After re-reading your question I realise I didn't entirely answer it.

As far as I know there is no specification for flow or backpressure. Of course the more flow and lower backpresure would be better for fuel economy and power. Any muffler will probably work, except mabey a glasspack because it will foul with soot.

I still say more noise comes from the engine compartment than the tail pipe with the stock muffler though.

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green 85 300SD 200K miles "Das Schlepper Frog" With a OM603 TBO360 turbo ( To be intercooled someday )( Kalifornistani emissons )
white 79 300SD 200K'ish miles "Farfegnugen" (RIP - cracked crank)
desert storm primer 63 T-bird "The Undead" (long term hibernation)

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