Yamaha 55hp lower on 70hp

[maartenk79]

Hello,

I've just put a 55hp Yamaha /Mariner shortshaft lower on my 70 to build a ces style motor. It works but it doesn't pump water.
Should I have used the 70 waterpump in stead of the 55? Or is there anothe issue i'm missing?

Also the exhoust doesnt line up so it uses the ehaust dump above water a lot. Is this a problem?
I'd like to run the smaller lower unit to get away from the 2.33 gears and the drag the big foot causes.

Thanks in advance

[maartenk79]

The lower unit is on and works. But damn, it's loud! I'm in a populated area so I have to work on the exhaust to line up.
Tried a chopper2 18" and got 50+ mph at 6500, not bad. 70604

[Hounddog]

You likely are not running the short shaft muffler. That is the reason for the noise. The muffler is still available. The 70CES model had no muffler. The 50G and 70B short shafts had the muffler. You will have to get the part from Japan, mega zip part sells for approximately $145.00 US. 6H3-14711-01-CA

[hydrospeed77]

I run a 55/60 2cyl lower on a 70hp 3cyl. It is also loud. I am pretty sure it is that the gearcase casing has a wall/baffle in a different location from the stock 70hp lower, does not line up with the 70hp mid baffles. This allows some of the exhaust to flow to the back of the top of the gearcase and exit the vent holes above the cav plate (you can see in the above pic).

Or, if you are running a surfacing prop at surface piercing height, the exhaust won't be fully muffled by the water.


Here is a pic of a strange 2cyl lower that has both baffles/walls. 2Cyl location is in between the bolt holes, the 70hp 3cyl location is back by around an inch:
http://www.boatracingfacts.com/forums/attachment.php?attachmentid=70606&d=1557263859

[maartenk79]

Here is a pic of a strange 2cyl lower that has both baffles/walls. 2Cyl location is in between the bolt holes, the 70hp 3cyl location is back by around an inch:

Mine only has one baffle. I'm a bit puzzled by the one with two baffles as the exhaust gasses have nowhere to go.
I was thinking about putting in the extra baffle at an angle and remove part of the front one to direct the gasses to the exhaust.

This is the 70hp 15" big lower
70612

And my 55hp 15" small lower
70613

[hydrospeed77]

So based on your two photos above, I have the exact same configurations..
You can see that some of the exhaust from the 70 3cyl mid will flow rearward of the baffle/wall between the two bolt holes, and not down to the prop shaft area. Whereas with the "big" 70hp lower, all exhaust will flow down to the prop shaft..
I'd be curious to see what a real 6J2 (factory 70 CES) gearcase casting looks like with regards to the above..

I think your idea of cut/add will probably work. My only concern would be does the smaller lower have enough exhaust flow down to the prop shaft for a 70hp 3cyl.. I mean I hope it isn't a restriction. Because the big case does have a larger passage all the way down to the prop shaft.. So for the small lower, maybe some exhaust is meant to be relieved out above the cav plate.. Not 100% sure on that.

Not sure why the strange aftermarket one I have has both baffles/walls, but maybe depending on what motor it will be used on, you are meant to grind/cut out the one between the bolt holes.. That's the only reasoning I can think of.

[Hounddog]

Currently on eBay there are mid 1990 55 hp lower units and they are both 2 bar units. My 1994 70CES lower unit is a 2 bar only the first bar is cut out a bit. These lower units were used with the 6F0 49 cubic inch motors as well so exhaust flow thru the hub should be OK with the triple. The 6F0 model has the shell style muffler like a 70 hp. Maybe the lower end of that muffler can be cut and welded on to a 6H3 muffler and then the exhaust will flow directly into the lower unit.

[hydrospeed77]

The muffler doesn't matter as much as the actual midsection "upper casing". It is the mid that has the baffles and exhaust outlet that determines the exhaust flow outlet location. It is also the mid that mates to the lower gearcase.. For example you could run the 70 mid either with or without the muffler and the exhaust still flow out the hole in the bottom of the mid.

The muffler might make things quieter if the configuration is all 70hp parts (big lower gearcase). But if the exhaust flow is open to the air and not muffled by the water through the prop, I doubt the muffler would make much difference.

[maartenk79]

Actually Hounddog may have a point as the difference in muffler could line up to the 55hp style lower?
Maybe the two baffle lower units are designed to be usable on the 60hp twocilinders and the 70ces?

[hydrospeed77]

I'm running a muffler and it is loud when running. It is a stock 20" 3cyl 70 with a 2cyl lower. No mods, all original.

Maybe no muffler affects the exhaust relief just below the lower cowl/pan (low speed/idle exhaust relief) so that noise makes it through.

When I trim down, so there is negative trim (like when getting on plane) it is quiet, but as soon as I trim up to running angle, it gets loud. I assume it is the vents at the back of the gear case, above the cavitation plate, plus the exhaust walls not lining up so exhaust is open to the air through those vents. Trimmed down they are likely blocked/muffled by water. It could be cooling water from the block, and not the lake water that muffles the vents when trimmed down.


Edit: oh Hounddog is probably referring to the long channel down the back of the mid (which leads to the vents at the back of the lower). My concerns are with the main exhaust exit at the bottom center of the mid being split by the wall in the 2cyl lower between the bolt holes, so some of that exhaust goes to the same vents at the back of the lower above the cav plate..