1984 Mariner 75hp 4-cylinder vs. '77 85 hp

Hi there. Been reading posts here for some time now and have a question of my own now. I have a 1984 75hp 4cyl Mariner outboard. After looking at my Seloc manual I see that some early 80's 80hp 4 cylinders, and the 1977 85hp all have the same displacement, carbs, and even jet sizes were the same. Where does the hp difference come from? Is it possible to get an extra 5 to 10 horses out of this 75hp model with some easy changes/adjustments? I've heard of some motors having down-tuned carbs and such on the same powerhead so as to give a bigger variety of hp choices. I was just wondering if maybe this was the case here. Any help would be greatly appreciated. Thanks, Chris

Re: 1984 Mariner 75hp 4-cylinder vs. '77 85 hp

Somewhere in that era the manufacturers changed from HP rating at the crank to rating at the prop, causing it to appear as if the identical motor lost HP. It didn't, it's just a different method of comparison.

Making changes to a motor in attempt to gain a couple of HP can get costly AND shorten the lifetime of the motor. OEM's have done a LOT of research in building these motor to run hard and last a long time. It is usually best to just stay with the original design. If you NEED more power, trade up.

Re: 1984 Mariner 75hp 4-cylinder vs. '77 85 hp

Thanks, I wondered about the method of hp rating change too. On paper they appear to be the exact same powerhead and I know the leg and lower unit are the same. I was a little concerned about making changes though because 1977 is the only year listed for the 4 cyl 85 hp, then it goes down to 80hp, then down to 75hp. Makes me wonder if there had been problems trying to run it at 85hp. But again it could just be the method of hp rating altogether. Thanks CharlieB for you input. If anyone else has any knowledge about this please let me know. Chris

Re: 1984 Mariner 75hp 4-cylinder vs. '77 85 hp

It's as Charlie said, just a different way of rating HP. The 85hp was crankshaft-rated and your 75hp is propshaft-rated as are all newer motors.

Essentially no difference between them except for whatever slight variances in ign, carb style, etc.

You're getting a similar level of performance out of your 75 as you would from the earlier 85hp.

BTW Mercury came out with the 85hp model starting in 1973, so it was in production for a few years before being marketed under the Mariner brand.

From '69-'72 the "800" (80hp) was sold. The 850 was an upgraded design from the 800, with power-ported pistons and block (same as the 1500) accounting for the increase in hp. Your 75hp has the same power-ported pistons and block design as the 85hp version.

Note: if you're looking for a bigger motor, your best bet would be either an 80's-vintage Inline Six, or you want something more "modern", a 90's-vintage 90hp 3-cyl would be a good upgrade from the 75hp.

I'd stay away from the later-model "big inch" 4-cyl's with the 2-stage "lean idle" carb on 2 cyl's, they idle real low but the sudden surge when the carb kicks in can be disturbing for watersports. Plus these 4-cyls have a lot of reliability issues.

HTH...........ed

Re: 1984 Mariner 75hp 4-cylinder vs. '77 85 hp

Thanks a lot! That took care of everything I wanted to know. The specs in the book looked identical except for slight differences in timing. Got my curiosity going. I'm fairly happy with the the motor anyway, except for the way it was taken care of before I got it, but that's another story. Thanks again for your time and info! Chris