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The 472 mph was supposed in very high alt with Nox injection.
...of 269 mph...
Here is the link with tables some pics (all DB enignes on the pics, though):
Daimler-Benz
Wonder whether someone has better quality copies of the tables, though.
So what kind of performance gains are we talking?
Without that huge rpm increase, going just from 3 to 4 valves (maybe with slight rpm increase), it may have gained 100-200 PS in both boosted and non-boosted modus operandi.