1991 70hp Force or 1975 75hp Chrysler which better

[fcross69]

I have just aquired a 1991 Force 70hp I have a 1975 Chrysler 75hp on my boat now which motor would push my boat better is the newer Force 70hp going to have more useable HP or will the old Chrysler 75hp going to be better? What do you all think?

 

[Frank Acampora]

Re: 1991 70hp Force or 1975 75hp Chrysler which better

Both engines have pluses and minuses. The old 75 has 3 carbs, a 13 inch prop, and probably better low end torque. It was, however rated at the powerhead and the gesars and water pump will absorb 10-15 horsepower leaving it with a net of somwhere around 60-65 HP. Props are becomuing more difficult to find and I don't think anyone re-hubs them anymore. Years ago I had a 75 on a heavy Chrysler 16 foot hull. It drove the boat to a respectable 30 MPH with good acceleration.

The 70 uses an itty-bitty 10 inch prop and has a single large carb. It is rated at the propeller though. You can use some mercury parts and propellers and it does have a superior ignition and water pump. I have worked on a couple and they do have surprising performance. The couple I rebuilt were on aluminum fishing boats and surpeised me with their acceleration and top speed the first time I tested them on the water.

While I do like the old style cowling of the Chrysler, if it were my choice, even if performance were the same, I would probably go with the newer 70.

BTW: the 70 uses a special shaped stainless plate in the exhaust cover to restrict exhaust. Simply switching to a flat plate off a 75 or 90 will give a couple of horsepower increase--not up to 75, but probably enough to notice a slight difference in performance.

I say, run the 75 and see what rpm and top speed you get. Then mount the 70 and compare. Come back and let us know your results. This should be a very interesting comparison.

 

[fcross69]

Re: 1991 70hp Force or 1975 75hp Chrysler which better

Frank I was getting 37mph on gps at 5100rpms with the 75hp on a balyliner 1750 that weighs around 1900lbs with motor fuel and 2 people. I did switch to 115hp lower unit like you sugested and did do some head an port work to this motor. I run a 13"x17" prop on that motor I just wonder if the Force 70 will have as much pull or not? I know the Force would have more on top end but like the idea of newer electronics and single carb I wonder how much the difference they will do on fuel? The force has a much more up dated tilt and trim system also. I like the exhaust mod on the Force also lol It's cause of you Frank I got addicted to the Chryslers and haven't been disappointed yet.

 

[jerryjerry05]

Re: 1991 70hp Force or 1975 75hp Chrysler which better

I'm not a big fan of the 70 and it's single carb. I've found the top cylinder doesn't last as long???? Or is that my imigination? And just my opinion.

 

[Frank Acampora]

Re: 1991 70hp Force or 1975 75hp Chrysler which better

Well, having done modifications to the 75, you are now not making a stock comparison. The 75 is undoubtably producing way more than stock horsepower.

As far as props alone, you simply can not beat the low end acceleration of the 13 inch diameter with a 10 inch diameter, especially since you have changed to a 2 to 1 lower unit. On the other hand, the 70 does have a more efficient lower unit and is slightly lighter than the 75.

Changing the engine is only a couple of hours work (usually) including the control box. At this time, it is really difficult to say whether or not the 70 will perform equally, less well, or outperform the 75. So, you may just want to experiment. After all, if you don't like it it is relatively easy to change back.

Fuel? If you are going to drive the boat at steady speed less than WOT, let's say 30 MPH, you will use approximately the same fuel with either engine. Flat out fuel mileage will depend upon how much horsepower the 75 is delivering at WOT versus how much the 70 is actually delivering. I never measured but I always felt that port work, in addition to increased horsepower at full throttle, gave me slightly better fuel mileage at partial throttle.

The other positive on your 75 that I never did mention is that you can, with a couple of hours work, upgrade the mid leg and lower unti yp a later model with a one piece lower unit and gain another 2-3 MPH. Everything will fit on the midleg from a 1979 up to 1992 Chrysler/Force 85-125

 

[RRitt]

Re: 1991 70hp Force or 1975 75hp Chrysler which better

frank,
does the '91 use metal or composite petals on the reeds? does it have detroit stator or mercury stator?
If mercury, is that during the self-destructing stator years?
IMO, both of those make an impact on long term reliability but I have no idea when or if the smaller engines changed.

As regards trim, which I know, the single ram 2w system will be better. The 2w electricals are superior to 3w, the
1991 70HP pump family was also used on mercury until 2008, the ram uses same seal as mercury 3ram center
piston, and the motor comes out of china for $110. Even though the 1970 trim system was better out of box it might be
a royal PITA to fix now ... due to parts shortages.

 

[fcross69]

Re: 1991 70hp Force or 1975 75hp Chrysler which better

Thanks you guys I have had alot of fun tinkering with my 75hp I guess thats just the gearhead in me I have always found my spot in the garage but I just have never had that Chrysler dialed all the way in I had to find a starter with 4 internal coils instead of 2 to get it to turn over with it having 180 to 185lbs of compression. I have polished the exhaust ports and plate to a smooth as glass finish I have also opened up the intake and gave it a partial smooth with ruff texture for better atomization of fuel. I am running hot fuel in though I mix 1gallon of turbo which is 113plus with 5 gallon of 93 amaco ultimate. I have it set with .066 mains and run all idle screws at 1 1/8 turn keeping it fueled up at a idle. But she just was be a cranky old beyatch sometimes. I have already replaced fuel pump with newer upgraded siera kit. For some reason she just wants to be temper mental til she is warm. I am talking 5 minutes at ramp warming up before I can put it in gear and go with out dieing. I have a new water pump and thermastat in her also so heat shouldn't be a issue. I guess I want new motors hassle freeness on a poor wrench heads budget lol.

 

[dropbear68]

Frank, can you go into detail the work done with pirting polishing etc i have a 75 id like get a bit more fang out of, what is the differance between a 75 and a 90, my 75 is early 80s

 

[jerryjerry05]

dropbear, you should post for yourself.
​FrankA also passed away.
​Start a post with the info you have and the questions you need answered.

 

[GA_Boater]

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