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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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    Well, first things first...port under load, 3/4 fuel and water came in at 2368...stbd came in at 2462 pinned...didnt get the fuel numbers as the clamp on the port blower let go and the hose blew off
  2. Boomer

    Boomer Senior Member

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    Beyond my know how...will need to get the mechanic back for that
  3. PacBlue

    PacBlue Senior Member

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    As far as order of precedence goes, you should have 100% confidence in your Fuel Rack settings and Tachometer readings before your start propeller tuning.

    That is why I would address these first, as you don't want to make a change to pitch, then make a change to your Fuel Rack, then make a potential change to your Tach/rpm sender and go back to find out you may need to make a change to your propeller all over again.

    It may be as simple as a mechanic re-adjusting the Fuel Rack, but it would sure be nice to know that your Tachs are reading correctly.
  4. Boomer

    Boomer Senior Member

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    Thank you.
  5. Capt Ralph

    Capt Ralph Senior Member

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    What were the rpm readings at purchase survey?
  6. Boomer

    Boomer Senior Member

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    Didnt get readings at survey as motor that was rebuilt was ill due to excessive oil dilution...
  7. Capt Ralph

    Capt Ralph Senior Member

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    There are a couple of items that still bug me;
    I must have missed it or it was a clandestine operation on your part, down sizing the injectors.

    Your Detroits have gear driven blowers (supper chargers). So until they go into bypass mode, they deliver the same amount of air to the lungs. You lowered the fuel / air ratio on the piston top.
    Lean burns may overheat the valves at mid range throttle settings.

    With less expanding gasses at the turbo, your not making the boost pressures nor loading the turbo or bypass blower as before.
    The compression ratio does not come up to help burn the fuel quicker at higher throttle settings.
    Extra slobber may result.

    Some 6s had a fuel servo that used turbo boost pressure and kept the rack from over fueling, if you have one, it may be keeping the rack to far back also, still lean.

    Factory tune is a must. If there is a tune for M15 injectors all components must be on the same page.

    OR, the mighty and forgiving 71 will run just fine and I'm blowing methane.

    Get those tachs checked and boost tube repaired.
    Regular hose clamps do not work on boost hoses.
  8. Capt Ralph

    Capt Ralph Senior Member

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    I want the next larger injectors for my 12V71TIs.
    I have to change 4 turbos (2 per engine) to ensure just a bit more air.
    FM
  9. Boomer

    Boomer Senior Member

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    Injectors were replaced before I took delivery of the boat so you didnt miss anything along the way....the mechanic is going to test the gauges at the motors versus bridge to check for accuracy as we speak
  10. Boomer

    Boomer Senior Member

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    The clamp that broke was a t bolt clamp...not the household kind
  11. MBevins

    MBevins Senior Member

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    I looked back at the the thread.
    I don't see any speed / RPM ratios posted. As an example, what is the speed at 1950rpm?
  12. Boomer

    Boomer Senior Member

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    1980 is 13 knots +- at 3/4 fuel and water load
  13. Capt Ralph

    Capt Ralph Senior Member

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    Tachs checked yet?
    Lets find the correct diameter of wheel you are supposed to have and compare to what you have now.
    Please re-read my #4 above.
  14. Boomer

    Boomer Senior Member

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    The boat originally came with a 3 blade...someone changed them along the way...24x27 sticks in my head but will need to confirm when home tonight...waiting on mechanic to verify the tachs
  15. Boomer

    Boomer Senior Member

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    Still waiting for the mechanic
  16. cleanslate

    cleanslate Senior Member

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    Boomer!! This sounds good to me, good RPMs, as everyone has said, as long as your Tachs are reading right. I would leave it be then if they are...you're splitting hairs on getting them exact when pinned. Sounds like your props are fine if this is the true RPM.
    I would also go back to the original injectors, there was a reason they were used , as Capt Ralph schooled us on.
    Here's my simple thoughts...If It ain't broke don't fix it, or try to redesign it . Your asking for trouble. Also from one older Ocean/DD owner to another I wouldn't run them much over 2200 RPM. See what happened when you ran it pinned.... hoses start blowing off, oil/fuel line start breaking filling your bilge with fuel or oil etc....You want to ''save'' the old engines, use the throttle to do that as was said before.
    Also check your entire throttle cables from the Morse controls to the engine , check to see they are both moving the same amount from idle to full tilt.
    I'm almost sure one or both were replaced some time and could be out of adjustment, including that Glendinning engine synchronizer unit , they have throttle cables ''33C red?'' to that can cause problems whether it's on or off. All of this can contribute to you engines being out of RPM when pinned.
  17. Capt J

    Capt J Senior Member

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    Those items would’ve broke anyways, sooner than later. I believe in running them wot for 4-5 mins every once in a long while to make sure everything is ok on them. If they’re starting to run hot at wot you know it’s time to do the heat exhangers
  18. Boomer

    Boomer Senior Member

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    New throttles and cables installed last year...throws are good...just trying to dial things in for the best speed, fuel economy possible without abusing things
  19. cleanslate

    cleanslate Senior Member

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    Got ya, but they could be out of adjustment by just a bit. It can be a bit challenging to get them both just right...At least for me the ''Meat Head'' yachtsman! LOL! I had a hard time with mine which I did on my own last year.