Mercury 650 and 850 Lower Units Interchangeable?

[mbrownone]

Hi, I own a 1967 Mercury 650 and saw a great deal on a 1977 Mercury 850 with a questionable lower end. Are these units interchangeable? I was thinking if they were, I could always upgrade my boat with the larger motor and use my existing lower case.

Thanks for the input

Here is the info on them both:
1967 Merc 650
Serial: 120346

1977 Merc 850
Serial: 7083040

 

[Texasmark]

Re: Mercury 650 and 850 Lower Units Interchangeable?

I'm going to respond to this and if I am wrong one of the senior shirts on here will jump on me....but you will get the correct answer which is what we are after.

I'm going to say yes with the only difference possibly being a different gear ratio. That would play into what is the correct prop for that unit in terms of pitch.....like if the gear ratio is different by 15% say, then the prop would need to be the opposite direction by 15% more or less to get the same performance.

Mark

 

[Chris1956]

Re: Mercury 650 and 850 Lower Units Interchangeable?

I think those gearcases are different. Your 850 will definately accept a gearcase from an inline six motor, with a prop change as outlined by Mark. I used to swap then between a '73 850 and '77 1500, w/o issue.

 

[79Merc80]

Re: Mercury 650 and 850 Lower Units Interchangeable?

The old 650 inline 4 used the same size grear case. The 500 and the inline 3 650's used the smaller cases

 

[194269]

Re: Mercury 650 and 850 Lower Units Interchangeable?

They are interchangable and the gear ratio is different. 650 is 2:1 and the 850 is 2.3:1, sooooooooo a prop change will be required.

 

[Faztbullet]

Re: Mercury 650 and 850 Lower Units Interchangeable?

They are interchangable and the gear ratio is different. 650 is 2:1 and the 850 is 2.3:1,

The 650 4 cylinder gear ratio is 2.3:1. Usually (I have seen and test ran one) a 2.1 on a inline 4 even with a lower pitch prop kills the bottom end grunt of theses engines. Below is a list of gear ratios for Merc inline engines:
40 & 402 2 cylinder............................................2:1
40/45/50/500 4 cylinder......................................2:1
50/60/650/700 3 cylinder....................................1:64:1
650/75/80/800/850 4 cylinder..............................2:3:1
900/90/1150/115/1400/140/1500.........................2:1
1970-71 1150 & 1350.........................................1:78:1

 

[Texasmark]

Re: Mercury 650 and 850 Lower Units Interchangeable?

The 650 4 cylinder gear ratio is 2.3:1. Usually (I have seen and test ran one) a 2.1 on a inline 4 even with a lower pitch prop kills the bottom end grunt of theses engines. Below is a list of gear ratios for Merc inline engines:
40 & 402 2 cylinder............................................2:1
40/45/50/500 4 cylinder......................................2:1
50/60/650/700 3 cylinder....................................1:64:1
650/75/80/800/850 4 cylinder..............................2:3:1
900/90/1150/115/1400/140/1500.........................2:1
1970-71 1150 & 1350.........................................1:78:1

Not hijacking the thread, but which of these engine/boats is the one in your Avatar and what will she do?

On the 650 @ 2:1 you mentioned dropping to a lower pitched prop didn't help. I have been curious for a long time just why that is....if you jack up the ratio 15%, why can't you "jack-down" the prop 15% and come out with the same results...Got any idea?

Thanks, Mark

 

[Faztbullet]

Re: Mercury 650 and 850 Lower Units Interchangeable?

The 13ft Allison with 150xs inline powerhead on 1986 midsection. It has had the following: bored .020 oversize,enlarged/modified boost port on block and piston,modified transfer port covers,thinned and enlarged reed blocks,modified megaphone exhaust tube, old style 150 carbs coverted to primer,1980 225hp spark advance box (26?), Xr4 1:78:1 lower unit and cut down 22 pitch big ear chopper prop and about 2 years work on engine. On good water 3-4 inch chop it will turn 6600rpm at about 66 mph. And below is pictures of a EFI inline in the work by a fellow iBoater . The lil inlines dont start making power and torque till about 2000 rpm and the 2:1 is not to noticable on a light hull but a ole sled trihull it shows up big time. Long story shorted,young fellow up road had a fabuglas 17 runabout, found a ole 850 rigged it out and kept bugging me that it was low on power and had hard time puller a skier up but ran great at WOT,we checked,carbs,linkage,etc,etc then ratio and was 2:1,loaned him a 2:3:1 unit and with same 17 pitch was huge difference.

 

[mbrownone]

Re: Mercury 650 and 850 Lower Units Interchangeable?

Thank you all for the great info and comments!

My boat is aluminum and relatively light. It can handle up to 100hp, though I don't want to max it out. The current 65hp is decent for my use, but not sure if changing to the bigger motor would make sense if the gears aren't correct with the current machanical setup. Still appears like the 850 may be a good deal to have around anyway for the price (can get it for $200).

Thanks again!