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Thread: Evinrude 31.8ci Mod Advice

  1. #1
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    Default Evinrude 31.8ci Mod Advice

    I posted a form a couple of months ago asking for peoples info on mods to a evinrude 31.8 cubic inch. over the winter we did some modifying, and were a little surprised with our results. we thought we would be in the high 6000rpms with the 10-15 ss and we were not close. what do you think our problem is?? I added some pictures so you can see how our rig is set up. Boat is 14 feet long with one person in it and motor and gas is 600lbs.

    NOW
    compression 147psi top, 152psi bottom
    port and polished intake and exhaust
    Wiseco pistons
    intake manifold increase in size
    boyesen reeds and opened reed stops
    tuner shorted an inch
    vent exhaust above the water
    1.50inch carb 83Main jet 40air jet


    BEFOR
    before we were running 125psi in both cylinders and the boyesen reeds. that's it
    1.50inch carb 77Main jet 40air jet

    The props are a stock evinrude 10-13 aluminum and a solas Stainless steel 10-15

    OLD TEST
    aluminum 10-13 prop @ 32mph at 6150 RPM (1 person)
    4people in the boat 100lb of gear 29mph @5800 RPM

    stainless steel 10-15 @ 33mph at 5550 RPM (1 person)
    3people in the boat 29mph @5200 RPM

    NEW TEST
    stainless steel 10-15 @ 34 5600 RPM (1 Person)
    3 people in the boat and 100lb of gear 31mph @5300RPM

    On the new test the engine has all kinds of power getting up and out of the hole, a lot quicker then before also runs the best it has ever but still thought it would have more power at top end.

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  2. #2
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    It looks like you could raise the motor up on the transom at least 2 or 3 inches, That will gain some in the top end.

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    I moved the motor up an inch with a block of wood and it would blow out on turns and off the blocks if I full Throttled it quickly, so I removed it. This was with the stainless steel prop.

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    You are reaching a pretty good speed for that motor and boat. You would need to jack the motor up and try a few props with a little more cupping if you are looking for more speed. The more prop out of the water usually will raise rpm's by a few hundred, just be sure you have adequate water pressure.

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    Quote Originally Posted by Emustard View Post
    I posted a form a couple of months ago asking for peoples info on mods to a evinrude 31.8 cubic inch. over the winter we did some modifying, and were a little surprised with our results. we thought we would be in the high 6000rpms with the 10-15 ss and we were not close. what do you think our problem is?? I added some pictures so you can see how our rig is set up. Boat is 14 feet long with one person in it and motor and gas is 600lbs.

    NOW
    compression 147psi top, 152psi bottom
    port and polished intake and exhaust
    Wiseco pistons
    intake manifold increase in size
    boyesen reeds and opened reed stops
    tuner shorted an inch
    vent exhaust above the water
    1.50inch carb 83Main jet 40air jet


    BEFOR
    before we were running 125psi in both cylinders and the boyesen reeds. that's it
    1.50inch carb 77Main jet 40air jet

    The props are a stock evinrude 10-13 aluminum and a solas Stainless steel 10-15

    OLD TEST
    aluminum 10-13 prop @ 32mph at 6150 RPM (1 person)
    4people in the boat 100lb of gear 29mph @5800 RPM

    stainless steel 10-15 @ 33mph at 5550 RPM (1 person)
    3people in the boat 29mph @5200 RPM

    NEW TEST
    stainless steel 10-15 @ 34 5600 RPM (1 Person)
    3 people in the boat and 100lb of gear 31mph @5300RPM

    On the new test the engine has all kinds of power getting up and out of the hole, a lot quicker then before also runs the best it has ever but still thought it would have more power at top end.

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    I've been watching this post in hopes that perhaps someone with more experience would chime in. With that said, I will address a couple of things that you've listed. First and perhaps foremost, excellent work in detail! Whether it runs or not, you've certainly spent some time on it. Very nice!
    Now, of the mods you've listed.
    The cylinder pressure is a good move that will help in the low to mid rpm range when it comes to needing more available torque to get a big prop to spinning. And if speed is your goal, you will need a bigger prop. But, once up to wot, I doubt the increase in pressure is going to help turn any more rpms. (Curious though, how much did you shave off the head?)
    The carb mods sound good but why such a major leap on the main jet? I believe that carb is standard with a 67D jet. My guess is that it's drowning the cylinder. More fuel isn't of much use if more air isn't added to maintain a proper air/fuel ratio.
    The tuner. The factory engineers developed the tuner length for good torque at or near 6000rpm. An inch is way up there. I would think trying to smooth and polish a factory tuner would serve you better. Certainly no more than about 3/8ths of an inch if you are looking to move up to the 7k mark.
    As far as expectations of high rpms, that motor came from the factory with a power pack with a rev limiter. Since you didn't make mention of any changes to the ignition, I thought I would throw that out there. Most will hold the R's down in the 6000-6200 range.
    As stated earlier, you are currently getting pretty good numbers out of that set-up. If speed is the goal, you will need to raise the motor. It's too low. The reason for your "blow-out" issue earlier is due to the type of prop being used. The stock design was never intended to be run at the surface. The motor will run much better if raised. The stock prop won't. Look into one of Ron's cleaver style props to correct the problem.
    Well, hopefully this will give you clues as to what areas to address. Perhaps someone with more experience will chime in if my thoughts are off course as well. Good luck with that and remember, more rpm is not always the answer. You may be better off strengthening the torque at 6k and going with a bigger prop.

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    Thank you
    we shaved 30 thousands of an inch and packed the head to get our compression around 150psi.
    To start off we use a blue point timing light to measure our RPMs which is very accurate so that being said the major leap in the main was because we put a 67D in and it ran a 31Mph and was to lean we confirmed this with the (spark Plugs, GPS and RPMs) we put in a 83D and we ran a 33Mph we confirmed this was our best power through (spark Plugs GPS and RPMs) we put a 87D and our speed dropped to an 32Mph, so it was running to rich we confirmed this (spark Plugs GPS and RPMs)
    when you talk about the tuner and the 3/8ths of an inch you mean of polishing inside? or do you mean to shorten?
    we are unsure if our engine has a rev limiter how do you know and can you remover it?
    so we are going to move the engine up more we are just trying to figure out how at the moment with out a jack plate, how far should we bring it up?
    we are currently looking for a new prop but are unsure what prop to go with should we go with a thru hub or over hub and if so do you thing we could go with a 16pitch cleaver over hub and keep our RPMs or will we lose More? what prop would you suggest?
    do you have any ideas to get more torque? or HP

    Thanks

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    Quote Originally Posted by Emustard View Post
    Thank you
    we shaved 30 thousands of an inch and packed the head to get our compression around 150psi.
    To start off we use a blue point timing light to measure our RPMs which is very accurate so that being said the major leap in the main was because we put a 67D in and it ran a 31Mph and was to lean we confirmed this with the (spark Plugs, GPS and RPMs) we put in a 83D and we ran a 33Mph we confirmed this was our best power through (spark Plugs GPS and RPMs) we put a 87D and our speed dropped to an 32Mph, so it was running to rich we confirmed this (spark Plugs GPS and RPMs)
    when you talk about the tuner and the 3/8ths of an inch you mean of polishing inside? or do you mean to shorten?
    we are unsure if our engine has a rev limiter how do you know and can you remover it?
    so we are going to move the engine up more we are just trying to figure out how at the moment with out a jack plate, how far should we bring it up?
    we are currently looking for a new prop but are unsure what prop to go with should we go with a thru hub or over hub and if so do you thing we could go with a 16pitch cleaver over hub and keep our RPMs or will we lose More? what prop would you suggest?
    do you have any ideas to get more torque? or HP

    Thanks
    3/8ths on the tuner would be in shortening it. (You're dealing with pressure waves or pulses traveling the speed of sound. So the difference between 3/8ths and one inch can be great in terms of working for you or against.)
    The limiter is built into the power pack. You might get the part number off of it and do a quick search. If it's factory original, I would say it's going to have the limiter. As for the tuner, I'd be curious as to how you came up with removing one inch. Did you get a baseline reading before cutting it? You might be better off replacing that one with a stock unit and checking it again. Just be sure not to change or adjust anything else. I have have been told by one of the old omc boys that those cross flows respond well to closing up the relief holes in the tuner. Your idle will be affected and you may end up re-jetting the carb if you go that direction. Perhaps closing all but the top two.?? As for the prop, I would highly recommend saving up for a trim plate first. With what you are reporting now, I would think you will settle in with a 16-18p prop. But only after you get the motor raised and decide between the over or thru hub design. If you are going to use the prop for all out speed with one person, the over the hub will probably be the better choice. If you will be loading the boat down from time to time, the 16 thru hub may be the ticket. Concentrate on twisting the props into the low 6k range and find the one that works. As a side note, what kind if fuel and oil ratio are you running?

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    If the power pack is rev limited, you will see the number "6200" printed right on it. And if you do have one of these packs, there is no question about it when you hit that 6200 rpm.

    We race these motors in our Mini GT (25hp) and GT Pro (30/35hp) classes. I've never had any trouble getting into the 6500 to 7000 range, and we run bone stock motors, right off the fishing boat so to speak. Ron has been working on some new props for these classes, but meanwhile many have done quite well with OMC props. A little thinning and sharpening, and some added cup will make a big difference.

    Our GT Pro boats run at a minimum weight of 650 lbs. (boat, motor and driver) and run in the 45-48 mph range. On a tunnel boat you can jack the motor as high as performance dictates and still get good water pressure. On v-bottoms height is limited -- water pressure drops of very quickly.
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    Polishing ports aint gonna do much...
    Its the height, shape and angles that make the diffrence.
    Calculate your compression ratio, adjust until you reached your desired values. 8.1/1 is good on europe 98 octane pump gas.
    Now you shaved the head, how is your squish? 0.9mm is nice and safe.
    And did you reworked the chamber of the cilinder head to its original shape?
    High exhaust port means more top end but needs rpms, also less torque so you bump the compression to compensate. (Compresion ratio)
    More exhaust area needs more flow of fresh gasses...
    And so you go on :-) .

    But a good prop and boat setup makes also a big diffrence

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    Where would you find a non rev limiter powerpack for this engine? Because I think your right, that it won't rev past 6200.
    We did not get a base line reading before, might have to get a stock tuner to test with. If we have a chance we will test blocking the relief holes in the tuner. I think we might buy a jack plate in the next week or two. And as you were saying we want a speed prop with one person in the boat so maybe are best bet is the 16pitch over hub cleaver from Ron Hill what do you think? Are his props good? Is there any body else that makes a prop like that? We are running pump has premium and 50-1 oil ratio.

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