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Thread: Help Modifying OMC 31.8 cid

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    Default Help Modifying OMC 31.8 cid

    Modifying a 1990 20hp 31.8 cid

    I have already changed the Carb and done some porting. I am looking for more hp, what are the best trick/tips to achieve this and does any body have any advice on the tuner (how should it be modify).

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    FFX-61
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    hey, some how i'm seein a crossflow motor, but that may not be right, got a shot of the powerhead

    Part one of "Secrets of the OMC Mod 50" : http://www.boatracingfacts.com/forums/showthread.php?t=2052

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    Ya your correct its a crossflow engine. I will try to get some pictures up tonight.

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    Team Member Smokin' Joe's Avatar
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    Crossflow 35 hp motor with the carb from the 35. Mike Petty

    bmh20h@att.net

    is the expert. Here's a photo of one of his 31.8 ci OMCs.
    He runs Mod C against Yamahas.



    Quote Originally Posted by Emustard View Post
    Modifying a 1990 20hp 31.8 cid

    I have already changed the Carb and done some porting. I am looking for more hp, what are the best trick/tips to achieve this and does any body have any advice on the tuner (how should it be modify).
    Attached Images Attached Images  

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    Team Member LakeRacer99's Avatar
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    Emustard...what boat are you running? Got any pics of your mods? What about raising the compression?
    Jason
    Outboard Junkie

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    I am running this engine on a 14 foot aluminum princecraft. I am currently in the Process of porting so I do not have any pictures. I am looking for a high compression head at the moment but can't find one anywhere, so will probably end up making one.

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    Team Member LakeRacer99's Avatar
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    Quote Originally Posted by Emustard View Post
    I am running this engine on a 14 foot aluminum princecraft. I am currently in the Process of porting so I do not have any pictures. I am looking for a high compression head at the moment but can't find one anywhere, so will probably end up making one.
    Good luck finding one. they do exist, but i've only seen one online in the last many years I think.
    What does your rig weigh and what speeds? What prop? Do you have porting specs?
    Jason
    Outboard Junkie

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    Ya I have been looking for one for a while and have not seen one. My rig weighs 275-300 lb boat alone with engine and gas tank 450 lb. 13x10 pitch aluminum OMC prop 32mph @ 6200RPM ( I can put 5 people in the boat and we slow down to 29mph) with a 15x10 pitch stainless solas prop 33mph @ 5500 by my self (two people in the boat 32mph @ 5300). I do not have porting specs.

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    Team Member LakeRacer99's Avatar
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    That's cool, post pics if you can.
    Look up JohnsonM50 and SmokinJoe on here, they've been modding up the 32ci OMCs a bunch.
    I have an old 14ft alum runabout (250#rigged) with a stock '57 18hp (22ci) spinning a 12p 2 blade that runs 28-29 with just me in the boat. but add another person and speed falls off to 24-25ish. I have a couple 32ci OMCs that I wanna play with and some 60s/70s 22ci that I want to modify also.
    Raising the compression, reeds and bigger carb is where I planned to start, just too many projects goin.
    Jason
    Outboard Junkie

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    Team Member Smokin' Joe's Avatar
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    Quote Originally Posted by Emustard View Post
    Ya I have been looking for one for a while and have not seen one. My rig weighs 275-300 lb boat alone with engine and gas tank 450 lb. 13x10 pitch aluminum OMC prop 32mph @ 6200RPM ( I can put 5 people in the boat and we slow down to 29mph) with a 15x10 pitch stainless solas prop 33mph @ 5500 by my self (two people in the boat 32mph @ 5300). I do not have porting specs.
    Porting is not the answer, prop work is. I run a stock 1983 35 with .060" milled head on a 350 lb Allison
    pad-v fishing boat, tiller handle. I run a Stiletto 10x14 ss prop(actually measures 10x13) 6150 RPM,
    41.7 mph 2 way GPS avg. A stock Stiletto ran 37 mph. It's tricky to get it right. I have a spare new
    Stiletto, can rework it and sell it to you. Forget al props. You need to turn 6000 RPM too. How high is
    the motor on the transom. I.e., what's the transom height from boat bottom to top of transom. The
    Allison has a 7" offset so I run 19-19.5" high. With a normal transom you'll
    need to run a shortshaft at least 18" high to get any speed. The stock motor will turn 7000 RPM
    then hits a wall because of the flat reed plates. You would need v-block reeds and better exhaust tuning
    to do better. Porting will not make much difference, and do not change the port timing. I run the milled head (150 psi)
    and 33 degree spark advance but have never risked pump gas. I run a mix of Sun 110 octane and pump 93 at
    1/5 (98 octane avg.) with Evinrude XD-100 oil@37:1 to avoid sticking a piston.

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