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Thread: Omc 3 cylinder comparison

  1. #1
    BoatRacingFacts VIP
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    Default Omc 3 cylinder comparison

    My 13 Allison (1972) is being restored as I get time, transom, core, the whole shabang. I have a question;

    I have an old (1980 ish) 75 OMC, not a nitro-cased motor and it has always been preached to me that it would be best to have a nitro motor, due to the 15 inch mid, as well as, narrower gear case. It is also my understanding that the later 56 inch race motor, is even better, but my question is; if I cannot readily purchase an old nitro motor, or a later racing 56 incher, wouldn't the standard later 56 inch, 70hp motor be better than what I have, and as far as top end goes, wouldn't it also be better than the 49 inch nitro motor, simply because it puts out so much more power?

    I am cost consious and not doing any sanctioned racing, but like to go fast. It simply seems that their is no good reason for me to spend $2,000 on a good nitro 49 incher, if i can pretty easily spend, $800 on a good, late 1980's 70hp that is "faster" still, though it will lack the 15 inch, on top end, I see this making little difference.

    Thoughts please?

  2. #2
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    Default Late Model

    I think you are doing the right thing going with a later model 56 CI. More punch for the bigger boat. Should be a decent rig for a lot less than modifying a 49CI.

    Good luck with your project !

  3. #3
    Administrator Ron Hill's Avatar
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    Default 56 Inchers

    Can you tell me how you know you have a 56 incher? Is there a serial number ID to look at? I want to buy a 56 incher.

  4. #4
    Team Member Travis Fulton's Avatar
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    Default

    the 56 cyl head is flat on top, the 49 has cut outs where the plugs go

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    Default 49 vs 56

    Ron, Even though the cranks will interchange as well as the carbs and ignition(with a little modification to flywheel for trigger) and also the front half/reeds will fit, the block and head are diff. With those two differences the rods and pistons will not fit the 49 in their stock form. Other than the 3.187 bore of the 56, you can also identify the block by not being able to remove the exhaust cover and have access to the ports. The 56's are "behind the liner" intake and sleeve set-up.Port timing will also be diff. than the 49.Other than the block-head-pistons-rods-powerhead adapter plate, everything else can be made to work on the 49. Phil

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    Default 49 vs 56

    Forgot to mention the diff. in the cranks. ---Earlier motors had 4 spline cranks as did/does the 49. Then they started with a 17 spline crank. All were forged. I understand from the SST60 guys that some earlier ones were lighter in stock form. Phil

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    all the 15" cranks are 4 spline and the 56 20" are fine I think
    ( or maybe they changed to fine not long after they introduced the 56)

  8. #8
    Sam Cullis Mark75H's Avatar
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    Which ones have the lost foam castings that look like the motor is made of painted styrofoam?
    Since 1925, about 150 different racing outboards have been made.


  9. #9
    Team Member mxp864's Avatar
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    Default foam

    the 56" from, i think 88-92 look like black foam....I think those are the years......

  10. #10
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    Quote Originally Posted by mxp864 View Post
    the 56" from, i think 88-92 look like black foam....I think those are the years......
    The 56 is a lost foam engine, thats one way to identify it.
    Cylinder head is one piece, exhaust is integral to the block
    49-75 was still being produced when the prop rated 56-70 came out so 86-88 you could get both
    EDIT sorry 56 came out as lost foam and perma mold

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