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Thread: Aftermarket intake/exhaust and fuel trims

  1. #1
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    Aftermarket intake/exhaust and fuel trims

    So I'm not posting about a problem per se but rather out of general curiosity.

    I installed a short ram intake (from eBay) and an HKS hi-power muffler for my P5. I like the sound it makes, not sure if there's a noticeable power increase but they say maybe a 5-10HP increase total. so not much. I bought it to make it sound and look better anyway.

    I noticed on my scan tool my Fuel trims are elevated now. STFT is mostly in the single digits but will spike to double digits and the LTFT is consistently in low teens after resetting ECU from the scan tool. I saw a reading of 18% briefly too. This was while driving at various speeds/RPMs/Loads. There is no CEL however. prior to changing the intake the fuel trims were less than +/- 7% all around. So is that normal for an aftermarket intake? Will driving around for a few days will the ECU "learn" the new intake? or maybe I have a vacuum leak somewhere although I doubt it because i made sure the intake was on tight.

    I'm kind of new to fuel trims but it makes sense that a better intake system= more air being drawn into the engine = more fuel has to be added to compensate which adds the 5 or so horsepower, but the ECU is factory tuned to the stock intake so the O2 sensors have to pick up on the changes.

  2. #2
    31N007 KingOfSpades007's Avatar

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    LTFT is what the ECU "learns" over time as it realizes it has to continue adding or taking away fuel. As long as STFTs are low/ in the single digits when the car is at temp/ relatively steady state, I wouldn't be too worried. LTFT can be adjusted by the ECU to a certain point, so there's some room for changes (as you've seen, increased flow). I would be inclined to agree with you that, over the next few days/ weeks, the ECU should learn to work around the increased flow and trims SHOULD average out. If you're not fussed about radio presets, you could unhook the battery for 5-10 minutes and let the ECU reset itself/ relearn faster.

    It's interesting to see how different cars work with fuel trims. My Subaru has STFTs all over the place (within the mid-teens pretty consistently) and when I looked at first I thought I'd had a vacuum leak.
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  3. #3
    Comp Engineering Student Riot_Polizei's Avatar

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    Quote Originally Posted by wrathofpaul View Post
    So I'm not posting about a problem per se but rather out of general curiosity.

    I installed a short ram intake (from eBay) and an HKS hi-power muffler for my P5. I like the sound it makes, not sure if there's a noticeable power increase but they say maybe a 5-10HP increase total. so not much. I bought it to make it sound and look better anyway.

    I noticed on my scan tool my Fuel trims are elevated now. STFT is mostly in the single digits but will spike to double digits and the LTFT is consistently in low teens after resetting ECU from the scan tool. I saw a reading of 18% briefly too. This was while driving at various speeds/RPMs/Loads. There is no CEL however. prior to changing the intake the fuel trims were less than +/- 7% all around. So is that normal for an aftermarket intake? Will driving around for a few days will the ECU "learn" the new intake? or maybe I have a vacuum leak somewhere although I doubt it because i made sure the intake was on tight.

    I'm kind of new to fuel trims but it makes sense that a better intake system= more air being drawn into the engine = more fuel has to be added to compensate which adds the 5 or so horsepower, but the ECU is factory tuned to the stock intake so the O2 sensors have to pick up on the changes.
    Fancy that, I just went through an ordeal like this :^)

    When I fixed my fuel trim issue, which turned out to be a vac leak, my STFT and LTFT remained beneath the 8-10% threshold. Thus, a CAI/SRAI shouldn't affect your trims if installed properly. Granted, I never read these when I had the stock airbox.

    What do your STFT and LTFT readings look like at idle?

    EDIT: Also, I believe the CEL gets thrown only when you consistently exceed 18% LTFT several times in a row
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  4. #4
    31N007 KingOfSpades007's Avatar

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    I've got stock airbox. I can update later this evening as I have an Aeroforce gauge. Pretty sure STFT at temp hovers +/-6-7%. LTFT sits at +/-6-7% too. You hit the nail on the head though as far as what trims should hover around.

    -6.2 LTFT highest in either direction.

    +/-7 STFT highest in either direction.

    Stock motor/intake. Removed VTCS at the head and LIGHTLY portmatched.
    Last edited by KingOfSpades007; 04-16-2019 at 08:51 PM.
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  5. #5
    Registered Member NaturalFlavored's Avatar

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    I can tell you that having a 4-1 header a CAI and lightly (can't stress enough lightly) ported intake, I took out the vtcs and plugged the hole I see I slight increase in power in High third and fourth gear. And a steady increase in fifth. I hit top end a little quicker 125mph.
    Depending on how I drive it, "like" racing from first to fifth I have no problem hitting 120+ but driving at 75 then downshifting and speed up I have a harder time hitting 120.

  6. #6
    Registered Member NaturalFlavored's Avatar

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    I highly recommend getting A 4-2-1 header, I want one and they are extremely important for a NA build if you don't turbo (even light turbo). I'm still working on getting one.
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  7. #7
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    the readings were pretty consistent throughout the RPM range. I checked it the next day and it was slightly lower. so I guess the ECU is indeed re-learning the fuel trims.

  8. #8
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    Quote Originally Posted by NaturalFlavored View Post
    I can tell you that having a 4-1 header a CAI and lightly (can't stress enough lightly) ported intake, I took out the vtcs and plugged the hole I see I slight increase in power in High third and fourth gear. And a steady increase in fifth. I hit top end a little quicker 125mph.
    Depending on how I drive it, "like" racing from first to fifth I have no problem hitting 120+ but driving at 75 then downshifting and speed up I have a harder time hitting 120.
    Is that easy to install? I've seen some header setups with no pre-cat installed (unless it was relocated) and I doubt I would pass NY emissions in that case

  9. #9
    Registered Member NaturalFlavored's Avatar

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    It's an easy swap, I'd recommend getting a non fouler online since it will look cleaner. The hardest part is making sure you get a header that has a egr valve connection that hooks up and then connecting them. Other than that it's an easy upgrade.
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