I’d heard they used old high-mileage road car blocks
this, they used to go cruising the taxi ranks looking for high mileage 3 series beamers, drivers would be offered a new car in a straight swap. Then they had detonation issues, that were finally solved by some old WW2 era engineering bod digging around for some Luftwaffe fuel recipes as he remembered the me109 had the same issues…
they know it made 1500 on the dyno,
Dynos were only accurate up to about 1000 or so and the very last M12 version (1987) put out an “estimated” 1400, in qualifying it ran at about 850 and race trim it was routinely run at about 650 , otherwise it would lunch itself, still good enough for over 220mph though…