Port size on Martin's conversion headers
 

Monster Miata customers, can anyone tell me what the port dimensions are on Martin's headers where they meet the heads, and/or what gasket number they closely match?

I am going to gasket match the exhaust ports on my head and want to be sure they match up to these headers. I especially want to be sure I don't get the ports larger than the header openings and create a blockage at the header flange.

I'll gladly measure mine later tonight for you....but how would you be able to line the 4 ports up horizontally? ie, port size verses actual port location on the flange?
I wrote down the gasket Martin recommends but will have to find it. He said it was in the manual but I never saw reference to an exhaust port part# in mine....maybe an older version?

I posed this question to Martin today and he was able to get back to me promptly. He indicated the header flanges were designed for a Fel-Pro #1415 gasket. The port size on that gasket measures 1.25" by 1.48".

I have a set of Martin's headers that are about 10 years old that I can measure, but I wasn't sure if the design had changed over time. I'm looking to get my head's exhaust ports well-matched to the set of his headers currently still installed on my '90, which were manufactured around 2011. In his reply Martin indicated that the flange design should not have changed over time.

The opening in the flange on my "old" header set is 1.25" by 1.875"...quite a bit taller vertically than the 1415 gasket he specified. Of course, the header's exhaust pipe walls taper downward pretty quickly after the entrance at the top of the flange, so a somewhat shorter port/gasket would not necessarily be bad as it would direct flow towards the "open" part of the tube rather than making part of it change direction.

To answer your question, you match up heads to header by lining up the bolt holes on the gasket with the bolt holes in both the heads and the headers. Comparing the fit you can "see" how the parts will mesh together when bolted up. With care and patience, the matching can be fairly precise.

If you want to get really precise, like I did when I had my lower intake matched to the head's intake ports, you can tape a long sheet of paper over the header flange. Then take a sharp-pointed #2 pencil, hold it at a low angle to the paper, and run the long surface of the lead around the edges of the ports and the bolt holes. A perfect pattern will emerge right at the boundaries of the ports and the bolt holes. Cut out those openings with an exacto knife, and "bolt" the paper template to the engine. You can then see exactly, probably within 1/32" or closer, how everything will fit.

Actually it doesn't hurt much if the opening you are flowing air into is a little BIGGER than the opening that's feeding it, as no "wall" or blockage to the flow is created. If it were a LOT bigger it will create turbulence. Conversely, you can actually have up to about an .080" "wall" sticking out into the flow due to a port mis-match without seriously hampering performance, as it doesn't protrude past the boundary layer of laminar flow.

If you get a chance to measure your headers, it would be interesting to see how close the flange dimensions are to mine!

Interesting read.
My "as close as i can get with a tape measure" measurements are 1 15/32" high by 1 3/32" wide.
Thanks for the reminder of the gasket # too.

The flanges on my headers were way off. They must have been for some type of high-port head. I had to re drill, or slot, the mounting holes & weld up the old ones in order to lower the ports. My flanges were off over 1/4". Martin informed me that his header supplier/manufacturer had used the wrong flanges. I made them work for me, though. There still may be some of those mismatched headers out there. Mine were from around 2006.