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TOP powerhead swap

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  • TOP powerhead swap

    I need to appeal to some of you with knowledge of the Tower of power Mercs. I have a 1980 90 hp that runs flawlessly. At the beginning of this past summer, I replaced the Stator and both switch boxes, put in new plugs, changed the lower unit lube and impeller, flushed and replaced the lines on my power trim cylinders, and mounted it on my restored pontoon boat. The motor is great and will push the 20 foot pontoon about 22 miles an hour. I have read that I can swap a 1983-88 115 hp power head onto my outboard for a horsepower gain of more than 25 based on the change in prop versus crank horsepower rating moving from pre '83 90hp to post '83 115hp. I have found a "naked" 1983 115hp long block on eBay for about 600 bucks from a reputable seller. My question is this: Can I take the carbs, ignition components, flywheel, etc. off of my 90 hp put that on that 115 hp block, bolt it to my midsection and have a 115 hp plus outboard? Thank you for your time, experience and consideration.

  • #2
    No. The carbs are different, the throttle cam(part of the linkage affecting the rate the carbs open in relation to spark advance) is different, you would also need the exhaust tuner(in the midsection) from a prop shaft rated 115

    The prop rated 115 does have a little less low end torque than your crank rated 90.

    FYI, mercury started prop rating the inline 6 in 1982
    Last edited by jimmbo; September 13th, 2017, 11:25 AM.


    • #3
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      Thanks for the reply Jimmbo. That's what I needed to know. Appreciate your knowledge and time.