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

  1. #11
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    Quote Originally Posted by Hounddog View Post
    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!
    Hi Hounddog
    Was there ever a spec sheet for the omc 45ci for t 750?
    They are 70hp at 7000 in sst45 spec which is stock porting which is faster?

    Did you get your carbon boat running?

  2. #12
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    APBA had a spec sheet for the T-750 45 cubic inch OMC when it raced in Formula 50. That sheet is basically the same as the SST 45. I have never seen a 70hp @ 7000 OMC 45. The port spec on the spec sheet is the same for both motors. My carbon fibre T boat is getting close. We saw a big crowd at our OPBRA Peterborough race this year.....over 8000 fans and we had a good boat count in both T-750 and T-850
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  3. #13
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    Quote Originally Posted by Hounddog View Post
    APBA had a spec sheet for the T-750 45 cubic inch OMC when it raced in Formula 50. That sheet is basically the same as the SST 45. I have never seen a 70hp @ 7000 OMC 45. The port spec on the spec sheet is the same for both motors. My carbon fibre T boat is getting close. We saw a big crowd at our OPBRA Peterborough race this year.....over 8000 fans and we had a good boat count in both T-750 and T-850
    Yes hard to believe the shaved head and custom megaphone will add 10 hp and lift the peak power up to 7000
    Al Stoker told me he has had many on his dyno and thats where they all are

  4. #14
    Team Member zul8tr's Avatar
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    Quote Originally Posted by Yellowjacket View Post
    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.
    This goes way back but speed varies as the cube root of power not the square root as noted above (support available). So a 10% gain in power will yield cube root (1.10) = 1.032 or about a 3 % gain in speed all else equal. In reverse a 10 % gain in speed yields (1.10) cubed = 1.33 or 33% more power all else being equal. Also reducing drag can achieve speed improvements without increasing power as much, it's a balance act.

    As stated there are many variables: performance efficiency, prop, boat design, weight, setup, weather conditions, fuel, jetting, timing, etc.......>

    Serious Testing with great notes reveals the reality.
    Last edited by zul8tr; 12-14-2019 at 02:32 PM. Reason: Added info
    " Three may keep a secret if two of them are dead" Ben Franklin

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