Hi Doug, 

My setup is for a Renesis so bit different to an 86 13b, what I found worked well is a short & compact muffler, all homemade, a 60mm perforated ss tube welded to the mazda renesis exhaust manifold that was shortened as much as possible. I perforated the 60mm tube with 10mm drilled holes, enough so area of drilled holes is same as pipe inlet.  A 100mm ss tube covers the 60mm tube and outlet of 100mm is at same end as inlet of 60mm, this is important to send sound waves back to source (works similar theory to ANR headset apparently). muffler is short, approx 150mm to keep it rigid & fit the space next to the psru. end plates of muffler were cut from 6mm plate & fully welded to both inner & outer tubes and covered with lightweight tin can as heatshield.

If I was using p port exhaust I would mount individual mufflers directly to each port, inner pipe same size same as port, would look like 2 cans protruding from side of engine, connect short tailpipes (same volume as inner perforated pipe) as close to engine end of outer pipe as possible to exit cowl. Idea is to send sound waves straight back to where they come from without restricting gas flow.

Probably as clear as mud, but my 2 cents worth, works a treat.