Very interesting observation

[Frank Acampora]

Regarding late model Force engines 1991 to 1996 or so: I have recently observed that the stainless exhaust plate on my 75 is different from that on my 90. The 75 has a significant bump out, reducing the volume of the exhaust cavity. The 90 has a flat plate. Part number for both engines of the same year is the same, however, the drawings are different.

Unfortunately, my 75 is nowhere near ready to run so I can not test any power difference by substituting the flat plate for the bumped-out one.

However, food for thought for any of you who have a 70 or 75. The plate is only about 20 bucks and the gaskets are only 3 bucks each. It might make an interesting experiment to see what happens.

To me, It looks like Merc wanted to use the same set-up and porting on all three cylinder blocks to save money. They then "strangled" the 70 and 75 exhaust to help reduce power output. I could be wrong, but manufacturers have done stranger things to save money. Unfortunately, I do not have a 90 block to set beside the 75 and measure ports.

The three carb 90 has about twice the venturi area (4.05 square inches) as the single carb 70-75 (2.07 square inches). Perhaps the reduced carb capacity and greater exhaust back pressure would account for the 15-20 horsepower reduction.

 

[Lyle29464]

Re: Very interesting observation

Not sure if that would work. I do know that the older OMC's used 3 or 4 things to pump the HP. different heads. different reeds, and the exhaust plate. It seemed like you needed to change all 3 to make it noticeable.