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  1. #1

    HP to cubic inch ratio - Is it right?

    I have read that the ratio of hp to cubic inch ratio can determine when an engine will need an overhaul. For example:

    CAT 3208 with 375 hp with 636 cu in overhaul would be about 5896 hours

    DD 6-71 TA with 450 hp with 426 cu in overhaul would be about every 1056 hours

    DD 6-71 N with 260 hp with 426 cu in overhaul would be about 6100 hours. A broker once told me that 6-71 N is the best engine ever made - they are bullet proof!

    Any opinions on this method? I have read online that heat is the big enemy of any engine. The more hp the more heat produced, and the sooner major overhaul comes around.

    I have also read that the cost of an overhaul can be around $3,000 per cylinder! If an overhaul is required every 1000 hours on a DD 6-71TA - I don't think I want DDs in my boat.

    AM I WRONG IN MY ASSUMPTIONS?
    Looking and learning about Hatteras

    Tuga

  2. #2

    Re: HP to cubic inch ratio - Is it right?

    Don't assume.
    Scott
    41C117 "Hattatude"
    Port Canaveral Florida.


    Marine Electronics and Electrical Products Distributor.

  3. #3

    Re: HP to cubic inch ratio - Is it right?

    Yes!!
    GLORY Hull # 365
    Northport, NY

  4. #4

    Re: HP to cubic inch ratio - Is it right?

    The only thing correct in your post is that burning fuel is what wears an engine. There is no magical formula. While the 671Ns will last longer than 671TIs, the TIs should get more like 3000 hours on average. Sure they'll be tired sooner if you run them hard all the time and don't take care of them but that's the same for any engine. 3K per hole is about right for a Detroit, closer to 4K from a Detroit dealer.
    Jack Sardina

  5. #5

    Re: HP to cubic inch ratio - Is it right?

    Quote Originally Posted by Miss Lisa View Post
    I have read that the ratio of hp to cubic inch ratio can determine when an engine will need an overhaul.
    Miss Lisa,
    Please put down whatever you are reading. HP and CU have absolutely no correlation on how often any engine requires rebuilding. How well one takes care of them has everything to do with how often engines require overhauling.


    Scott & Malina

  6. #6

    Re: HP to cubic inch ratio - Is it right?

    My 6-71TI's (390 HP)with 3000 hours are running strong with no issues. I expect at least another 3000. I spool them up once a day just to clean the turbos. I agree that the 6-71 was the best one ever. But not at 450HP

  7. #7

    Re: HP to cubic inch ratio - Is it right?

    It's kind of right in part, if that can be said to be "right". Lots of things contribute to TTBO: use pattern, maintenance, engine design. The amount of fuel burned is a pretty good indicator of TTBO, because it takes into account a lot of those things- if I am not mistaken, fleet operators base overhaul intervals on total fuel burn, among other things.

    All other things being equal, you could argue that, for example, a 6-71 putting out 250 hp will require less frequent overhauls than a 6-71 putting out 485 hp... but all other things are seldom equal.

    You could also argue that getting more hp out of fewer cubic inches means that the engine is working harder, and that squeezing more power out of fewer cubic inches is going to mean more frequent overhauls....possibly. Maybe, maybe not.

    Modern common-rail diesels squeeze more power out of four-stroke diesels than I thought was possible. And yet they seem to be lasting okay... for now. I think the jury is still way out on those engines. I know that CR engines are much more efficient in using fuel to make power than older mechanically injected ones were. Whether that translates to increased longevity.. who knows?

    There is no formula, not that I've ever seen, for predicting engine useful life on the basis of displacement and horsepower. What you want for that is an engine survey done by someone who knows that particular model engine- and has looked at a lot of them.

  8. #8

    Re: HP to cubic inch ratio - Is it right?

    Quote Originally Posted by Scott & Malina View Post
    Miss Lisa,
    Please put down whatever you are reading. HP and CU have absolutely no correlation on how often any engine requires rebuilding. How well one takes care of them has everything to do with how often engines require overhauling.


    Scott & Malina

    Put down the bottle. Even a overly maintained engine will wear.
    Scott
    41C117 "Hattatude"
    Port Canaveral Florida.


    Marine Electronics and Electrical Products Distributor.

  9. #9

    Re: HP to cubic inch ratio - Is it right?

    I have a friend who lives in Destin, and he recommended that I talk to his mechanic. Long story short the mechanic said that the hp/cu in ratio is BS! He said that total fuel flow thru the engine determines overhaul times. 30,000 gallons is usually when an overhaul is needed. Of course, the maintenance of the engines has a great deal to do with it - he said!

    Sounds like he agrees with some of you guys.

    Thanks for helping me learn.
    Looking and learning about Hatteras

    Tuga

  10. #10

    Re: HP to cubic inch ratio - Is it right?

    So an engine that burns 150 GPh will only last 200 hours but an engine that burns 10gph will last 3000 hours ?! Wrong!

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