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  1. #1
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    1985 Chris Craft Stinger 390
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    Mercruiser Horsepower Ratings

    This might be a dumb question, but when Mercruiser rated horsepower on their stock 454, 7.4L engines through the mid-80's and early 90's , were the horsepower ratings given at the prop or crank? Specifically asking about the 454 mag bravo's, MCM 370, 400, 420, 440, 460 etc.

    If these are prop ratings what is the loss generally from the crank? 30 hp or so sound right?

    Thanks

    Happy New Year to all........

    Todd

  2. #2
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    1989 Chaparral Villain III
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    Re: Mercruiser Horsepower Ratings

    The old 330hp was at the crank and that same engine became 310hp at the prop. That works out to be an efficiency of 94%.

  3. #3
    VIP Member VIP Member MILD THUNDER's Avatar
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    Re: Mercruiser Horsepower Ratings

    Hey Todd, I got the miami vice complete first seasson on dvd for xmas. Last night I watched the episode where they race from miami to bimini. The stinger 390 vs. 50ft Cary. Awesome!

    I would highly recommend throwing a set of roller rockers on, Edelbrock intakes, your holley 800 carbs, and with a good exhaust, Id bet you would be around 400HP. I have twin 425HP in the same boat as you, mine may even be a little lighter, and I feel it needs more HP. I think with the stock 365's. your boat will run about 55. With the extra HP from the upgrades, should get you around 60. A better cam would be a big help, but I know you dont want to tear into the engines. My old 23ft boat with a stock 454 365HP picked up 8 MPH from a cam/exhaust change.

  4. #4
    VIP Member VIP Member MILD THUNDER's Avatar
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    Re: Mercruiser Horsepower Ratings

    Here is the cam used by merc in their 400, 420, 425 motors.

    http://www.cranecams.com/?show=brows...tType=camshaft

    So to make 365HP engines to a 420Hp, you need the above cam, valve springs, crane gold roller rockers, gil manifolds, and a holley 800 DP with 79p, 94S, and a 6.5 pv. Those jets may be on the rich side, but thats what it came stock with.

  5. #5
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    1985 Chris Craft Stinger 390
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    Re: Mercruiser Horsepower Ratings

    Hi Joe, I just sent you an email. Thanks for the data, but I'm just wondering where the HP is rated at on the Merc engines in the 80's and 90's. I figured others may want to know as well, since I could not find any information on this question on this site. Maybe I just did not look hard enough.

    The Miami Vice DVD's are a real blast from the past. Season 2 is out now. I just got it yesterday and right away started watching the episodes with the most Scarab KV coverage. Good show, but I always just watched it for the boats (ok and the bikini's, and cars etc....)

    I like my Stinger, but nothing looks like that KV!
    Last edited by stinger390; 12-29-2005 at 11:13 AM.

  6. #6
    Registered Big Block Billy's Avatar
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    Re: Mercruiser Horsepower Ratings

    When they went to the gen 5 in '92 , the rating changed to prop Hp. BBB

  7. #7
    VIP Member VIP Member MILD THUNDER's Avatar
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    Re: Mercruiser Horsepower Ratings

    Heres a question. My 454/425HP motors came with Gil exhaust stock. I currently have 3" Log iron manifolds. What would be a guess on my prop HP? 375?

  8. #8
    Registered Big Block Billy's Avatar
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    Smile Re: Mercruiser Horsepower Ratings

    Manifolds rob about 50 Horsepower. Log ones probably more. BBB

  9. #9
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    1996, Donzi, 22 Classic 502
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    Re: Mercruiser Horsepower Ratings

    The 1991 Mercruiser catalog, which I have in my hand, rates hp at the prop.

  10. #10
    Registered Big Block Billy's Avatar
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    Arrow Re: Mercruiser Horsepower Ratings

    Yeah I guess they changed the hp rating around 91 then.


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