Interesting that you would mention 928 and Mercedes V8 in the same thought. The relationship between Porsche and MB has been a productive one on the performance end of the spectrum. The 'guts' of the first automatic trans in the 928 were sourced from the MB 6.9 automatic, we all know about the 500E joint venture, and (on point to your discussion) it is said that AMG used dual 928 catalysts in its free flow exhaust for high-output, mid-80s MBZ V8s.

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Richard Detoy
'84 300SD
'76 Moto Guzzi 850 Le Mans