Howdy Ronn and All:
Cam Profiles- Ronn, If you can imagine a comparison of an egg and a pointed dunces cap. Imagine that they are the same height and the same width across the base. IF you were to superimpose the profile of the egg over the cap, note the area of egg that overlaps the hat. An egg profiled cam, with the same degrees of duration, lift and lobe seperation, will out perform a cap shaped cam. The duration at .050" begins to tell the rest of the story.
Back in the old days of serious "stock" car drag racing, stock cams were determined by duration and lift alone. It didn't take the boys long to figure out that a fatter profile out performed the stock profile. Thus cheater "Stock" cams were born.
Shimming stock springs- Stock springs are measured for seat pressure/valve closed position and Valve open. They are rated at a given height of 1.22" open and 1.69" closed. Stock '78 and later springs are rated at 150#@1.22" valve open/spring compressed. And 54#@1.59" valve closed.
Wear of the valve face and seat, valve stem/rocker arm contact, and valve spring pockets cause the actual valve lengths to grow. As these dimensions grow the spring has less compression and, consequently less tension. These factors all add to less spring pressure and premature valve float at higher rpms.
Given your investment in your engine it seems to me that starting with new stock springs, or at least, tested-up to specs, old springs, and shimmed at least to specification heights is cheap insurance. Spring shims were initially designed to allow head rebuilders to get springs back to stock specs after machining seats and valve faces. Exceeding spec heights is good to a point. That point is coil bind, or compressing the stock springs so tight that the coils are closed and therefore solid. Shimmed stock springs with a higher lift cam should be checked carefully before start up.
Also, with a higher lift cam, make sure that there is clearance between the bottom of the retainer and the top of the valve stem seal.
Anymore spring pressure than you need to control valve action at your max rpm is wasted, and causes premature wear.
Lighter weight or lightened components should also be considered when planning a performance valve train. Adjustible rocker arms, and corresponding pushrods, will also help in avoiding valve float caused by hydraulic lifter pump-up.
Dual Pattern Cam design- In theory, the longer exhaust duration is supposed to compensate for exhaust flow deficiencies. NASCAR engine builders found some value in dual pattern cams at restrictor plate tracks. Even with headers, the downward kink at the port exit in six cylinder heads impedes exhaust flow as rpms go up. A dual pattern cam can't hurt and may help in some areas.
It's what I'm using next. I'd love to hear from anyone who has already gone that route.
Adios, David
