|Many Pontiac enthusiasts seeking more power look to the heads for the answer. Milling, longer valves, taller springs and porting may necessitate a cam change, higher ratio rockers or both in order to capitalize on the added flow and power potential. I am one of these power-hungry Pontiac guys.
For example, my 406 (+.030 400) sports a Comp Cam 270H Magnum hydraulic cam. With .313 lift at the lobes, a stamped rocker arm provides a theoretical lift of .470. (.313 x 1.5 = .4695) With a 1.65 ratio rocker the lift approximates .516. Considering the extensive porting work performed on my d-port heads, the higher ratio rockers are a good bet to help the engine develop its potential.
When a high ratio rocker is used, the pushrod, which transmits the open and closing events of the cam, moves closer to the rocker arm stud (intake side of the engine). Consequently, all pushrods holes (except heads #614 or #722 requires the pushrod holes to be elongated 1/3rd of the way down toward the intake side to avoid any possible interference between the rod and head. Every pushrod must move freely! Elongating the pushrod hole is performed off the engine. It is done with stones or high-speed carbide bits.
However, valve train geometry can be altered when changes are made to the heads. Valve train geometry must not be overlooked and one must never assume that it is okay. The object of proper valve train geometry is to get the most out of your heads and that includes durability. When the geometry is not correct, overloading may result shortening the life of the valve guides.
What is proper valve train geometry as it applies to our traditional Pontiac engines? If we look at the valve tip surface imagine it being divided in half. The ideal contact point is slightly before the centerline on the intake side (when the valve is closed) and slightly after on the exhaust side (when the valve is open). The contact point of the rocker arm moves away from the intake side to the exhaust side and back. Below are simple illustrations.
How do you go about checking valve train geometry? Its not a difficult process. Below are the steps to follow:
About | Contact PSP
To get the ink on the marker to stick to the top of the valve, you'll need to clean the flat surface of the valve tip. I've found that Brakeleen works best in removing oils.
Color the top of the intake and exhaust valve completely. With roller tip rockers or true roller rockers, the contac pattern will appear but could take more than two full crank rotations of it to do so. If in doubt rotate the crank again by hand. The fricition between the roller and the tip is present to remove or lighten the ink. In either case, you'll need to look closely. Stamped rockers will reveal a pattern, it may be appear more erratic looking and less defineable than with rollers.