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Thread: Coverting from Bravo 2 to SEI 121

  1. #1
    Join Date
    Sep 2011
    Posts
    3

    Default Coverting from Bravo 2 to SEI 121

    I just purchased an 1989 Invader v210 cuddy which somehow was originally equipped with a 5.7L Bravo two 2.00 ratio. The lower is in questionable condition and was wondering if a SEI 121 would work in this application and if so what size prop. The Previous owner says 52 MPH 4400 RPM with a 17 1/4 25 P prop. Also being and older drive what are the mods that are required. Thanks in advance for the advise.

    Mike

  2. #2
    Join Date
    Nov 2006
    Posts
    661

    Default

    A Bravo One lower should bolt directly up to the bravo upper. It is interesting that it has a bravo two drive. My understanding was the bravo two was mainly used on work boats. My concern is that the bravo two will have a lower x dimension, so by putting a bravo one lower on it, you are essentially raising the prop centerline. If it is already mounted at the right spot, then a bravo one will put the prop too high and it will not perform properly at all.

    I am not that familiar with x dimensions and the measurement you would be looking for, but I would suggest finding a boat that has a bravo one, then measuring from the bottom of the transom assembly to the bottom of the boat ( the bottom of the "V") and compare that to yours. If it is the same, then it is a safe bet that a bravo one would work. If yours is mounted higher, then I am not so sure.

    Tech Support

  3. #3
    Join Date
    Sep 2011
    Posts
    3

    Default

    First thanks for getting back to me so quick. I was so concerned with gear ratios and prop pitch that I did not even think about the difference in dimesions between the B1 and B2. I memory serves me the cavitation plate is just about even with the bottom of the hull and since that is part of the upper it should not change regardless of which lower is being used. I did get the original installation manual that was issued when the boat was purchased new so I will look in there for dimesion differences when I get home.

    Let say the x dimension is the same and I bolt a B1 on. This brings me back to my original question. The B1 was never offered with a 2.0 ratio upper and when I run some prop pitch calculation I come up with 27 - 29 this does not sound right??

    What effects the gear ratio used in a drive? Length, weight, engine size/HP
    unfortunatly I can find very little info on how similarly sized boat are equiped.

    Thank again

    Mike

  4. #4
    Join Date
    Sep 2011
    Posts
    3

    Default

    Been doing some more research!! First I checked out my cavitation plates and they are right at or near the bottom of the boat. Just like the Alpha gen 2 on my stingray (For Sale). Then I check the Merc Installation Manual for the 5.7 liter Bravo which shows a standard X dimension for all Bravo drives. So the 121 should work just fine.

    On a different note I have discovered is the ratio is based on a combination of both the upper and lower gear sets so if the SEI 121 has the same gears as the Merc Bravo 1 it would make my drive a 1.5 ratio. Which them put me into 19 to 21 pitch prop.

    Hope to order my new lower when funds allow. Winterizing is coming soon so I have time.
    Mike

  5. #5
    Join Date
    Nov 2006
    Posts
    661

    Default

    Sounds like you got it figured out. It is a little odd that the bravo two has the same x dimension of a bravo one. A bravo two will definitely swing a bigger prop, so the prop centerline has to be lower than a bravo one. Maybe they want more of the bravo two in the water since they are typically used in house boats and work boats. You might see a nice performance increase since there will be less in the water with the bravo one. Make sure and update this when you get it going.

    Tech Support

  6. #6
    Join Date
    Mar 2013
    Posts
    2

    Default Do you still have the Bravo 2 housing? I have to replace my Bravo Rod 727 452 4507

    I have to replace my Bravo Rod 727 452 4507

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Sterndrive Engineering, Inc. (SEI) manufactures a stern drive designed to replace Mercury Marine's MerCruiserŽ Alpha OneŽ, R, MR, Gen II and Bravo OneŽ stern drives. MerCruiserŽ Alpha One, R, MR, Gen II and Bravo OneŽ are trademarks of the Brunswick Corporation. SEI has no affiliation with the Brunswick Corporation. Consequently, any warranties governing products and or sold by Brunswick Corporation will not apply to products manufactured and or sold by SEI, or to products damaged due to the operation of an SEI product.