We should be using advertised intake closing points to calculate dcr. As pat Kelley states, true compression can not start until the intake closes. Using a formula to generalize @ .050" valve events to closing points completely ignores the lobe profile and introduces error into the calculation. The advertised event will be much closer to the actual closing point of the cam. Depending on lobe separation, a mild cam with advertised durations in the 260* range can drop the dcr two full points below static. Cheryl
That Wallace calculator and compression estimator is junk. Our little 283 makes 170#s with 7.7 dcr and 9.8 static compression.
I believe the industry standard is 0.006" lifter rise? Buick used 0.002" for their advertised specs which is practically useless information.... no one else uses that spec. Pat Kelly calculator: http://cochise.uia.net/pkelley2/DynamicCR.html
the point of posts were to say even with squish and the pent heads, you still need to watch how high you go with your compression. I think the design of the pent chamber head will only let you get away with 1/2 point extra compression at the most over a non nailhead motor, even with quench. that's just my opinion, with the engines I have had.
The 425+.030 that I just completed ended up at 9.97-10.03. Running a fairly aggressive timing curve with NO pinging/knocking type noises after driving for about 300 miles for proper break-in & checking for any problems.
Not likely to diagnose knock problems on build specs alone. Things like high coolant or air intake temp will undermine the best specs. Restricted exhaust, heavy vehicle weights, low gears, etc. have way more influence on where that "line" is. A real huge one is the width and diameter of the exhaust valve seat. That one feature alone can reduce the temp of the valve 100* per (x width) increase. Type and brand of spark plug becomes important when you are close to the line. That choice alone might vary depending on how radical of a cam you have. Beyond incorporating the most optimal specs at the front end, one needs the ability to read what the engine is telling you as well as identify the problem areas. Spark plug is a good place to start looking.
