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Detroit 671ti's proper propping?

Discussion in 'Engines' started by Boomer, May 22, 2019.

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  1. Boomer

    Boomer Senior Member

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    Not to hijack this thread, but my detroit 671ti s only burn 23 gph for both motors under load at 15kts...sounds like I am under propped? 4 blade 24 x 20p
  2. Capt Ralph

    Capt Ralph Senior Member

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    What do they consume at WOT or max HP? What is your hi idle (max rpm no load) and max loaded rpm?

    Ships total fuel use at 15 kts; 23 GPH; 230 HP total; 115 HP per engine. Just over 1.5 gallon per mile.
    What RPM is that, 16-1800?
  3. Boomer

    Boomer Senior Member

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    1990 rpm...will get readings this evening on full throttle no load...under load full throttle will have to wait until the weekend
    Last edited: May 22, 2019
  4. Capt Ralph

    Capt Ralph Senior Member

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    In my years of purchasing old twin engine inboards, I've always got stuck with the old props that were rebuild so many times, the diameter was way gone. One boat, the previous owner even reduced his pitch because of the stuff he had on board (junk), removed it and the boat was under pitched.
    At 1900 to 2000 you should be making some HP and burning near 23 GPH per engine, also boat speed should be up also.
    You have the 380 hp beast?
  5. MBevins

    MBevins Senior Member

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    My 6-71s burn 16 gph aside turning 1950rpm , pushing 44K lbs. to 19knts
    J&T 'd to 450hp each.

    I think you are way under propped.
  6. Boomer

    Boomer Senior Member

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    I had the props repitched last year as all the blades were different and took a guess at what it should be...my motors were 450hp that I detuned with m15 injectors....my guess is 410hp
  7. MBevins

    MBevins Senior Member

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    That changes things.
    Any particular reason why you detuned them? Ralph might have insight but I suspect you may be having to adjust these props after checking WOT this weekend.
  8. Boomer

    Boomer Senior Member

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    Just older motors and didnt want to stress them out as much anymore
  9. Capt Ralph

    Capt Ralph Senior Member

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    Now I'm coming down there to beat you with some old & stale noodles.
    If you didn't want to stress them, keep the throttles back. Now you have a non factory config vs your serial number.
    So, Back to it after your lashings; That's 410HP at the flywheel, 380 at the prop. You lost 70HP off the top. I'm familiar with the tune as long as the timing height is correct. But you may still be under propped. What diameter wheels do you have? Pitch? Reduction?
  10. MBevins

    MBevins Senior Member

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    Boomer..the problem is that in time you won't be happy with the speed so you're going to push them past 2000 rpm and now you're going beyond 80% which isn't good for them.
  11. Boomer

    Boomer Senior Member

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    24x20pitch...4 blades
  12. Boomer

    Boomer Senior Member

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    Nope..80% max..I get it!
  13. Boomer

    Boomer Senior Member

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    Motors with no load....2650 port and 3062 starboard
  14. Boomer

    Boomer Senior Member

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    I had the new injectors put in when I bought her...it has been a year now and both motors adjusted accordingly to the smaller injectors
  15. Capt Ralph

    Capt Ralph Senior Member

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    Pretty tall on the stb but o k for now.
    Next go for a WOT run, We want to see no more than 2450 mid loaded boat.
    2400 fully loaded.
  16. Boomer

    Boomer Senior Member

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    Agree the stbd is a little high...only held it there for a second or two before I backed off...was a little surprised it went that high...I am fully loaded with fuel and water so will look for that 2400 number on top end...thank you
  17. Capt Ralph

    Capt Ralph Senior Member

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    Don't forget the bait. As long as your off da dock!!!!!
  18. Boomer

    Boomer Senior Member

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    Yes sir!
  19. PacBlue

    PacBlue Senior Member

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    I would be concerned with that "tall number".

    The Fuel Rack on the Governors for the 6-71 Detroit's would be set at 200rpm over rated rpm, so a No load Idle rpm would be set at a max of 2500 + 200 = 2700rpm for your engines.

    A 3062rpm at No Load would flag an issue or two:
    1. The Fuel Rack needs to be reset
    2. The Gauges may not be reading/operating correctly.

    3000+ rpm and you are testing bearings and connecting rods, if in fact the gauge was accurate :(. Sample specs for the 450hp version of the 6-71TI below:

    upload_2019-5-24_14-15-30.png
    Ertle moore likes this.
  20. Capt J

    Capt J Senior Member

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    I agree with Pacblue. Buy a photo tach and put the reflective tape on the harmonic balancers and have someone shoot them at WOT and know exactly what your rpms are.