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Thread: My Kilo Motor

  1. #201
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    Default To bridge or not to bridge

    Quote Originally Posted by pdt View Post
    what is the height and width of the exhaust ports on the bridge port engine ?
    I will have to measure my oval spare block tomorrow as im sure its bigger, but you have worked on them so should know.
    I am realy interested to know the differences now.

    I do understand a bit about the port timming, but some of this is new to me that's why I am asking.
    if you know the exact sizes outboard ,then please put them down here.
    .
    .
    Suggested reading is Gordon Jennings "Two stroke tuners handbook" where port-time-angle equations will help you understand one aspect of 2-stroke power delivery.

    Specific to the topic at hand, manufaturers eliminated bridges because when overheated they expand inward and scuff pistons. As Fred points out, a common means of reducing the problem is oiling the bridge through drilled ports in the piston skirt. The factory method was to eliminate the bridge entirely, thus eliminating interference problems.

    While reading Jennings, take time to study maximum port width as a function of bore diameter (there is a formula). I've successfully removed bridges from OMC Permold blocks and made more power the result of larger port openings with less restriction. Take care not to widen ports further as ring drop will be induced: Bridge supports rings.

    Regardless, there are two major downsides to the OMC 56: Lack of sufficient cylinder cooling, and too long inter-cylinder merges. The Mod-50 and all late-model triple use shortest possible merges. Improving the exhaust would make more power than all the porting in the world.

    Good luck with your project!

    Tim

  2. #202
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    yes I have a copy of the 2 stroke tuners handbook. a member here sent it to me.

    So what stops the so called oval port engines exhaust ports being enlarged (up or down)and the radius made better ? is there a reason this cannot be done to good effect.


    what exactly is inter cylinder merges ? (sorry to ask but its another usa thing we have never heard of) probably know it when told what it is in uk English ha ha .

    .

  3. #203
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    Just a thought...
    fastest outboard ever, a bridgeport
    fastest F3 49ci 31M also bridgeport
    coincidence
    OR
    is that just a red herring as they have proper exhaust systems???

  4. #204
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    Default Merges and port widths

    Quote Originally Posted by pdt View Post
    yes I have a copy of the 2 stroke tuners handbook. a member here sent it to me.

    So what stops the so called oval port engines exhaust ports being enlarged (up or down)and the radius made better ? is there a reason this cannot be done to good effect.


    what exactly is inter cylinder merges ? (sorry to ask but its another usa thing we have never heard of) probably know it when told what it is in uk English ha ha .

    .

    A stated previously, port width is limited by the geometric formula precluding ring drop. If you want to be wild, try going beyond formula some time. Be sure to buy another block and pistons; you'll need them in 5 or so hours.

    OMC 56 blocks have cast-in exhaust ducting that merges 5-8 inches downstream (depending in cylinder) of the ports for best low & mid torque. Tuning for best top end demands closest possible merges, hence the stunning performance of the Mod-50's.

    Tim

  5. #205
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    you can mill the exhaust on the 56er and fix it. set it up like a Mod50, it's been done.

    150psi or 28cc head chamber is about the limit of pump gas.

    time to go fix some boats

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

  6. #206
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    Default 56 exhaust mods

    Quote Originally Posted by Fast Fred View Post
    you can mill the exhaust on the 56er and fix it. set it up like a Mod50, it's been done.

    150psi or 28cc head chamber is about the limit of pump gas.

    time to go fix some boats
    Correcting interconnects is not as easy as milling the passages. To do it right, a casting needs to be made and it must be gasketed and attached securely with very limited space for fastener support. The fact it has been done does not mean it's been done right, or effectively. A big problem is connecting then routing back to the center outlet - a very tight space with sharp turns. A customer asked me to do it right but was not willing to spend the $$$ needed. Can post pics of the milled-away block, the operation breaks through water jackets and not pretty.

    Fred and I generally agree on compression limits, but you need to realize that effective pulse tuning increases cylinder pressure dramatically. Some of my best motors deliver only 120 PSI cranking. This is deceiving because the exhaust ports are a mile high: Top end compression is a function of the top edge of your exhaust port. Because pistons/blocks are expensive to fix, and the season is short, I recommend 100LL avgas with minimum 16:1 oil regardless.

  7. #207
    pdt
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    Yes the triple I have is restricted as far as doing anything with the exhaust housing for sure.
    its not like the older type that could come off and be given a different exhaust manifold.

    the 120 psi really does surprise me, if mine was 120 I would dump it....I have 148..149 ..148.
    somewhere on here iwas told I need to get to atleast 165 psi and that maybe getting rid of the small reliefs could give me 10psi to help me raise it up a touch.

    im finding it all very confusing , but great reading

  8. #208
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    done at the factory, the way i did it was to mill it off, put it in the freezer, tig on it , back to the freezer, more tig, then mill the welds then cap it.

    i don't think he is goin to change the exhaust so he would be ok with the fuel said at that comp spec.

    good stuff

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

  9. #209
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    cool, seem gots the right people lookin in hear maybe some more ausome notes

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

  10. #210
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    fred, clear out a spot for me.
    out front again

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