Early Merc 3 cyl lower unit choice

[Texasmark]

Ok you guys in the know, I'm going to stumble through this with several inaccuracies i'm sure. My intent here is to answer the question for me as well as others that may have had the same curiousity not focus on beating me up. Grin

Somewhere back in the '70-'80 timeframe, Merc decided to come out with a new mid-range looper and made it with 3 cyl rather than the usual 4, probably because OMC had developed the technology and pulse timing on a 3 cyl was easily achieved, and you just eliminated 1/4 of your engines internal cost. Great.

In their selection of lower units for those mid-range engines, they used the smaller lower units turning 10" wheels at a sizzling 1.78 gear ratio rather than doing as OMC did back in '69 when it introduced the Triumph 55hp 3 cyl looper with a large gearbox, 13" dia prop and more like a 2.3 ratio. I know that the old '60's OMC 4 bangers produced used smaller props running at high rpms, but apparently they wised up and realized the benefits of a larger prop turning slower for the consumer market.

So the question is why the smaller lower unit?

Thanks,

Mark

 

[Texasmark]

Re: Early Merc 3 cyl lower unit choice

Bump

 

[arimatation]

Re: Early Merc 3 cyl lower unit choice

I have a 1994 60HP Merc 3 cyl. I has a 1.64-1 ratio. I don't know why they have varied ratios other than to be able to match the HP to the lower unit and prop size.

 

[Chinewalker]

Re: Early Merc 3 cyl lower unit choice

The '70s and '80s Merc 3-holer was not a looper like the OMC. It was essentially 3/4 of the 4-cyl., or 1/2 of the 6-cyl., as it used the same bore/stroke with the same rods & pistons. They discovered early on that the 3-cylinder Merc didn't like to be lugged, so they went with the small gearcase (same housing as the 40/50 series) and a unique gear ratio to allow it to spin.

 

[Texasmark]

Re: Early Merc 3 cyl lower unit choice

The '70s and '80s Merc 3-holer was not a looper like the OMC. It was essentially 3/4 of the 4-cyl., or 1/2 of the 6-cyl., as it used the same bore/stroke with the same rods & pistons. They discovered early on that the 3-cylinder Merc didn't like to be lugged, so they went with the small gearcase (same housing as the 40/50 series) and a unique gear ratio to allow it to spin.

Chine, you obviously know what you are talking about but I could have sworn that I read sales brochures in those days that said those engines were loopers......I thought it was Merc's retaliation for OMC's loopers. So I guess they were "Direct Charged" like my '89 Tower was. On that process, and that engine I had, you couldn't ask for a sweeter running, more fuel efficient engine than that. Don't know how looping could have improved the engine had it been looped.

I agree that the small gearbox/prop would reduce the opportunity to lug these engines.

Thanks for the reply.

Mark

 

[OldMercsRule]

Re: Early Merc 3 cyl lower unit choice

Hmmmmm....... That's somethin' I didn't know.

I always thought Merc went ta three cylinders ta build a reverse engineered copy of the OMC looper with enough difference ta avoid the patent, although I knew the Merc threes were not nearly as good as the Johnnyrude loopers, (one of the best outboards made, [especially in that mid power class], IMHO).

Seems like three is an odd number unless ya have a reason fer it as OMC did.

I never did like the Merc 3s so much; 4s and 6s have always been me favorites. JR

 

[Faztbullet]

Re: Early Merc 3 cyl lower unit choice

Dont knock them little 3's as Merc made a 650 Xs version (cowling and jets) and a Hi-po version with 3 tiliston's and 1.1 speed master. This engine would whup up the OMC boy in the tunnel boat series and will hold up to 7500 rpms if jetted and timed correctly, try that with a OMC looper 3 cylinder and see if rods hold...

 

[Chinewalker]

Re: Early Merc 3 cyl lower unit choice

The high performance version of the Merc 3-holer shared very little with its service motor brother. They had to completely redo the intake manifold and main bearing arrangement to accommodate the three carburetors, as opposed to the two on the service motor. If they had started with the three-into-three idea for the service motor it probably would have been a much better motor in the end. As it is, most Merc mechanics would include the 3 cylinder Merc 650 as one of the worst Mercs to ever wear the badge (right up there with the 4hp Gnat!).

 

[Faztbullet]

Re: Early Merc 3 cyl lower unit choice

Yes the Hi-po was very similar to the T2 and Twister engine in way front crankcase half ,carbs and bearings. I had 2 T2x's back in the 80's and sold both (divorce).. I wish someone would have kicked me in the a** when I got that idea...

 

[arimatation]

Re: Early Merc 3 cyl lower unit choice

Do you guys know anything about the newer Mercs?

I have a 1994 Merc 3 cyl 60HP. The brochure says it's loop charged.
Should it be a decent engine?
I bought it in September with a 16 ft boat. I have not taken it out other than the test ride. I am trying to freshen things up on the motor, trailer and boat.

Our family had a 1976 650 Merc that was bought new. I remember my Brother lent it to a friend about 1986 and he blew the engine. I think it had too much prop. I tried telling my Brother that. So it did not last long, but I think it was lugged too much.

 

[Texasmark]

Re: Early Merc 3 cyl lower unit choice

Well, I'm currently running a '02 90 hp 3 that I know without a doubt is looped is part of the family you mentioned. It has the traditional 3 cylinder exhaust hum at mid rpm's that 3's are noted for.

This is a good engine, not my best, but a good engine. Having dropped down from 4's and 6's I will admit it's not one of them, but I like it. About I can say for now.

Mark