Depends on engine manufacturer and sometimes even product line.
The Detroit numbering you mention specifically pertains to the two-strokes. The first number is the number of cylinders; no intervening letter is a flat configuration, and a "V" would be a V-configuration, and the last digits refer to the displacement of each cylinder in cubic inches. Thus a 6-71 is a 426 cubic-inch (~7 liter) in-line six motor, and an 8V92 is a 736 cubic-inch (12 liter) V-8 motor.
HTH.
-Sean
http://OurOdyssey.BlogSpot.com