Quote:

Perhaps I should have called it a "rule of thumb", not a "magic number", this I'll concede.


Perhaps...
Quote:

This number is based on the physics of volumetric transfer. I don't think these mathematical equations "expire" with time.


What are these equations to which you refer and how do they really apply when considering that (as pointed out above in Polyspheric's reply) the conditions under which the intake and exhaust ports operate in the real world are so vastly different that it's almost a shot in the dark to try to attach an arbitrary # based on some perception of similarity?
Quote:

Why does just about every internal combustion head ever manufactured feature an exhaust valve approx. 75% of the intake?


Is this 75% of diameter, or 75% of valve curtain area? And which would really be more suited to a comparison of flow #s?
Quote:

You may really mean to point out that this rule of thumb isn't foolproof every time??. To which I'd still ask you to come up with a fact based equation that outweighs it.


There isn't one, which is why saying 75% is invalid. What do you think a modern Pro Stock head flows percentage-wise on the exhaust side? It's about 60%. If 75% was the trick to making more power, wouldn't you think those guys would find a way to increase their exhaust port efficiencies to achieve that, rather than simply adding another 10+ degrees of exhaust duration?
Quote:

Bottom line, heated exhaust gas needs approx. 75% of the volumetric space to travel at the same velocity as the cooler, wetted intake charge.



Bottom line is the exhaust needs of a Pro Stock-type high compression engine, or anything along those lines, aren't the same as something that you or I might build in our garage. Trying to box in the camshaft parameters by quoting the old 75% ratio is oversimplifying the situation and the solution.

Enough of this theoretical I have real engines to work on.