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Thread: Mercruiser Marine intake injectors

  1. #1
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    Default Mercruiser Marine intake injectors

    i have purchased a used Mercruiser MPFI intake for my 383 Vortec and am wondering which injectors i should use with it. I plan on 500 horsepower and according to the injector calculator: Brake Specific Fuel Consumption is roughly 0.50

    So 0.50 x 500hp= 250 lb/hr of fuel used
    8 cylinders 250/8=31.25lb/hr

    Duty cycle should be at the highest of 0.8 so 250/.8=39lbs/hr injecotrs

    Does 39 lbs/hr injectors seem applicable for this engine? its not like i'm going to be running at WOT all the time so i think i would be ok with the factory Mercruiser injectors of 29lb/hr. What do you guys think?

  2. #2
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    I am also running the marine intake with the factory injectors with my 383. I haven't come close to running out of fuel at all yet or even maxing out the duty cycle on the injectors I think it gets to about 60% at WOT, But I will go and double check this to make sure. I have a pretty mild cam though. zz4 350 cam with RHS heads, 10.5:1 CR N.A.
    Robby

  3. #3
    Lifetime Member hquick's Avatar
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    I'm running the marine intake with 42lb injectors on my SC'd 350 and only see about 30% max duty cycle.
    I'm happy having the 42's though...plenty of room for growing.
    Howard

    YOU ONLY NEED TWO TOOLS IN LIFE - WD-40 AND DUCT TAPE. IF IT DOESN'T
    MOVE AND SHOULD, USE THE WD-40. IF IT SHOULDN'T MOVE AND DOES, USE THE
    DUCT TAPE.
    IF YOU CAN'T FIX IT WITH A HAMMER, YOU'VE GOT AN ELECTRICAL PROBLEM.


    98 K1500 'HOLDEN' Suburban.
    Custom Whipple SC, Mercruiser Marine intake, 0411 PCM, 4L80E w/shift kit

  4. #4
    Lifetime Member Aloicious's Avatar
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    Quote Originally Posted by hquick View Post
    I'm running the marine intake with 42lb injectors on my SC'd 350 and only see about 30% max duty cycle.
    I'm happy having the 42's though...plenty of room for growing.
    you're only seeing 30% max duty cycle? I wonder if the injectors being too big might be causing some of your transient fueling issues? what are your minimum pulse widths/duty cycle? just a thought sorry to hijack the tread

    back on topic though, I'm running a MPFI intake on my vortec 350, its not the marine though, I'm also using a adjustable FPR, some specs in the sig.

    I'm currently running some bosch 890 24# injectors at 45 psi on a vacuum referenced return system. I also had the injectors flow tested and bench tested for offsets and what not by yaw power, I typically see around 72% max duty cycle at WOT pulls, and around 3% min duty cycle.

    I don't think I'm anywhere around 500hp, and its not a 383 (yet). your math sounds about right to me, I would think some 39# injectors would do pretty well for you, are you planning the engine to be n/a, or forced induction, etc?
    1996 c1500 gen1e 5.7L - the "LS31", 24x CNP, LS2 coils, modified T56, 12200411PCM running COS3, zz4 cam, custom MPFI, etc. coming soon: Twin Turbos

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    any updates ?

  6. #6
    Lifetime Member hquick's Avatar
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    From me...no.
    I'm still having fuel issues but not sure if its pump related.
    Doesn't hold pressure after I turn the key off and can have perfect AFR in the morning and way lean in the arvo.
    Howard

    YOU ONLY NEED TWO TOOLS IN LIFE - WD-40 AND DUCT TAPE. IF IT DOESN'T
    MOVE AND SHOULD, USE THE WD-40. IF IT SHOULDN'T MOVE AND DOES, USE THE
    DUCT TAPE.
    IF YOU CAN'T FIX IT WITH A HAMMER, YOU'VE GOT AN ELECTRICAL PROBLEM.


    98 K1500 'HOLDEN' Suburban.
    Custom Whipple SC, Mercruiser Marine intake, 0411 PCM, 4L80E w/shift kit

  7. #7
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    Quote Originally Posted by hquick View Post
    From me...no.
    I'm still having fuel issues but not sure if its pump related.
    Doesn't hold pressure after I turn the key off and can have perfect AFR in the morning and way lean in the arvo.
    injecters bleeding off ??

  8. #8
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    Quote Originally Posted by stroker97k1500 View Post
    I am also running the marine intake with the factory injectors with my 383. I haven't come close to running out of fuel at all yet or even maxing out the duty cycle on the injectors I think it gets to about 60% at WOT, But I will go and double check this to make sure. I have a pretty mild cam though. zz4 350 cam with RHS heads, 10.5:1 CR N.A.
    Robby
    I went back and checked again to see what the actual duty cycle of the stock marine 24# injectors were at WOT and I saw up to 73 %....

    Howard are the temperature differences in the morning and evening making the iat sensor read hot air going in and leaning it out perhaps?
    Robby

  9. #9
    Lifetime Member hquick's Avatar
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    I've considered those possibilities. The temp's aren't different enough to cause a 3 point lean out Robby. I was just watching my fuel pressure guage and I saw it change from sitting around 60psi to sitting around 40-45psi.
    I don't think the injectors are an issue....they were brand new when installed.
    I think I'll drop my tank and check the harness at the pump as these vehicles are known for having issues there.
    Howard

    YOU ONLY NEED TWO TOOLS IN LIFE - WD-40 AND DUCT TAPE. IF IT DOESN'T
    MOVE AND SHOULD, USE THE WD-40. IF IT SHOULDN'T MOVE AND DOES, USE THE
    DUCT TAPE.
    IF YOU CAN'T FIX IT WITH A HAMMER, YOU'VE GOT AN ELECTRICAL PROBLEM.


    98 K1500 'HOLDEN' Suburban.
    Custom Whipple SC, Mercruiser Marine intake, 0411 PCM, 4L80E w/shift kit

  10. #10
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    I hope you will figure it out soon. I have faith you can figure it out! Good luck man.
    Take care,
    Robby

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