Force HP rating question

[Bigprairie1]

Just curious but does anyone know what changes Force made to (supposedly) 'up' their hp on the 90's 90hp from the previous 80's/85hp? ...and conversely what they did to the 125hp to 'down' the hp to 120hp?
(..minor carb changes?..timing?....or?)
I'm no expert but there doesn't appear to be a lot of physical/parts differences on these motors.
Any opinions on this?
Thanks
BP

 

[Nate3172]

Re: Force HP rating question

I had wondered the same thing a while back and could not find any substantial difference, if any at all, between the parts either. The only conclusion that I have come up with (since I don't have access to a 90hp) is that they possibly changed the port timing on the cylinder sleeves and may have also enlarged the ports. I don't think the actual ignition timing is any differen't either so I ruled that out.

 

[roscoe]

Re: Force HP rating question

The ignition systems are different on the models that did away with the blue cd boxes. And the timing is different on the Thunderbolt and cdM ignitions.
Carb jetting is different, depending on model. ANd I believe the "rated at" rpm is different.

Porting may be different, not sure.

 

[Nate3172]

Re: Force HP rating question

The only difference between the blue cd boxes and blue coils versus the black and black is the resistance accross the ignition sytems. Less resistance on the blue for prolonged life span because of less heat from the lower resistence. So, if it has prestolite, it has the same timing from an 85 to a 90.

 

[roscoe]

Re: Force HP rating question

Yes, timing was 28, 30, or 32*, depending on ignition type.

 

[Frank Acampora]

Re: Force HP rating question

The old 85 (and old Chrysler 75, 85, 90) was 2.80 inch stroke by 3.3125 bore. Horsepower was generated at different rpm with different carbs and, I think, different compression ratios.

The newer 90 is 2.80 stroke by 3.375 bore for about 2 cubic inches more. The old 85 had regular aluminum reed blocks with 5 reeds each side, the 90 had rubber covered reed blocks with 4 larger reeds each side and much smaller reed stop plates for better induction. Havn't had my 90 apart yet but porting may also be slightly different.

The 120 is 2.80 stroke by 3.3125 bore. the 125 is 2.86 stroke. by 3.3125 bore. The ports are .060 higher in the cylinder to compensate for the longer stroke and, of course, the carbs are different.

 

[Nate3172]

Re: Force HP rating question

The bore and stroke are probably what are creating the majority of the difference. Unfortunately, my Clymers manual does not give me bore and stroke specs so when the difference between the two question popped into my head I didn't have a whole lot of info to go with. Thanks for the specs on that Frank, they could prove to be useful for me later down the road.

 

[Frank Acampora]

Re: Force HP rating question

I had a 125 that scored a crankpin. I did not have a 2.86 crank to put in it so I put in a 120 2.80 crank. The engine ran quite happily but my top speed was down about 3 mph which would translate into maybe (guess) 15 horsepower. Of course, part of the loss would have also been due to the (now) more aggressive porting. I'm sure on a light boat with the engine revving a lot higher, those raised ports would have translated into more horsepower but at 4500 rpm, they hurt it. You see, porting is in some respects scientific but it is also esoteric and occult. Some is cut and dried and some is an educated guess. Factories can afford to play and experiment. You and I can not.