OK,
TDC refers to the piston position and then relates to cam and ignition timing for that cylinder. If you were timing a V configuration engine then you would need a reference from the crank to each bank of cylinders, a radial engine you may need to time each cylinder.
For general tuning the TDC timing mark is for number 1
Engines that use sequential multipiont EFI can inject after the overlap to avoid the polution thing and I guess the technology may and probably has gotten to this piont, hence the price and rarenes of aftermarket cams for the BP
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