Why marine headers Have so large collector diameter? Just bought two pairs of stelling white steroids, primary tube id 2", collector id 3.8". What you guys think, Would smaller 3" collector work better on 502cid engine?

I think it has something to do with introducing water. But I’m not sure. It is an interesting difference to note.

That might Be The case, But isnt The stelling taill pipes usually dry?

Unfortunately these stelling whites Are For TRS drive boats and The collector IS too far Back. How about The primary tube Inside diameter? IS 1.7/8" primary tube too small For 670hp BBC, looking at stelling greens, merc cmi, usually 1.7/8" headers.

Choking down the exhaust is never the answer.

I respectfully disagree.
But i guess it depends on what you mean by ‘choking down’

Tuned header systems use exhaust energy to draw intake charge early in the int valve opening to get the intake charge mass moving earlier...so it can move more later on in the charge timing.

The exaust energy needs velocity and tuned length to achieve the desired harmonic at the desired rpm.

Conversely, a log style exhaust is more brute size and getting it into the exhauat tails. But it still can't be too large.

Header collectors are sized to effectively bring the 4 tubes together and merge the exh flow. It's sized a little smaller to increase velocity for the merging. Similar to how the intake valve throat (seat and shape/diameter) promotes flow around the valve head.

Theres a good thread on Speed-Talk about a NASCAR header design/fab guy dynos full tube headers vs log manifolds. They headers helped out..but not by much.

This assumes the engine is well setup and the parts complement the intended use. Generally, a bit smaller than optimal intake port CCs (CSAs) are better. It promotes port velocity, and marine BBCs don't spin up very high, demanding less of the port area.

1 7/8” is perfect. 1 3/4 would maybe even be a little better but it would be close

Exactly what I said. Going from 3.8" to 3.0" choking it down. We can get into as much pipe tuning dynamics as we want. I have never seen any dyno or real world application where "backpressure" makes more power. I have seen hundreds of header and exhaust dyno's to prove other wise. Collector length yes, smaller collector never. 2 strokes and tuned pipe expansion chambers, yes. But a backyard mechanic such as myself erroring to smaller collector, no.

Seriously?...
It makes more power throughout the power curve.