degree of crankshaft rotation that the valves are in motion, and the most common forms of it are advertised and 0.050-inch lift. Advertised duration is the point that tappet lift begins and ends, but because each manufacturer measures in its own way, the values are virtually meaningless when comparing. The aftermarket began rating duration at 0.050-inch tappet lift, the point that significant airflow is believed to begin. By using this value, we can accurately predict a camshaft's effect on performance.

Two other camshaft specs often overlooked that can have a significant performance effect are intake centerline (ICL) and lobe separation angle (LSA). ICL is the crank angle of the intake lobe at peak lift. It can be repositioned once the cam is ground with an offset cam key or adjustable timing set, changing the intake-valve opening and closing points. LSA is the degree of separation between the ICL and exhaust centerline (ECL). Repositioning it changes the point at which the exhaust valve closes as the intake stroke begins. But since LSA is a fixed value, it cannot be modified once the cam is ground.

To better understand the effects of ICL and LSA, we must define the two types of compression within an engine: static and dynamic. Static compression is a fixed value calculated from such parameters as bore, stroke, and combustion chamber volume. It is simply the volume compressed during piston travel from Bottom Dead Center (BDC) to Top Dead Center (TDC). Dynamic compression is related to static compression but is more closely associated with cylinder pressure. It is the volume trapped at the point the intake valve closes and actual compression begins. It can, however, be modified by repositioning the ICL.