Yet more confirmation that 1.25" is the right choice for the primaries:
Exhaust Header Design Comments
Same formulas as in the early posts of this thread, but makes a specific point to say "Note that 1-1/2” pipe is large enough for a 273” motor with max torque @ 4000 RPM." ... for reference, a 273" has 559cc per cylinder compared with a D16's 399cc.
On a side note, does anyone have an idea how much dyno time (and cost) would be involved in doing a full BSFC map for several throttle positions? I have an annoying feeling the answer is 'a few hours' and 'pricey' :-/ but I'm thinking A-B-A testing at cruise won't really show up header differences very well w.r.t. torque curves etc., and I'm not sure A-B-A testing going up hill would be very easy to get consistent (doubly so as I have no cruise control). Not to mention that A-B-A testing on a header is a fair bit of effort and waiting for the thing to cool down enough would put a bit of time between runs...