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injector question

Discussion in 'Engine Building' started by Bobby17772, Jun 16, 2010.

  1. Bobby17772

    Bobby17772 New Member

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    I was doing some research on injectors for when I decide to boost my car. I was messing around with the RC injector chart and I was messing around with some numbers, like if I was to get bigger injectors or stay stock or use an FMU (I know I know I've read up on them) and it seems like if I was to go for a tune for 200 crank hp I would need like 388cc injectors...so like 390's or whatever or even 450's.

    But when I put the numbers in for using a 10:1 FMU at 6 psi that would bring the rail pressure to ~105psi and it says I would only need 254cc injectors (or pretty much stock honda injectors)

    can anyone tell me if this is true? if so how?

    thanks for the help
     
  2. Bobby17772

    Bobby17772 New Member

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    bump, anyone
     
  3. phunky.buddha

    phunky.buddha Admin with a big stick Admin VIP

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    Using a rising rate FMU to tune (which you shouldn't if you can avoid it) basically works by cranking up the fuel pressure in your fuel rail. The injectors are rated for a certain flow rate at the car's normal fuel pressure- which for most Hondas is around 40-45psi. At that pressure (43psi on my Civic), your injectors will flow their stock rated 240cc. Double the pressure and you'll get about double the flow since you're pushing twice as hard- but the rating is still based on the stock fuel pressure.

    So talking about an ideal case with no losses, added friction, blah blah blah- if your injectors are rated at 240cc at 40psi, they will flow the same as a 480cc rated injector when your fuel rail is pressurized at 80psi.

    A 10:1 RR FPR will up your fuel pressure 10psi for every 1psi of boost, so at 6psi of intake manifold boost, you'll see an additional 60psi of fuel pressure. 100psi pushing an injector rated 240cc at 40psi will make it flow like a 600cc injector.

    Understand?

    Now this isn't really all that great for the injector- it's better to have one that's rated for the proper flow and tune for it rather than to just crank up the fuel pressure for certain amounts of boost. You can overstress and destroy your injectors if you up the pressure too much. It does work though if you're in a pinch.
     
  4. Briansol

    Briansol Admins Admin VIP

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    With so many (free) tuning solutions available, using an FMU today is just plain stupid and hacked.

    dsm 450's cost about the same as an fmu. if you don't have one, you can borrow a laptop, and get tuning with crome for free. it's not more expensive, so there's no valid excuse to even consider an FMU anyway.
     
  5. Bobby17772

    Bobby17772 New Member

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    yeah I didn't really plan on using an FMU I was just messing around the the chart more or less as a "what if" thing and I was just wondering like why it would be that way.

    on a side note, how long do you think stock injectors would run pushing that much pressure.

    this is more or less just for learning
     
  6. Magana559

    Magana559 The Warden

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    Fuel pump will fail before the injectors. Most pumps only handle a max of 100PSI before being overloaded and fail. if your fuel pump fails you run lean and running lean makes pistons melt and detonation occurs.

    FMU will fuck up you engine in no time. injectors are cheap, $2-300 for a nice set why would you risk a 2k+ engine on a $40 fmu?
     
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