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Thread: Bigger injectors needed after new performance cams?

  1. #1
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    Bigger injectors needed after new performance cams?

    I have swapped in a 240/240 combo camshaft on my frankenstein M50B22 engine, and now waiting for it to get tuned. I've been searching around the net about this topic but have yet to find a definitive answer (or i suck at googling). Are bigger injectors needed for this setup to work? I know some folks swap in M3 cams into their 325i with no issue, but I'm not sure about my special little frankenstein engine. Thanks and yes still a newbie to this forum (sorry).

  2. #2
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    Bigger injectors needed after new performance cams?

    The 3.00” diameter MAF was good for S50 in stock tune, should be good for your 2.2.
    What is a 2.2L? Bore and Stroke?
    What have you combined to make it?


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    Last edited by bluptgm3; 04-27-2019 at 03:58 PM.

  3. #3
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    Bigger injectors are ONLY needed when you've reached 90-95% duty cycle (valid injector open time, often around 14ms duration max), to be able to fuel enough for the air intake.

    Your cams will not give you that much more capacity to overrun the injectors. Don't waste your money. Just because they're bigger doesn't translate to more power.

    Same goes with MAF. If you're not reaching its max, there's no need to "upgrade".
    Of course, you can do anything you want, but make sure MAF and injector size is reflected in the tune, or else it will actually lose power. You can't just slap things together in hopes that it will work better simply by chance.
    -Abel

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  4. #4
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    Thanks for your input. Basically I'm running Megasquirt on this setup on a speed density tune. The tuner said that the injectors are too small, exceeding the optimal 80% DC after free-revving it (almost 90% DC). From my knowledge, the green-tops (which i am using) are perfectly fine on even more powerful M/S5x engines. Am I missing something here? The car is yet to be dyno'ed because of this.

    M52B25 rotating assembly except the pistons (still stock 80mm). Base engine was M50B20 btw. Blew the bearing on the last one, and couldnt resurface it any more.

  5. #5
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    90% isn't ideal, but is typically OK if you're able to hit your air/fuel ratio targets at that DC. But the injectors are close to topped out - and if that's 90% DC and not at full load, you'll definitely want an upgrade. If it's a burst at 90% from your acceleration enrichment, and goes back to something more reasonable, it's less worrisome.
    Matt Cramer
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  6. #6
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    Quote Originally Posted by Matt Cramer View Post
    90% isn't ideal, but is typically OK if you're able to hit your air/fuel ratio targets at that DC. But the injectors are close to topped out - and if that's 90% DC and not at full load, you'll definitely want an upgrade. If it's a burst at 90% from your acceleration enrichment, and goes back to something more reasonable, it's less worrisome.
    I'm not savvy in terms of tuning terminoligies, but can the fuel mapping affect this? In other words, could be possible that the map the tuner had for the initial startup too rich?

    Correct me if I'm wrong, but the car would have less rotational momentum while free revving compared to when on the dyno (in theory). So duty cycle will be lower on the dyno, right?

    Also thanks for everyone's response

    - - - Updated - - -

    Quote Originally Posted by 328 Power 04 View Post
    Bigger injectors are ONLY needed when you've reached 90-95% duty cycle (valid injector open time, often around 14ms duration max), to be able to fuel enough for the air intake.

    Your cams will not give you that much more capacity to overrun the injectors. Don't waste your money. Just because they're bigger doesn't translate to more power.

    Same goes with MAF. If you're not reaching its max, there's no need to "upgrade".
    Of course, you can do anything you want, but make sure MAF and injector size is reflected in the tune, or else it will actually lose power. You can't just slap things together in hopes that it will work better simply by chance.
    Thanks for your input. Basically I'm running Megasquirt on this setup on a speed density tune. The tuner said that the injectors are too small, exceeding the optimal 80% DC after free-revving it (almost 90% DC). From my knowledge, the green-tops (which i am using) are perfectly fine on even more powerful M/S5x engines. Am I missing something here? The car is yet to be dyno'ed because of this.

  7. #7
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    If it's hitting 90% DC at no load, the AFRs are not super rich, and this isn't just a short burst of extra fuel from the acceleration enrichment that's making it bounce off 90% duty cycle (a competent tuner should definitely know the difference), then yes - I'd say you are out of injector here.
    Matt Cramer
    1997 BMW 328i convertible, 1972 Chevy C10 pickup, 1966 Dodge Dart slant six
    BMW - where "Why doesn't everybody build cars the way they do?" meets "Why can't they build a car the same way everyone else does it?"

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