Hello woody,
thank you very much for the pictures and comparisons. BTW nice shoes.
What i was not aware of, when writing my last post, is that the US 986 00-04 headers have an additional catalysator. The ROW headers don't have that. They look more like the 97-99 header manifolds. The 97-99 exhaust system seems to have a different designed muffler system than the later cars.
I think it's pretty obvioius thet the 987 and 996 have a bigger diameter. In general the diameters in automotive industry are measured in inches (exterior diameter). So i would suspect from your measuring the 986 systems are a 1,5" (38,10 mm) system whereas the 987 / 996 systems seems to be 2" (50,08 mm).
If you can delete catalysators that is always a good idea to gain power. But you have adjust the ECU tuning, else there won't get a big benefit (in general stock catalysators are 400 cell catalysators).
What does the exhaust tip diameter has to be if have two 2" (50,08 mm) inlets? So one inlet has an area of 1970 mm2. Now we have to calculate that back for 2 inlets and get a diameter of 70,82 mm or 3940 mm2 for an optimal exhaust tip diameter. (Please note that all values are rounded.) So if the singular exit tube is 70 mm it's absolutely OK and correctly designed.
In my personal opinion 2 separated exhaust tips won't be a plus because of the x-pipe factor.
A before and after dyno run is always a plus.
Regards, Markus