Finally I have the ability to add real figures to this thread! By my calculations the following should be adopted for optimal performance. Oh and these sizes are MINIMUM ID (NOT OD as pipes are normally specified). Include lots of bends (and not straight-through mufflers) in your design and you should go even bigger....
Twin system (ea): 59mm (~2.5")
Single: 70mm (2.75")
Twin system (ea): 58mm (2.25")
Single: 69mm (~2.75")
PS I had to take an average port size on the 560 since they're oval and not round. Hence there's not much difference due to the port sizing.
The more I think about it, the more I think a single 3-3.25" OD would be more desirable than a dual exhaust. It would weigh less, flow the same or better, would be easier to make, use less material and therefore cost less. It would also eliminate the need for a balance pipe.
I think dual exhaust was used due to ground (and other) clearance issues and the possibility 3" mufflers weren't common (or available)? There is also the possibility duel exhaust had the perception of power similar to quad exhaust now and 4 stacks on the old ocean liners!
So my next exhaust will be a single where possible, with a dual outlet.
I’ve just been doing some calculations & if you believe one school of thought, exhaust primaries should be a minimum length for optimum flow. Of course space is the biggest issue here, but by my math (using optimum diameter pipe), the minimum primary lengths are as follows: