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Thread: Will blueprinting a 50D Tohatsu result in a 10% performance gain??

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    Question Will blueprinting a 50D Tohatsu result in a 10% performance gain??

    Maybe I am expecting too much out of my Tohatsu 50D powered T750 boat??
    We have been racing T750 Class in Canada for a few years.
    The OMC’s as a group have really improved performance wise and are difficult to beat.
    The driver is not the issue. He makes us competitive. The boat is set up well it is not a concern.
    We are currently running 100% stock. I need a 10% improvement over the stock motor to
    run with the top OMC’s.

    QUESTION: For those who started stock, what % improvement did you see when you spec’d
    out the motor?

    Here is what I can legally do by our rules:
    I can pin the crank.
    Balance the crank and rods
    Port the motor to spec
    Run any tuner
    Cut the head to produce 160 lbs compression.
    You can remove the oil injection
    You can run plastic reeds
    You can run a nose cone but not LWP.

    The T750 Tohatsu speed record is currently over 66 mph.
    We are at 54 mph on the race oval.

    I need to be 58 to 60 mph.
    WILL THE MOTOR IMPROVE 10%?

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    Even if you improved the motor 10% your speed won't improve by 10%...

    Speed of a planing boat is a square root of the power. That is, to improve your speed by 10% you need 21% more power. So even if you increase the power by 10% you are likely only going to see about a 5% increase in speed. It's a bit better than that since you are accelerating and the power increase does improve acceleration but that also assumes that your prop is perfect matched.

    You can probably expect an improvement primarily from the blueprinted ports and from the cutting of the head. The plastic reeds aren't going to produce much power but if you eat one it won't be expensive. The tuner would help, but you've got to either know what you need, or spend a lot of time on a dyno testing to get that right. You probably need to do it, but you need someone who knows how to do exhaust tuning and has a dyno in order to do it.

    While you may well be down on power, you need to understand that you most likely aren't going to get it all from one big tweak like building a motor.

    When you assume that you "can't do any more" to improve your setup or boat you are doomed. There is always more. If you stop looking at any one area you've stopped improving and you won't go any faster.

    You need to look at your aerodynamics, the separation zone at the back of the boat, any stagnation areas in the front ect...

    Props are always key, small increases in efficiency, acceleration and top speed can always make big differences.

    If you make changes to the aerodynamics and prop then the setup needs to change to take advantage of it.

    If you want to win you need to get busy and start testing.

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    Thank-you for the reply. Let me supply some more information because
    I do not want the thread to end at this point. First I have been racing in these type
    Classes since 1964, so I have several years of experience. What I am looking for here is to hear from people who started stock and then blueprinted their motor. It costs a lot of money to do these mods and I maybe better off to go with a different motor. There is not a lot of information on the 50D. We know it is strong in SLT and was tops in Formula 50 years ago. The current world speed record for T750 was raised from 55 mph ( I can beat it ) to over 66 mph, both with a 50D.
    I spend a lot of time trying to get the most out of a 70 CES T850 combo. I just do not think I can get the 50D up to par with the OMC. There is no sense wasting money if I can't get there.

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    3E3B871100 Full Dressed Power Head 50D/2-STROKE (Racing)
    Superseded to 3E3B871100R
    $2,555.28
    Tohatsu Nissan Genuine OEM outboard motor part number 3E3B871100
    The part superseded to 3E3B871100R
    3E3B871100R is the latest current part.

    Looks like they are still selling the complete powerhead.

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    have you done the pen and paper exercise to see if its possible to compete with an OMC?
    Total piston area/stroke/rpm/BMEP
    etc etc
    and gear ratio

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    Yes, I have. I have also spoken to two non racing people who did a spec motor. Similar performance results in each case. They improved approximately 2 mph with a noticeable improvement throughout the power band. That is what I expected. So I am going to spec my motor for next year.

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    how tight the specs are has lots to do with it, like deck height if that where they measure the porting???

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    I have the UIM spec sheet for the motor. Many times people forget about the centre line crank to deck measurement. I have purchased several legal passed inspection power heads only to find they are not legal had that measurement been taken. The inspection is only as good as the inspector doing the inspection. Currently we are competitive running a tired stock 50D against most of the race prepared OMCs. The top boats have the good package (good set ups, motor and driver). I have just finished a race prep OMC which I will run on a very light Crichfield. We will be able to compare the two outfits. I also have a carbon fibre Critchfield in the works which will run a race prepared Hydro Tec 60D Yamaha. The power head is done, I just need to decide on the seat position in the boat. I am thinking of moving it well back same as the British T850 boats. Last resort is running our 60 hp race 4 stroke Mercury. I think we have covered all options!

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    Default 160 Pounds of Compression

    How many pounds of compression does your stock motor have?

    I really know about the 40 D (two cylinder) but a two cylinder can pick up water from the cav plate which allows you to run the prop shaft even with the bottom. No low water pick ups, you said, but what about a water pick up behind the prop???? (On the cavitation plate.)

    Have you thinned and sharpened the skeg???

    What prop are you running?

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    Quote Originally Posted by Ron Hill View Post
    How many pounds of compression does your stock motor have?

    I really know about the 40 D (two cylinder) but a two cylinder can pick up water from the cav plate which allows you to run the prop shaft even with the bottom. No low water pick ups, you said, but what about a water pick up behind the prop???? (On the cavitation plate.)

    Have you thinned and sharpened the skeg???

    What prop are you running?
    Ron The current motor is a tired untouched motor off a pontoon boat. It was my parts motor. I sent my other motor to an engine builder to have it blueprinted. That was over 6 years ago and I have not got it back. I just got another motor so it is the one I will blueprint. We must use the original water pick up position. The compression is 135 on all three. We run 12 x 18, 12 x 19 or 11.5 x 20 thru hub props. The skeg will get done once the nose cone goes on.

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