A single 3" tube flows close to 2 x 2.5" tubes , so that would be fine for c 500 bhp.
I would agree with the <1.75" for this displacement and RPM. There are a lot of examples of folks going too large on the primaries in respect to the motor needs. Every calculator I've ever seen assumes dual collectors, so that may be something that will be hard to determine for a single collector. I've seen some stuff that indicates the collector diameter should be larger than intuition, and primaries small than intuition.
For a 383 a 3.75" OD x your calculated Primary Pipe length tailpipe will suit for an 8 into one system, in most rear engine cars this is too long and requires some clever pipework to fit. Unless your an absolute balls to the wall racer going longer & smaller dia is usually best.
For a 383 a 3.75" OD x your calculated Primary Pipe length tailpipe will suit for an 8 into one system, in most rear engine cars this is too long and requires some clever pipework to fit. Unless your an absolute balls to the wall racer going longer & smaller dia is usually best.
So Im about to order materials and the collector, in your post above are you recommending a 3.75" exhaust diameter or were you perhaps referring to the 1.75" primaries? I was planning on running a 3" exhaust pipe. I can go larger but prefer not to if I dont need it.
You might find it better to fabricate your own collector, the one shown in your photo looks pretty, but due to that it does not give a definite end/length to the primary pipe.
If you choose to go 3" tailpipe it might need to be longer and could kill a bit of top end power... depending on what your rev limit is.
Pardon perhaps a uneducated question: If an 8-to-1 collector is used, wouldn't the exhaust timing pulses be evenly spaced as each passes through the collector (assuming equal length primaries), and thus wouldn't the collector then become an extension of any single tube of the primary system, and only need to be of similar sizing or larger by one or two steps? I would assume the scavenging benefits would be pronounced as well.
Will the old headers that came with the car fit your 383 heads, if so why not simply join those two collectors into one large pipe, rather than waste a lot of time and effort building a complete new system based on a lot of unknown factors for the 383 engine. Even if the head bolt pattern is different you could chop the old plates off and fit correct ones. Then if you like the sound build a correct system later.