Perhaps it is a better head that is adapted for Europe's diesel? maybe they have better fuel than us?
I suppose the pistons would also be easy to modify for higher compression too, but I'd be shaving the head down if I wanted more compression from an engine that already had a set type of piston, otherwise I'd have to add metal to the tops of the pistons or machine the tops down and install longer conrods.
Turbo pressure could also account for part of the power difference.
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