In the Late 70s through into the 80s there were 3 inline 6s, a 90, a 115, and a 140. Although all of them were the same Displacement, and shared a few Parts like the Crankshaft,, there were many differences, Porting and Port Timing was different, in some cases there were what was called Power Porting, which required a different Block and Pistons. The Number of Reeds could vary, Carb Size would change, as well as Exhaust Tuning.
In the early 80s, OMC also had 3 engines of the same Displacement, again they were 90, 115, and 140. Porting was different, as was exhaust tuning, Carbs were different sizes, and the Cylinder Heads varied in Combustion Chamber Volume.
These OMC Engines did offer the Option of, by changing some Parts, and some machine work, the 115 could be converted into a 140, and the 90 could be converted to either a 115 or a 140. That wasn't possible to do with the Mercs, oh they could be hopped up, but they wouldn't be like a Production Motor
On some of the OMC 9.9 and 15hp, it was simply a carb Carb Change, on others it was a Carb, and Exhaust Tuner. On the OMC 22in Motors, known as the 18/20hp also shared the same Displacement. On these the Blocks and the Porting were Identical, but in this Case, the Port Timing was changed by using a Piston where the Location if the Piston Pin was different. This also reduced the Compression Ratio, so a different Cylinder Head was used to regain it. Crankshaft was different, as in being much more accurately Balanced, as the 25 WOT Range was 1000 rpm higher than the 18/20. Putting a 25hp cylinder Head on a 18/20 would result in too much Compression, Detonation, and Engine Damage
