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Thread: exhuast out the water ?

  1. #11
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    when the 70/75 has a nitro gearbox ( small one) its a bit restrictive to the exhaust so the holes offer an alternate path

  2. #12
    pdt
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    so is this better if using an over hub propeller just for the holeshot or better all round for the over hub and thru-hub props?
    its definitely making sense to un-restrict the exhaust as much as possible.
    its also a cool look I think with the large holes.
    is there any down side to doing this though ?

  3. #13
    pdt
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    I can now see it being better to breath and run better yes, but would it also make it better to have NO exhaust running over the blades of a over hub propeller ?
    I know that the over hubs are designed for surface piercing so do work with a certain amount of air mixing with water, but if the exhaust gases are
    now non existant is this going to give even better results for this type of prop. or is there still a certain amount of through the gearbox exhaust ?

  4. #14
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    Quote Originally Posted by pdt View Post
    is this the type of thing you mean by drilling holes above cavitation plate? the two big holes next to six smaller ones
    is this just an empty exhaust cavity then , and this lets the chopper prop bite pure water instead of an exhaust mix ?


    ( By the way this is NOT my engine its a pic I have used I think from this forum, nice looking old engine from the seventies)
    Just a FYI. The 1989 and 1990 EP nationals were won with a motor similar to the one pictured. The winning motor did not have the exhaust holes as shown, others in the race did have them.

  5. #15
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    Quote Originally Posted by Popa Sam View Post
    Just a FYI. The 1989 and 1990 EP nationals were won with a motor similar to the one pictured. The winning motor did not have the exhaust holes as shown, others in the race did have them.
    so the winner didnt sound as good as the rest? lol

  6. #16
    pdt
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    Quote Originally Posted by Powerabout View Post
    so the winner didnt sound as good as the rest? lol
    ha ha brilliant ha ha
    winner didn't sound and good

  7. #17
    pdt
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    opened up the gearbox to have a look in the area to drill the holes for exhaust vent/relief.
    Was surprised as ive never even bothered looking down there and taking much notice before, its just a big empty.
    I see its a nice flat area with no webs to worry about so I will be drilling those out as real soon.
    had a look at the water pump while there and managed to have a bolt break in the gearbox !! now I am f-------d, need to get that out now.(separate topic)

    When drilling the holes is there a good size to do them ? ive seen some with 2 holes and a couple with an elongated hole !!!! is one better than the other or
    just as good ?
    looking forward to having the holes now, sound of a 2 stroke is nice even when loud, and if it makes the engine go a touch better than great.
    i am always prepared to try anything to make it run a bit better, and this is definitely an easy job to do, just the type I like.


    .

  8. #18
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    Quote Originally Posted by Coastal 60 LO View Post
    I personally don't think they offer any power increase on most motors. Some will disagree. Sometimes opening up the exhaust like that will require you to re-jet your carbs because it can cause a lean condition. Also, by cutting the mid you weaken it if done incorrectly. Some people have lost lower units due to the mid breaking from exhaust cutouts. One of the best things about opening up the exhaust like that is that for some it makes the races more interesting. Races would be boring without the noise. I prefer to drill the lower unit for some added noise. Drilling the lower unit above the antiventilation plate seems to hold up better. JMHO.
    I agree 100%
    Helping folks out around the globe.

  9. #19
    pdt
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    on the pic I posted of the white engine with the holes above the cav plate, what are the six small holes for ?
    I cant see what they are there for ?

  10. #20
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    Quote Originally Posted by pdt View Post
    I don't know the correct term for this, but i have seen lots of race engines with the exhaust coming out of the back of the
    mid-section instead of through the propeller.
    is there a great benefit to this ?
    is there a downside apart from noise ?
    and does it make a difference to a over the hub propeller or is this not advisable to do with such props.

    .
    "Above water exhaust". The real benefit to having an exhaust seperate(above the prop), is to incorperate a slimmer, more streamlined racing foot with less displacement/less drag. The idea for the underwater/thru-prop design was about quiet operation in recreational use. It would be hard to build a thru-prop exhaust gearcase that is very small in diameter, as there has to be a sufficient area for the exhaust to exit. Furthermore, the idea of thru-prop exhaust would be for nothing if the prop-shaft were run up to the water surface. There would be noise anyway. The seperate exhaust tower/racing type gearcase engines can be run very high up for minimum drag. Over the years, underwater(thru-prop) units have been utilized in certain classes of racing though. There are cone-nosed/race-like versions of these that are made as well. However, I don't think you will see the two types raced against each other much, if at all. Thru-prop race types are more of a utility/racer class of today. Unlike the shortened, "above water exhaust" race engines, the high performance thru-prop models can be used recreationally.

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