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Thread: Help Modifying OMC 31.8 cid

  1. #21
    Team Member Smokin' Joe's Avatar
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    Quote Originally Posted by Emustard View Post
    JohnsonM50 what kind of compression do you get with that performance head?

    The aluminum sticks a little to the water but not bad. I have played around with the weight in the boat and trimming it and still think I need it up higher but will have to wait till the ice goes away. I use this engine almost every weekend for fishing so would like to keep the stock lower unit.

    So I replace the OMC steel reed with boyesen reeds do you think I should switch back to the steel reeds? I heard some one say you should take out your water pump and you will gain 3-7 hp what are your thoughts on this, is it true if I can still cool the engine.

    I’m just looking to get into the 40s with her and need help getting there.
    I left out the fundamentals. You can't jack the motor up and prop it unless the
    bottom is smooth and straight to within a few thousandths of an inch. Any hook or rocker and
    you're dead. Smooth al won't drag more than any other smooth surface (water sticks to all surfaces and
    drags the first monolayer, which drags the 2nd and so on), and by smooth I mean gelcoat
    or gray primer on gel coat. Trailering picks up oil from the road, the bottom must be cleaned
    regularly as well if you trailer it. There are a lot of details that have to be right before even a fast
    boat can be made to run up to its potential.

  2. #22
    Team Member LakeRacer99's Avatar
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    Lots of good suggestion here on this thread. Pics would help us see what you are working with also.
    Jason
    Outboard Junkie

  3. #23
    Team Member JohnsonM50's Avatar
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    Thanks Joe, its fast but then again the Bezoats its on is always fast. It weighs in at about 430# including me. The Yamato foot is les than 2" wide & 3/4 deep at prop shaft center. A good running stock used powerhead would make about 58-59 on that tower so 65 is realy only up about 6mph. Thing is I know theres more. Whether to spin more prop or more R's is the Q. Im guessing it to be about 7200 to 7500 now.
    Eric.. When you look at various 2 strokes theres motorcycle motors with ridiculous power, so why not outboards? They need the torque & have to trade off some hp to get it. Water resists almost 800 times air. A set of wheels on pavement rolling on bearings is nothing compared. An outboard has to be tuff-r too for the rigors of being run like its always going uphill. If you took a really hot MC motor removed the 5 speed gearbox & attached it to a prop it would not be likely to get the boat over the hump & on plane. That being said the best thing is for it to be right, piston clearaces, ring gaps, tuning the list goes on. Check your 20 over for built in restriction, I run time at about 29 30, never saw much difference as long as its 30/35. the motor I mentioned with the wiseco's & 3 cyl rods has 150psi with a regular head cut a bit.
    When I got my Airborn done I started out with a 22ci 25hp OMC. had it running nice but was stock. It did about 33 tops. Then I got a 31.8 [big block]. It's very similar, like a big cuz. That made 36 1st try & the torque was a seat buster LoLs Keep in mind thats a flat bottom racing runabout at about 475 total. I stared messing with carbs & set up & props, tweek-d it up to about 39, & actually got a 40 once. [It was a conditions permitting moment] Then I got my hands on this 13.5 P 2 blade righty speed prop. I adapted it to an old club foot

    Yes, its modified a bit but is still 12:21 That made 44 1st try then 46.1 soon after. [Some said 'that'll never go'] Its on my pals hydro now & has made 52gps at about 500# @ well over 7G's.
    That's about where I stopped working with stock gearcases & never had a thru hub 31.8. for all the work done I have to say besides putting a racing gear case on & running it on a race boat, the most speed Ive ever seen gained was from a prop & subsequent set up change. Theres 2 ways to approach boat motor performance.. you either turn more R's with the same prop or you spin more prop on more hp. if your 31.8 is open & flowing like a 35 it already has the capability to just a matter of the right prop. over-rev Check also into whether or not you have a rev limiter built into your power pack. thats an easy change.
    Joe, I tried to PM you long ago but didn't hear back.. had some Q's

  4. #24
    Team Member JohnsonM50's Avatar
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    Default 31,8 build help

    Hey Jason, how are you? Did you say pics? How bout a RaT's history?..but 1st a radar run. clik the pic, blow it up & crank it up. Had a little carb burp at 1st but nursed it up to speed:

    I made a trade for this nice Yamato Pro case.. Stared at it for awhile & started chopping up parts.

    & fitting them up.

    It took a little work to eliminate the 'clamshell' lower mounts.

    & fit the exhaust.

    But in the end its turned out to be quite a motor. It has a waterpump for the 'slo-zone's so is really a good play motor.
    Last edited by JohnsonM50; 01-10-2013 at 05:24 PM. Reason: mistakes

  5. #25
    Team Member Smokin' Joe's Avatar
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    Quote Originally Posted by JohnsonM50 View Post
    Thanks Joe, its fast but then again the Bezoats its on is always fast. It weighs in at about 430# including me. The Yamato foot is les than 2" wide & 3/4 deep at prop shaft center. A good running stock used powerhead would make about 58-59 on that tower so 65 is realy only up about 6mph. Thing is I know theres more. Whether to spin more prop or more R's is the Q. Im guessing it to be about 7200 to 7500 now.
    Eric.. When you look at various 2 strokes theres motorcycle motors with ridiculous power, so why not outboards? They need the torque & have to trade off some hp to get it. Water resists almost 800 times air. A set of wheels on pavement rolling on bearings is nothing compared. An outboard has to be tuff-r too for the rigors of being run like its always going uphill. If you took a really hot MC motor removed the 5 speed gearbox & attached it to a prop it would not be likely to get the boat over the hump & on plane. That being said the best thing is for it to be right, piston clearaces, ring gaps, tuning the list goes on. Check your 20 over for built in restriction, I run time at about 29 30, never saw much difference as long as its 30/35. the motor I mentioned with the wiseco's & 3 cyl rods has 150psi with a regular head cut a bit.
    When I got my Airborn done I started out with a 22ci 25hp OMC. had it running nice but was stock. It did about 33 tops. Then I got a 31.8 [big block]. It's very similar, like a big cuz. That made 36 1st try & the torque was a seat buster LoLs Keep in mind thats a flat bottom racing runabout at about 475 total. I stared messing with carbs & set up & props, tweek-d it up to about 39, & actually got a 40 once. [It was a conditions permitting moment] Then I got my hands on this 13.5 P 2 blade righty speed prop. I adapted it to an old club foot

    Yes, its modified a bit but is still 12:21 That made 44 1st try then 46.1 soon after. [Some said 'that'll never go'] Its on my pals hydro now & has made 52gps at about 500# @ well over 7G's.
    That's about where I stopped working with stock gearcases & never had a thru hub 31.8. for all the work done I have to say besides putting a racing gear case on & running it on a race boat, the most speed Ive ever seen gained was from a prop & subsequent set up change. Theres 2 ways to approach boat motor performance.. you either turn more R's with the same prop or you spin more prop on more hp. if your 31.8 is open & flowing like a 35 it already has the capability to just a matter of the right prop. over-rev Check also into whether or not you have a rev limiter built into your power pack. thats an easy change.
    Joe, I tried to PM you long ago but didn't hear back.. had some Q's
    Try

    outboard_services@mccauleyandson.com


    I wouldn't think that small fishing gearcase would work well when jacked
    up to 21" tr. hgt. or better, too little rudder/skeg to correct against
    prop walk. Even at 19.5" tr. hgt. the 1976-79 35 hp straight skeg
    model gearcase failed completely because the bullet and 'rudder' above the
    bullet are too blunt. I could only eliminate blowout by straightening the
    motor, and then the boat was in a sharp left turn at 40 mph. The more streamlined
    1980-83 gearcase runs 41.7 mph.

  6. #26
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    Do you guys have a relief cut in the side of your cylinder wall or is that just something they did on the 20hp or is it on all the 31.8cid 20,25,30,35hp.

  7. #27
    Team Member JohnsonM50's Avatar
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    I've read of that but never saw one, most of my stuff is early 80's or older. Best reason I think noted was easier starting & that it had no effect on performance. If anything raising the exhaust port time slightly would be typically for more top end. The rings & compression stroke 'cc's are the key to compression. It can vari from motor to motor about 120-135 w/o any significance in speed. 150 from what Ive seen takes milling. 2 of mine are to the point where the pistons will hit the head w/o the head gasket. I don't know how much can be milled but be advised they are already close.

  8. #28
    Team Member Smokin' Joe's Avatar
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    Quote Originally Posted by Emustard View Post
    Do you guys have a relief cut in the side of your cylinder wall or is that just something they did on the 20hp or is it on all the 31.8cid 20,25,30,35hp.
    Where, for what?

  9. #29
    Team Member LakeRacer99's Avatar
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    Quote Originally Posted by JohnsonM50 View Post
    Hey Jason, how are you? Did you say pics? How bout a RaT's history?..but 1st a radar run. clik the pic, blow it up & crank it up. Had a little carb burp at 1st but nursed it up to speed:

    But in the end its turned out to be quite a motor. It has a waterpump for the 'slo-zone's so is really a good play motor.
    Very nice work, I admire your initiative. love the pics and video...did he say 65mph? That's awesome.
    I don't wanna derail EMustards thread, but what waterpump did you use with the yamato case? I have a Konig lower that I have wanted to graft on my 31.8, but I wont do it without a waterpump.
    Jason
    Outboard Junkie

  10. #30
    Team Member LakeRacer99's Avatar
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    emust...I still go back to the rigging set up and prop work, then carbs, compression, exhaust- squeezing every last mph out of the setup before i go grinding on the ports. Probably because I wouldn't know where to start there also. But I have seen many good running motors sacrifice low end and midrange post grind over the years. pop up a few pics of your rig if you can.
    Jason
    Outboard Junkie

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