Lynn,
The addition of a star to a WW2 British vehicle designation just indicates that it is a slightly different version of that vehicle, and in almost any way ! They really did get themselves into a bit of a mess with these designations so don't look for patterns, there aren't any !
The GM two stroke diesels in Valentines were, as mentioned above, of various different power outputs. These were standard variations available from GM from the start and basicly differed in the settings of the injectors and govenors. The M3 / M4 Medium tank twin 6-71 unit was the most powerfull available from the start but again was a standard variant, designed initially for generator and pump sets and even as quad sets. After the war these engines were offered with different cylinder liners with better gas flow and then four valve heads (all exhaust) and eventually with turbochargers as well as the superchargers. Finally they were killed off by emissions regulations.
It should be mentioned that the fitting of the GM engine instead of the AEC engine in the Valentine resulted in almost the entire instalation being redesigned, including a new (much better) gearbox. It was never possible to swap engines into an existing tank as is hinted at by a reference above.
David
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