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  1. #1

    Would changing to smaller injectors save fuel?

    See if this makes any sense: my DD 8v71 Naturals have N70 injectors. I believe that N55 injectors are also available for these engines. Now, '55' is 21% less than '70' arithmetically. Would it therefore follow that I might get 21% more (better) mileage if I changed out to the lower rated injectors? Apples for apples, I cruise at 1400 RPM, so would I likely get 21% better mileage if I stayed at the same RPM using the N55 injectors? Does this make any sense? Thoughts? Thanks.

  2. Re: Would changing to smaller injectors save fuel?

    Fuel demand is directly related to load demand. You cannot cheat physics. Your fuel burn will be identical. You will just have to push the throttles further forward to get to 1400 rpm and then you will run out of throttle should you need full rpm. Diesels do not have throttling loses like a gas engine does. You can get better mileage on gas engines playing around with carbs and fuel injection throttle bodies, but this does not work on diesel engines. Gas engines control the amount of air that enters the engine, diesels control the amount of fuel. Distinctly different mechinisms.
    Chris
    1973 48' Yachtfish
    "Boss Lady" my other expensive girlfriend.
    Follow the refurb at www.starcarpentry.com

  3. #3

    Re: Would changing to smaller injectors save fuel?

    Boss Lady, agree with everthing you noted, but have one question. Given the variable speed governors on these DD would you not advance the throttle controls to the same physical position for 1400 RPM, and the governor would then open the injectors more to get the required fuel?

    Pete

  4. #4

    Re: Would changing to smaller injectors save fuel?

    The rack would need to advance more to "open" the smaller injectors farther. It still results in the same amount of fuel, and the amount of fuel is what counts.
    Like Chris said, doing this would limit your ability to use full rpm if you needed to, it would act in the same way as installing a mechanical limiter on your "throttle" handles.
    "The older I get, the faster I was......."

    1979 60C "Ohana" hull# 331

  5. Re: Would changing to smaller injectors save fuel?

    You would run out of available fuel adjustment before you get to 2300 rpm, with all things being the same, e.g. props, hull weight, etc. since the engines can no longer make full rated power (I think these make 330 HP with that injector) with the smaller injector size, he may only get to 1800 or 1900 rpm under load. Either way the rack runs out of room.
    Chris
    1973 48' Yachtfish
    "Boss Lady" my other expensive girlfriend.
    Follow the refurb at www.starcarpentry.com

  6. #6

    Re: Would changing to smaller injectors save fuel?

    Let me try again. I am just trying to verify my understanding of how our governors work. If you change to a smaller size injector then the injector will need to "open" more to deliver the fuel required to achieve any given RPM. Since the throttle control at the steering station is just an "RPM request" to the governor, you would put in the same position as you always did for a given RPM. The governor would then open the injectors to the point required to achieve the requested RPM. Thus, the only difference seen at the steering station would be at some point pushing the throttle further forward would not do anything since the governor would already have the injectors fully open.

    Pete

  7. #7

    Re: Would changing to smaller injectors save fuel?

    If you were fine with lower speeds, though, and detuned your props to make full rated rpm you would definitely save fuel.
    -- Paul

  8. #8

    Re: Would changing to smaller injectors save fuel?

    All previous posts are acurate in a real world terms you proubably would not notice any differance in fuel consumption. There may in fact be a very slight improvement caused by the smaller orfice in the 55 injectors they do atomize a little better. If you were to go from 90 injectors to 55 you would see a cleaner runninnig engine but even then the fuel savings would proubably be so slight it would only be noticable in a labratory.

    If your looking for maximum fuel economy and you don't care about reducing the maxiumum HP the engine will produce. Then you could de rate the engine by re setting the governer. If your currently set for 2300 max RPM you could re sett to 1800 RPM. Then spec out a prop for the new lower RPM and lower HP the de rated engine will call for a prop of larger diameter or greater pitch. You will gain more by increasing diameter but you have to have the hull clearance to do that if not adding pitch will also work just not quite as well. The result will be a lower RPM for the same cruise speed and an engine closer to rated load at that speed. With this you will see a noticable savings in fuel consumption.

    Brian

  9. #9

    Re: Would changing to smaller injectors save fuel?

    Thanks guys. What I am trying to accomplish is the best fuel economy possible running my 50' Yachtfish at about 9 nmph. I don't care about going real fast, and if I have to outrun weather, or whatever, then jumping up to maybe 13 nmph is just as good to me as going 18 nmph (current top end, blowing $100 dollar bills out the exhaust with fuel consumption).

    I am generally OK with trawler speeds and am looking at the next five to fifteen years when it looks like the forces of darkness are going to try to force the price of diesel up to European levels. I know that I have unnatural levels of masochism in me as it is but if diesel goes toward $5 a gallon then I won't be able to mentally tolerate it and my beautiful Hatty will be gone/sold and I am then looking for a single engine trawler. That entire situation would make me miserable so to stave it off as long as possible I want to be able to extract as much economy and mileage as I can from my DD 8v71Ns, hence the question about changing the injectors.

    I do appreciate all your input and I am wondering if the post about de-rating the engine, maybe using smaller injectors and re-setting the governor / rack and changing the props might lead me in the direction of accomplishing my goal - which in the long run might enable me to keep my Hatty.

    Again guys, great responses. I love your ideas. Please keep them coming.

  10. Would changing to smaller injectors save fuel?

    Brian says: "...The result will be a lower RPM for the same cruise speed and an engine closer to rated load at that speed. With this you will see a noticable savings in fuel consumption..."

    If this means there would be some (minor, I think) improvement in fuel efficiency due to the somewhat greater efficiency of a larger diameter prop, that sounds good. But to go 9 knots the engine would still need to develop the same horsepower as before except for the "slight" improvement in prop efficiency.

    I swing 28" x 32" props with a 8V71TI...even if I could get another 2" diameter, and I don't have enough clearance for that, that 1/14 diameter increase gave a 1/14 improvement in efficiency, it would not be much...unless somebody was running lots of annual hours....I guess it might be 5% more efficient???....anybody know more about prop efficiency???

    And another question: If I take off my turbos, cut my injectior size...ADJUST GOVERNOR...ETC...I'd have to reduce pitch OR DIAMETER...does that make me less efficient?? How can that be?? ... I find that hard to believe given that a prior long thread suggested turbos was 10% more efficient...

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