Oh lord.

The B16A/B17A (PR3) head and B18C1 (P72) heads flow almost identically. The biggest differernce at any point on that graph is ~2.5 CFM which is completely insignifigant.

The PR3 head has a higher combustion chamber volume. This means that it provides a slightly lower compression ratio than the P72 head. Similarly, the Type-R motors use a hand-ported version of the PR3 head with their own intake manifold. The larger combustion chambers mean a larger surface area which in turn makes the head a bit easier to repeat the same port job over a production line. One could also argue that the PR3 head provides more generous "squish" area for high-CR running.

If you put a Type-R PR3 head on a flow bench I guarantee it would produce noticeably better numbers than a PR3 off a B16A or B17A.

Now I swear that I've typed this exact reply in any number of threads, even down to responding to dubster99's freakin flow bench graph. Geez people, yall aren't very creative at ALL.