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Thread: Evinrude 31.8ci Mod Advice

  1. #11
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    Thanks for the tips with the prop, I will check the rev limiter tomorrow. Where do you get as non rev limiter?
    Is there anything else you do to get your boats into the 40+ mph what prop would you recommend I go with? Thru hub or over hub

  2. #12
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    I will check are compression ratio I know it's 147psi top and bottom 153psi. We run premium fuel but it 94octane. We have about 1 millimetre clearance. We did not rock the top of the piston, we just reshaped the head in the chamber it's almost Identical to the high compression head OMC once offered. We are looking at getting a new prop soon.

  3. #13
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    "Rapair" makes CDI modules for outboards. Look them up on Google and a number of dealers will come up. Or if looking at used ones, any 2-cylinder OMC pack with connectors compatible with your stator and coils should work. Make sure the wire harness is long enough to reach the coils.

    I've had very good luck with my 10x18 Ron Hill chopper on my v-bottoms. That's an overhub prop. Not sure which is better generally, over- or thru. If you do any idling (ie trolling), the overhub will fill with exhaust and have rather little thrust.

    I don't know of anyone other than Ron making speed props for these motors.
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  4. #14
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    Ron is your best bet and he is dedicated to the sport, go for it and I'm sure the price will be right.

  5. #15
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    I'm thinking your on the right track & all the advise given is good. 1 thing I know for sure is it is difficult to get real increase from the 31.8. I would suggest closing off the above water vent & try closed as it was set up for 'open'. Could be the wrong way to go. Its a 1st for me to read of closing the tuner holes too & is something I will try [Thanks]
    Some older power packs are not limited, work or not? From what Ive seen they are wired the same, I eliminate plugs & hard wire them because Ive found them to fail under 50-65mph stress, gounds go bad too.
    Props.. RPM's, lower pitch & or less dia will raise RPM period. Ive been pleasently surprised to get better acceleration and top speed from a good prop vs a bigger one. Another inportant thing I've learned is never think going the 'other way. on something won't work.
    Reeds. On bending the dampers too far the reeds instead of curving out were hinging on the thinnest part of th petal. This experiment made the motor FAST! for about 10-15 minutes. being a metal reed I had to replace a piston. Later I tried Boyesen 2 stage & was disapointed. I removed them while intact.. too expensive for no noticable power.
    On the bench & nearly done I have a .030 over 35hp. Its got square topped ports, the discoloring on the rods honed off, the piston assemblies weight balanced [a few grams off is common]. A problem being ring end gaps of .017+ on new rings usually. I bought a pair of .040 overs & cut them to fit for an .008 gap. for each top ring. I bought a set of reeds from Carlson, single stage composite. Hope they're better. The head is cut to the point where it 'touches' w/o the gasket, the gasket is .050 & the resulting squish is about .062. That might need more clearance & I'm going with new gasket [measuring from an old one].
    Will it work? I'm hoping to approach if not break 70 with a Yamato hatchet 1:1 on a C hydro but no promises yet LoLs
    Another thing that is FOR SURE.. All the motor mods available or inventable will be slighted by a boat not set up for maximum efficiency & a GOOD prop. Keep taking notes & Best of Luck.

  6. #16
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    Good results !!
    On my OMC 25 hp . I did all that mods . and found the best reeds is a CCMS with a ported cages and stock reed stoppers ( donīt mod that !!!...cost me a new sleeve!!), Cut & balance the flywheel, CDI Non limiter pack , Champion QL82C ( gapped 0.030") with a Taylor, 8mm ignition cables, exhaust releiving and a Turbo1 SS Prop ( 3 Blade). Stock timing !!! Waohhhhh!!!! and excellent Punch !

  7. #17
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    I'd be interested in seeing how the reed plate was ported. Something that I've not seen mentioned concerning the intake is the two stud "ears" that protrude into the intake. It appears that they would deflect a lot of air from filling the void in the outside areas of both reed blocks. I took the time to smooth both "ears" on the sides nearest the carb and give them a more blended radius. The intake is still on the bench and I'm still getting baselines on the motor so I can't say that it will be of much benefit too me. But for you guys running 7k and above, I can't see where it wouldn't help. Has anyone cut a new blank reed plate and tried a different reed configuration? The stock design should leave a healthy sized low pressure area near the center (right behind the reed attachment screw, or between the heels of the reeds) that I can't see being filled by an incoming charge until the piston stops it's compression stroke or actually begins to compress the primary. Looks like the motor could take a deeper breath if that center area could get part of the incoming charge. .??

  8. #18
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    Theres not much room to port the intake, unless you got oversized reeds? The plate is beveled on the outer edge & can be beveled the full perimeter.

    If you look at the blue area, that can be cut back.

    The ears can be cut & do look like a large obstruction but won't make much difference. In fact none of this does, maybe a little but I have no accurate means to measure flow. If you look backwards thru the whole induct system [reeds out] & into the crank case cover you can see that whats going on is a motor designed to run good while capable of idle/ low throttle all day. Opening up the upper crank case window can be done to match the lower but remember increasing crank case volume iisnt good for fuel mix velocity.

    As far as RPM.s Ive run 72-7500 on several motors for extended time no problem, I doubt they.ll last 30 years LoL like a stock can but several no problem. I've heard of 8g's but never saw it.
    As with any performance craft or all out race rig, the prop is one of the most important things.

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    JM50, I see by the pics that you are paying attention to detail. Question for you. Looking at your top pic. See how the 3 reeds of each cyl, closest to the center would provide the shortest pathway for air to travel? What do you think would happen if we had a stiffer reed in that area. Along with a moderate reed at 12 & 6 o'clock. And a more flexible reed covering the outside three.??

  10. #20
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    Quote Originally Posted by TorqueXrpm View Post
    JM50, I see by the pics that you are paying attention to detail. Question for you. Looking at your top pic. See how the 3 reeds of each cyl, closest to the center would provide the shortest pathway for air to travel? What do you think would happen if we had a stiffer reed in that area. Along with a moderate reed at 12 & 6 o'clock. And a more flexible reed covering the outside three.??
    Thanks, TXR, Been at it a long time. I'm not sure if it would help but aiming to even the flow around the reeds is interesting. I do know that when I broke one it was right in the 'straight in path' & could imply higher pressure. Figuring how to fit V reeds w/o adding crank case volume would be the trick because the v block takes care of that situation by its shape. I looked into a set from the 4o-50 looper OMC. too big dimensionally & too small if cut down I thought.

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