Since Markus was in the enviable position of having all four types of E30 M3 headers in his
possession at one time, he was able to make reasonably accurate and very comparable measurements of the pipe
diameters. Both primary and secondary outside diameters were measured, as well as the tubing thickness.
This is extremely useful information. Markus' measurements are summarized in the table below.
I have added in two headers which were not part of this measurement set.
The 215HP 2.3L S14 header is the same as the standard 2.3L header. This is known from Markus,
who is quite familiar with the configuration of the European 215HP E30 M3.
Also, the Italian 320is "M3" header dimensions are added on the basis of information obtained from the ETK.