I see that these have a different part number but it is not obvious that there is actually any difference. Does anyone know if they have a meaningfully different fitment or ID/OD? IE, can I bolt NA mufflers up to a turbo Y pipe without issues?

 

the turbo mufflers are physically larger, the cans are bigger, but the fit is the same

 

What mufflers are you talking about? The stock ones, a specific aftermarket brand, etc. The stock ones will bolt up. Stock replacements should as well. I'm not sure of the difference beyond I think the turbo's piping is a larger diameter. Before they went to the same muffler for both Racing Beat's TII ones had larger diameter piping but would bolt to either stock Y-pipe or their own. Except their Rev TII ones only work with that particular system, It won't bolt to the stock Y-pipe. All other aftermarket currently available are either singles or use their own Y-pipe so they're not compatible with the stock one.

 

OEM. Since the heat shields etc are all in good shape on my car but the mufflers are EOL due to internal condensation, I'd like to maintain correctness. I am pursuing lines on good used ones from an 88 GTX and might've found a couple NOS NA ones but no S4 TII ones as yet.