Originally Posted by 1965tripleblack
There is a huge thread on this topic that has been moved to "The Dump". Why?
I didn't have time to go through the entire thread, but I immediately noticed that the duration used for the calc was the wrong duration.
The only way to figure DCR is by using the duration where the intake valve is effectively closed, which is the "seat-to-seat" duration, AKA, "advertised" duration. Comp Cams uses duration at .015" valve lift, and sometimes .012 amd .010 as well. This is known in the SAE as J604d duration, which is duration at .006" valve lift.
Was this ever addressed in the thread?
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.