The 85 300SD has higher HP than the 81-83 300SDs and 82-83 300DTs. 81-83 SDs had 123HP, as the 82-83 300DTs did, I think 84-85 300SDs and 300DTs had 125 HP. They also have different gear ratios, so you get lower RPMs high speeds compared to older 300SDs and 300DTs. I don't know if the takeoff speed or top speed is any higher though. Someone correct me if my data is flawed.
-Joe

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.