My Rabbit is an 81 NA diesel with a compression ratio of 23:1. the only turbo engine in my manual is an 83 which is also 23:1. it also says that all diesels up to 83 should get 406psi compression minimum. Mercedes 617s are only 21:1 and only 284-327psi compression. It seems like a lot of room for improvement. I guess thats why the engines last so long. Accounts for why nearly twice the displacement only offers 50% more power.

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81 VW Rabbit diesel (247k kms)
76 300D (119k miles)