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Thread: Different Outdrive Ratios

  1. #1
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    Different Outdrive Ratios

    Looking for some guidance here if anyone can help. I have searched through many threads looking for information on the difference between outdrive ratios but have not been able to find one discussing repercussions of slapping on an outdrive with a totally different ratio than previously ran. I am looking to purchase an SS outdrive for my 18 TR. I hear it is a bolt on 5-8 mph which excites me. that being said, the one I am looking at has a different gear drive from what I am running now. My first generation Alpha has a 1:5. the particular drive I am looking at is a 1:32.

    Is it as simple as re-propping to compensate for this different ratio?

    Am I going to lose any top/low end versus going with a 1:5 ratio ss?



    thanks in advance!

  2. #2
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    I don't have any personal experience w/a SS, but I don't know anyone who has seen those increases w/no other changes.
    In fact, some have seen none at all.

    IMHO, a stock Alpha w/an additional 100 HP will generate a pretty big increase.
    SEI sells their after market Alpha replacement drives w/an incredible warranty for $1,295.00.
    I think it may be cheaper to go the huskier engine route than trying to find and maintain an unobtanium SS.
    George Carter
    Central Florida
    gcarter763@aol.com
    http://kineticocentralfl.com/


    “If you have to argue your science by using fraud, your science is not valid"
    Professor Ian Plimer, Adilaide and Melbourne Universities

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    I really don't want to burst your bubble, but on a 20' MINX. I saw NO increase in speed.

    I did however suck up every possible weed getting into my boathouse.

    Ken

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    So long as you are reasonably within the range of available prop pitches, I'd think you could just re-prop and find something that works. (Whereas, if your prospective 1.32 gears would require dropping down to a 15 inch pitch or something, that'd likely be more of an issue.)

    The more I hear people report about their prop observations, the more convinced I am that you just have to try a few. So many variables.

    I would not expect anything near a bolt-on 5-8 MPH. I suppose it's possible, but I would try not to get my hopes up on something like that. Back to the props/variables thing, there seem to be a lot of very different results for different people. Some alpha SS setups sound like they work wonders, and as some have noted already, some don't seem to do much at all.

    There may be different schools of thought about turning steep props more slowly versus shallow props more quickly, though the former would seem to offer at least one clear advantage. Dropping from 1.5 to 1.32 and changing nothing else would be going the opposite way.

    Best of luck with whatever you decide. Curious to hear how it turns out.
    "I don't have time to get into it, but he went through a lot." -Pulp Fiction

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    ????? not sure what just transpired. So an SS is a shorter Alpha drive than a the standard Alpha drive. It is designed to be more of a surface piercing drive. It is using the heavy duty gear set with the ratio 1:1.32. This ratio is designed for high torque motors such as a big block. In theory the prop is going to slip more because it's so close the surface of the water. Therefore the even though the the ratio is greater the prop slip should null the Rpm loss. If any thing you'll need to increase pitch to get the slip out. Because you need to add more pitch to prevent slip your speed would increase. Hence the gain in mph.Note what George said parts are scarce and expensive. But I think if your running it on a small block the 1:1.32 should hold up pretty well
    machinist ,bore it deeper,ream it bigger, and lap it to a fine finish



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    My 87 alpha1/454 330 hp setup has the 1.32:1 ratio. I run a quicksilver 21p (I cant remember the size, maybe 14.5??) cleaver @ ~63-65 mph @ 4300-4500 rpm. I can feel the prop spinning 90 to nothin when idling around the docks. Original parts since 1987 and the internals still look new.

    I hear the reason for the 1.32:1 is because the two gears are closer in size, making them more similar in strength than a 1.5 or 1.62 gear ratio. It sounds feasible, so I'll accept it.
    Never Kick a opossum at 40 mph!

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    Hadn't ever heard that about the slip going up that much, but it's interesting. (I have to say I'm skeptical that the slip would go up so much with the 1.32 alpha SS that the prop pitch should be the same as a 1.5 alpha, but I have absolutely no experience with them to actually know. Just seems like jumping from say, 10% slip at WOT, all the way to 20% slip would be a lot.) Hmmmm...
    "I don't have time to get into it, but he went through a lot." -Pulp Fiction

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    Let me say it like this, these hulls really won't go faster than about 65 until at least 400 or more HP (for an 18) is utilized to lift the hull further out of the water and reduce wetted surface.
    I don't know of anyone who had any real, consistant, measurable gain just by adding a shorty and maintaining stock power.
    The reduced surface area of the shorty only adds to the reduced area of the planing surface benefits when enough power is available to get the hull "over-the-hump" in the 75-85 MPH range (for the typical Donzi hull).
    There're several here that tried this route and ended up w/quite a bit more power to make it work.

    Besides, I think you'll find there's quite a few more props available for 1:47-1:50 ratio drives.
    George Carter
    Central Florida
    gcarter763@aol.com
    http://kineticocentralfl.com/


    “If you have to argue your science by using fraud, your science is not valid"
    Professor Ian Plimer, Adilaide and Melbourne Universities

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    Wow, this is all very good information here. I have been told by several donzi owners that this would be the answer to get me into the 60's. For some reason, I can run at only about 57 no passengers, half a tank of gas. Maybe I should try to play around with some different props first as I ahve an awesome hole shot presently.

    How much top end would you folks say I should expect form my boat? stock 350 mag 18', only 300 hrs on the engine runs very strong, almost perfect compression, normal conditions, no passengers half a tank of gas.....I thought 57 was kinda tame. With the correct prop, can I achieve the 60's in your opinions? Should I try that first?

    CHACHI, I am sorry to hear that! what a waste of time and $. I would obviously want to avaoid that scenario lol. Is there anyone on here who has had success with slappin on an ss drive? Anyone? I feel like I have come across this topic so many times on thi ssite saying how great they are.....


    Thanks all!

  10. #10
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    Patrick, you really need over 300 HP to get into the 60's.

    A new engine w/at least 400 HP will/should get you into the 70's w/some prop shopping.
    George Carter
    Central Florida
    gcarter763@aol.com
    http://kineticocentralfl.com/


    “If you have to argue your science by using fraud, your science is not valid"
    Professor Ian Plimer, Adilaide and Melbourne Universities

  11. #11
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    You need to evaluate what you got for numbers. Pitch, RPM, MPH then you determine how much slip you are getting from the theoretical MPH to the actual MPH. A prop calculator will figure it for you.


    You can never go wrong with a top end kit to increase HP
    machinist ,bore it deeper,ream it bigger, and lap it to a fine finish



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  12. #12
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    To give you a comparison, my 18C with 350 mag mpi, 300 hp, with 1.5 drive ratio, and a 23 pitch mirage, would do about 68 mph.

    Bob
    Member - WAFNC
    1997 22 Classic (sold)
    1997 Formula 271 Fastech
    502 Mag MPI

  13. #13
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    "To give you a comparison, my 18C with 350 mag mpi, 300 hp, with 1.5 drive ratio, and a 23 pitch mirage, would do about 68 mph.

    Bob"




    Seems to disagree with many above posters..
    Cheers,
    Pismo
    1996 22 Classic
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    74.5mph best @ 5050rpm GPS (Speedo said 80)
    27" Labbed Mirage Plus
    75.5mph best @ 4800rpm GPS (Speedo said 82)

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    I'm scratching my head--is it possible there's some 18C versus 22C confusion going on in this discussion?

    "You really need over 300 HP to get into the 60's" sounds more like a 22 than an 18 to me.

    "These hulls really won't go faster than about 65 until at least 400 or more HP (for an 18) is utilized to lift the hull further out of the water and reduce wetted surface." I thought 415/425 should make for maybe 68-72 in a 22, not an 18. I'd think an 18 would reach 65 with far less power, more like Bob's numbers (18C, 300 HP 350 mag, 68 mph).

    I wonder if the "True Lies" thread has some 18 numbers to work from...
    "I don't have time to get into it, but he went through a lot." -Pulp Fiction

  15. #15
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    Actually, I was using my Minx experience as a basis. It was 2' longer and 200 or so #'s heavier than an 18.
    A minx really needs 300 hp to do 60.
    A stock 260 hp 350 would not do it unless everything was optimistic to the extreme.
    Mine had a new 350 Vortec which everyone agrees produces 315-330 hp or so and would do 63-65 MPH on most days.

    But I still believe if the 18 in question goes 57 and it's propped correctly, the answer isn't an SS.
    George Carter
    Central Florida
    gcarter763@aol.com
    http://kineticocentralfl.com/


    “If you have to argue your science by using fraud, your science is not valid"
    Professor Ian Plimer, Adilaide and Melbourne Universities

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