Cam Quandary

The T/A 6.6 CamshaftsSince factory engineers must balance such characteristics as idle quality, throttle response, full throttle performance, and emissions and fuel economy when designing a camshaft for a specific application, the industry saw some very unique profiles emerge at the beginning of the emissions-conscious performance era. To maximize these characteristics in the Pontiac V-8, engineers redesigned the entire camshaft line for all 301, 350, and 400ci engines produced from 1977 forward.

There were two different grinds for the T/A 6.6 in '77, No. 549112 for automatic-transmission engines and No. 549431 for manual-transmission engines; and another, No. 10003402, for all '78-'79 T/A 6.6 engines. Valve timing of the three varied slightly, and each had its own effect on engine operation. Until now, the only details known about them was printed in vintage Pontiac Service Manuals and MVMA documents. This included advertised duration, intake and exhaust valve opening and closing points, and gross valve lift. From these, we calculated the ICL and LSA of each.

We know that the No. 112 and No. 431 grinds contain the same lift and duration, but the ICL and LSA are different. Because automatic and manual transmissions load an engine differently, manual transmission engines can typically tolerate more aggressive cam timing without compromising low-speed performance. And it appears that this was the case with the No. 431 cam. However, the No. 402 '78-'79 camshaft appears to be a compromise of the two, with significantly less exhaust duration.

Those familiar with cam specs may notice that the T/A 6.6 units have an ICL greater than the LSA, indicating that the intake lobe is slightly retarded. Presumably this would have a unique effect on engine operation, but what? Was it to improve performance and overall efficiency? And what are the 0.050-inch duration specs, identification stamps, and color codes of them? With so many unanswered questions, we were determined to find out.

Physical ComparisonThe first step in answering these questions is to obtain each T/A 6.6 camshaft. But since they are no longer available from GM, we had to locate used examples in excellent condition. Quick calls to Dave Hall, owner of Lincoln, Nebraska-based RestoreAMusclecar.com, and renowned W72 Performance Package-guru John Witzke produced an assortment of units that had come from their original applications.

A visual inspection revealed that the No. 112 had faint traces of its green color code, and a stamped "O" on its rear journal. The No. 431 cam had traces of its orange color code, and a stamped square on the rear journal. And the No. 402 cam had traces of its white color code, and was stamped with a circled letter "A."

We then measured the physical characteristics of each by inserting it into a block and rotating the crankshaft while recording tappet movement at specific lobe lifts with a degree wheel and dial indicator. This would accurately provide the 0.050-inch duration and lobe lift values that we were most concerned with. To maintain comparative accuracy we reused the same components and carefully replicated the testing procedures to measure two sets of lobes on each camshaft.

Although our recorded lobe lift varied from what was printed in factory documents, we're confident that the duration and lift values were measured correctly. We then loaded the information into our Performance Trends Engine Analyzer dyno simulation program. Our experience is this program can accurately predict how slight differences might affect engine operation and performance. And since we use it on a regular basis for myriad projects, a number of stock Pontiac engines are already on file. So we selected our stock T/A 6.6 engine profile and inserted the different grinds.