I was wondering about this myself. My owners manual for the '72 4.5 call for a minimum octane level of 91 RON. The compression ratio is called out as "8". Is the "8" a MB short-hand for 8:1? That doesn't square with a 91 octane requirement.

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Litton
'90 420 SEL (sold)
'72 280 SEL 4.5
'98 ML320 (for sale)
'86 560SL
'05 Jeep Grand Cherokee Ltd (offroad in style)
'87 Chevy Blazer (AZ Pin Strips)