Originally Posted by oldbogie
You are pretty much correct on this. The rising piston is applying a lot of pressure to the mixture in a situation not unlike the exhaust side during that cycle. So having the intake closure point at .015 inch and gussing an additional 15 degrees if that's even done is simply not adaquate as the pistong will be forcing much greater reverse flow than one would think if that information was related to intake flow at a depression of 28 inches of water.
Different manufacturer's give absolute degrees in some value other that truely shut valve degrees these are often measured from .001 to .006 inch from the seat. While not "perfect" this is a lot better situation than from .015 inch or guessing that the cam actually seats the valve with an additional 15 degrees as many cams, especially older grinds have extraordinarily long ramps and 15 degrees doesn't get close to true valve closure.
Old flat tappet cams, like the "30-30" have ridiculously large spreads between .050 (254) and seat-to-seat (310) durations.