Injector Size

Discussion in 'Forced Induction & Tuning' started by CanaryGT, Jul 27, 2007.

  1. CanaryGT

    CanaryGT Well-Known Member

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    I wanted to start a thread to just simply ask questions about Injector size.

    I have a completly stock engine and I am going to be running say at most 12 Lbs of boost.

    What size injectors do I need and Why?

    My buddy has a built 347 stroker and he only uses 30# in his. He is all N/A....

    Please someone shed some light on this....
     
  2. S281 #39B

    S281 #39B Member

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    Hope this helps,

    FORMULA:
    (horsepower X .5 (for N/A) or .6 (for power adder) ) Divided by (No. of cylinders X by 85% duty cycle)

    The proper injector for this application would be
    (350(hp) X .6 = 210) divided by ((8 X .85) = 6.8) = 31 lb injectors = 30 lb injectors with no room for improvement
     
  3. 19mustang95

    19mustang95 Legend

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    30lb injectors wont handle a blower at 12lbs of boost, if that is what you were trying to say.(not sure)
     
  4. quickstang_1994

    quickstang_1994 Well-Known Member

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    Its the formula for 350hp for a blower this is an example.
     
  5. quickstang_1994

    quickstang_1994 Well-Known Member

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    the answer was 31 and in the old day when 36's were around they would have been used. and yes they will Saleen uses 30's on their SC cars just saying, but I wouldn't recommend it. I would still go with 42's.
     
  6. quickstang_1994

    quickstang_1994 Well-Known Member

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    This is the correct formula to use.
     
  7. CanaryGT

    CanaryGT Well-Known Member

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    I looked into this and I found a site with a calculator built in...

    http://www.injector.com/injectorselection.php

    It makes more sense except why do you put in a higher multiplier for a forced induction car as opposed to a NA??
     
  8. Greggerex

    Greggerex Active Member

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    Not just the old days! ;) I just got a set of Ford Bosch 36# blue-tops from Racetronix - there are still a few kicking around brand new (they were used in the 94-95 Thunderbird Supercoupes). They are going in the Stang tonight cuz my 30#-ers aren't doing their job.

    And you can also easily get 39# injectors.

    However, I agree with whoever said 42#-ers - they are a good place to start for a stock engine getting some boost. And according to the given formula, on a V8 they'll run out at around 475 hp (boosted).
     
  9. quickstang_1994

    quickstang_1994 Well-Known Member

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    some injectors are different like the voltage reading are different(can't think if the word). plus ford blues are 24.
     
  10. rgs

    rgs Well-Known Member

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    they can get away with 30's because stock all their cars run low low boost
     
  11. quickstang_1994

    quickstang_1994 Well-Known Member

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    9-12 depending on which option you chose.
     
  12. Greggerex

    Greggerex Active Member

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    Ford also had blue top 36# injectors - I think they were a darker blue (leave it to Ford). :D

    As for voltages, I believe all injectors accept a PWM (pulse width modulation) signal for operation - their flow characteristics may be slightly different, but electrically they should be the same.
     
  13. Paul

    Paul Legend

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    It has to do with something called Brake Specific Fuel Consumption. This is basically a ratio of fuel consumption to engine output. Basically, a naturally aspirated motor can make the same power with less fuel than a forced induction (turbo, blown) motor of the same horsepower output. In the formula above, you'll notice that Saleen put up a .5 BSFC for N/A, and .6 for blown. These are decent round number figures that can be used when calculating fuel injector requirements for an engine.

    It is always better to "over-inject" a motor (within reason) since you can just limit the pulsewidth of the injector via the tune in the ECU. Very large injectors (e.g. 160 lb/hr) can sometimes be a bit difficult to tune for idle/drivability due to the very large amount of fuel dispersed in a very short time.

    However, although 42s may be a little larger than necessary (not by much) for you application, it will easily be compensated for by the tune, and will allow you some room to grow if necessary.

    Oh, and you never want to run an injector past 85% of its maximum duty cycle. That is, past the maximum amount of time it can stay open when spraying fuel into the intake manifold.

    Hope that helps.

    Paul.
     
  14. quickstang_1994

    quickstang_1994 Well-Known Member

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    but its the signal that counts and the v8 ford 36's were bright red.
     
  15. Greggerex

    Greggerex Active Member

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    Wow, so Ford in all their wisdom made light-blue-tops (24#), dark-blue-tops (36#), red-tops (30#) and bright-red-tops (36#) - guess they ran short of colors! :D

    I've never heard of the red-top 36#-ers... interesting.

    Either way, like I said, the signaling should be the same. O0
     
  16. quickstang_1994

    quickstang_1994 Well-Known Member

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    they were used before 30 came out. Saleen used them in the S351 cars.
     
  17. Paul

    Paul Legend

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    The Ford 36s on my silver car are dark blue, not red.
     
  18. quickstang_1994

    quickstang_1994 Well-Known Member

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    Hell I just looked threw some old catalogs and seen the changes dark blue, bright red, and even white. so wtf
     
  19. Paul

    Paul Legend

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    Probably three different manufacturers that made them for Ford.
     
  20. CanaryGT

    CanaryGT Well-Known Member

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    It does, thanks!