Super mod 15hp omc help

[Glenno]

Hi all,

I was wondering if any one out there has modified the older smaller capacity 15hp omc.
I would apreciate any input as I have started a few mods on a junk motor I have.
I will start with twin 9.9 carbs and go from there, still have the standard reed plate and standard head, no porting.
I have adapted the current model 15hp exhaust tuner though as it looked heaps better the the old one.

I know it may seem like a waist of time to some but the motor was free. I race 550 super sport in Australia and want to learn more about the way things are modded and why my current 550 motor is built the way it is (built by someone else).

Any input would be really apreciated.
Glenno

[rumleyfips]

There is an interesting engine on pg. 23/4/5 of the thread "name that outboard" on this site.

I couldn't find it with the BRF search bur Google got it with "name that outboard".

John Mcmanus

[rumleyfips]

Also, try thread "1992 15hp Johnson performance hop up" at Hydroracer.net.

John

[Glenno]

5930359304593055930259289592905929159292
Just thought I would post some pics of the finished motor. I ended up putting 2 15hp carbies on, high speed jets took a while to sort and had to put on some velocity stacks as I was getting sprayed with fuel as I tiller steered.
Stats are 32kph (19.8mph) before any mods on fiberglass dinghy.
And 50kph (31.07mph) GPS after mods, same boat, standard depth (not surface piercing), i did put an 11pitch stainless prop which was off a merc 25hp apparently. Sounds like about 7500rpm, havn't put a tach on.
Just need to try jacking it up now and need to find a chopper or cleaver around 15- 16pitch.
I really think the power head is pretty good now but the gear case even with the low pick ups will be the let down for speed with this rig. Would love to try one if those omc race legs on it.

As long as I am on the water, playing with outboards or boats or propellers I am happy.
Glenno

[Fastjeff57]

G'day!

Love to know how the power band was effected by those long intake runners. (I'm thinking about doing something similar to a Merc triple.)

Impressive performance improvement, by the way.

Jeff

[Glenno]

Hi Jeff
I am not sure about the affect on power band from the long intake. I only did them so I could fit the 2 carbies on top of each other (there was no room tight to the reed cover). The power head seams to have great get up and go, maybe loses a bit of torque in the higher revs (that may be the standard exhaust tuner though, may need to be shortened a bit). It does use a whole heap of fuel though, probably more than double a standard one.
Cheers,
Glenno

[ice_spy]

Dual carbs will always suck more juice. However your results seem to show an improvement with more fuel entering the motor. Air speed has a lot to play with intake lenghts, you may want to try some Reed cage stuffers, or adjustable cones at the velocity stack intakes to speed up air flow. Often 1x decent carbie can perform the same as twin carbs With less fuel consumption and half the tune problems.
This knowledge I can only extend from years of playing with jetski engines, twin cyclinder single and twin carb 2 stroke motors.
I was under the impression also that a nose job on a leg with a fishing prop at speeds below 80klm is just adding drag. When you fit the surface prop you do need it to flow water to the intakes though as you already know.
Good job so far, cleaned up very nice in the pics.

[Glenno]

True, at the level this motor is modded a single carb could do the same job for sure. But two carbies is really cool, just something I wanted to do. I had a pile of old omc 9.9hp and 15hp junk and wanted to do something cool with it. The motor does have low pick ups) in pics futher up on this page) but as you thought they have not yet been needed. I also think that the small skeg could be a problem once I surface pierce to the height I am happy with. Dual carbs will work better than one if this had dual tuned pipes, that might be going to far as I don't have any one to beat with this motor.
Some would say why bother with this, just get a 25hp, they don't have outboarditis like me. Hey if you enjoy playing with outboards then go for it I reckon.
Glenno

[ice_spy]

It's awesome Glenno, the twin carbs certainly has the wow factor. Thats what I was thinking about too, twin exhaust pipes out the back. It would scream. Lol. Fun factor build 10 out of 10.
You might be able to build up the skeg on one side to help combat the prop walk, that is as you say if there's any skeg in the water. :cool:

[Tiller guy]

Wow 11 mph gain is huge! Very cool little project motor.

[Powerabout]

I would be working on getting some v style reeds in there instead of the flat plate ones