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Thread: Intakes, Fuel Trims, and AFR

  1. #1
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    Intakes, Fuel Trims, and AFR

    Those who have a wideband in their car will notice that at idle or cruising (closed loop) the AFR is at 14.7:1, i.e. the ECU is telling the injectors to spray 1 part of fuel for the 14.7 parts of air that are flowing through the MAF.

    So why 14.7:1?

    14.7:1 keeps the CAT happy, emissions low, and provides decent fuel economy. The best fuel economy comes at 15.2:1, but that will damage the CAT. Now that the target has been set, how is it achieved?

    The front O2 sensor of your Evo has the important job of sending a signal to the ECU telling it if the AFR is in closed loop. Think of the front O2 sensor as a switch. When the AFR is at 14.7:1 the switch is turned off. When the AFR is either too rich (<14.7:1) or too lean (>14.7:1) the switch is turned on. When the switch is on you can have the following two conditions:

    1. The AFR is too rich (<14.7:1). The ECU receives the signal from the front O2 sensor and then sends a signal to the injectors telling them to spray less fuel at idle or cruise to bring back the AFR to 14.7:1.

    2. The AFR is too lean (>14.7:1). The ECU receives the signal from the front O2 sensor and then sends a signal to the injectors telling them to spray more fuel at idle or cruise to bring back the AFR to 14.7:1.

    Luckily for us we can log these conditions with Evoscan or any other generic logger (PCMScan). These parameters are known as fuel trims. There are short term fuel trims (STFT) and long term fuel trims (LTFT). The STFTs change rapidly and feed into the STFT which on the Evo change every 4 minutes. There are three LTFTs, our main concern is with the LTFT-low (idle) and LTFT-mid (cruise).

    On the Evo the LTFT ranges between +/-12.5%. What does that mean?

    Let us say that your driving your stock Evo and logging the LTFT cruise. At a steady speed the LTFT cruise changes every 4 minutes. After two 4 minute cycles, the LTFT cruise is +5%. What does that mean? It means that the ECU was told by the front O2 sensor to add 5% fuel to bring back the AFR to 14.7:1 at cruise. Basically, the O2 sensor noted that the AFR has leaned over time and told the ECU to compensate to get the AFR back to 14.7:1.

    You stop and fill gas from new gas station and then you drive some more at a steady speed. After 3-4 cycles you look at the LTFT and all of a sudden it is -3%. So the O2 sensor noticed that the new gas is causing a rich condition and told the ECU to tell the injectors to spray 3% less gas. Basically, the cruise AFR was richer than 14.7:1 and the ECU was tasked with bringing it back to 14.7:1.

    The above process goes on and on and on. The ECU never stops adjusting the trims to achieve 14.7:1. This is done to keep the CAT happy, emissions down, and fuel economy decent.

    The problem is that LTFT cruise carries over into WOT operation. Let us say that your LTFT is at +8%. The fuel map in the rom is telling the ECU to adjust AFR to 10:1 @ 4000 rpm. Let us assume that the target AFR is the actual AFR. I know it is not, but play along or assume it is a Subaru where the target AFR is very close to actual AFR. Your logging this AFR with a wideband and the log says that the actual AFR is 9.2:1 and not 10:1. Why is that? Here is the math: 10x0.08=0.8-10=9.2:1. The rich trim has made the actual AFR richer than it is in the map.

    Conversely, let us say that the LTFT is -8% and the target AFR in the map is 10.5:1 @ 3500 rpm. The wideband reads 11.34:1 AFR. Let us do the math: 10.5x0.08=0.84+10.5=11.34:1. So now the negative LTFT at cruise has carried over to WOT and made your actual AFR leaner than the target AFR in the map.

    As long as the LTFT do not swing widely, then the actual AFR does not swing widely from the actual AFR.

    Then, you go an install an open filter intake. Now all bets are off. Your LTFT go nuts. Why? Your MAF is now seeing a totally different airflow through it and is telling the ECU to adjust the injectors accordingly. I have logged Evo Xs with intakes that have drifted the LTFT to +11.9%. YIKES!!! Basically, the O2 sensor was sensing a lean condition and signaling the ECU to have the injectors pump more fuel to combat that condition.

    Let me give you a real life example: The chart below is from an Evo X that has an intake, tbe, uicp. The table on the left is the AFR and on the right is the LTFT. This is one 4th gear WOT log.



    The AFR was set to peak at 12:1 and taper to 11.2:1 by redline when the fuel trims were at 10.2%, i.e. the ECU was adding 10.2% fuel to bring back the AFR to 14.7:1 in closed loop.

    The chart below is on the SAME car after the trims fell to 6.3%. Basically, the ECU reduced the fuel addition by 3.9%, 10.2-6.3=3.9%.



    Look what happened to the AFR as a consequence of the change in trims. It leaned out all across the rpm range. The fuel map was not changed on the car and the car was hitting pretty much the same load cells. Take the 5000 rpm 220 load cell, for example. It was at 11.5:1 when the trim was 10.2%, then the trim fell by 3.9%. Let us do the math: 11.5x0.039=0.45+11.5=11.95:1 AFR. Now look at the “new” AFR in load cell 220 @ 5000 rpm and it is 11.9:1. The 3.9% fall in LTFT leaned out the car from 11.5:1 to 11.9:1.

    Here is another example that shows the impact of fuel trims on the AFR. This is Gear Grinder's Evo X. It has a TBE, famous CARB legal intake, UICP. We flashed the car back to the stock 020 rom id when the trims were at +6.4%. They went to 7% by the time we logged. We ran two back to back WOT 4th gear runs.

    Then we ran droive the car at a steady 60 mph on the freeway and headed back to the same on-ramp that we did the first two logs. By the time we got to the on ramp, the LTFT was maxxed out @ 12.5%. We did 2 back-to-back WOT logs

    Here are the results, confirming the previous results



    The AFR became richer when the trim went up from +7% to +12.5%. At 6500 rpm the AFR richened up from 11.05:1 to 10.6:1, 0.45 of a point richer. The math supports the findings: the trim rose 5.5%. 11.05x0.055=0.6-11.05=10.45. 10.45:1 is very close to the 10.6:1 that we logged when the trims maxxed out at 12.5%.

    Let us take a more hypothetical example. You go to a shop and install a very popular CARB legal intake, then the tuner proceeds to tune the car and the trims were +2%. He sets the AFR at 11:1. Gradually, your trims drift to 12%. You log your AFR and you find that your AFR is a rich 9.68:1. Let us do the math: 11x0.12=1.32-11=9.68. You think to yourself what the hell happened to my AFR? Well, the tuner failed to adjust the injectors/MAF to the new airflow from the intake.

    Whenever an intake is installed on an Evo X, the fuel trims must be logged and if they have drifted beyond +/-5% (Cobb suggests +/-8%), then the injector latency or the MAF scaling MUST be adjusted to bring back the trims into line. Unfortunately, this takes a lot of time. Consider that the trims change every 4 minutes and you must log them until they settle, then change the latency and log them some more until they settle close to +/-5%. It takes at least an hour to do all this.

    Evo X owners are lucky, because most of the intakes that I have read/logged make the trims positive, i.e. over time the AFR that was set by the tuner becomes richer. The Subaru and the Mitsu Ralliart intakes make the trims negative, i.e, over time the AFR that was set by the tuner at say 11:1 becomes leaner. Check out this thread on Evom about an intake on an RA and what the tuner ended up doing because he failed to check the trims and adjust the tune. BTW, I am nj1266 on that forum.

    http://forums.evolutionm.net/09-rall...ms-intake.html

    For the past three Evo Xs that I have tuned I have been doing this. Not all Evos with intakes require it. I have noticed that Evos equipped with an Injen intake do not drift the trim beyond +6%. This is a fantastic intake. Not only does it keep the trims in check, it also lowers the IATs during WOT more than any other intake that I have logged while road tuning. I highly recommend this intake for the Evo X.

    If you do not want any of this mess, then your best bet is to use a drop-in filter or keep the stock paper filter. These do not drift the trims by much and lower IATs during WOT, but you will not hear the noise from the turbo*

  2. #2

    Re: Intakes, Fuel Trims, and AFR

    Haha!! That's some good knowledge. I'm still really knew when it comes to this stuff and at least now I gave an understanding. Good wrote up naji!!
    REESE TUNED!!<br /><br />referring to redline energy drink <br />Anonymous said... <br /><br />THis stuff made me feel like i was jacking off for 2 hours with out a break in between

  3. #3

    Re: Intakes, Fuel Trims, and AFR

    Great reading Naji. When we spoke about getting a "fuel economy tune" you would set the cruise settings of my injectors to target a 15.2:1 ratio? The Evo VIII, IX, and X ecu is a pretty good work of art when you start to understand it more.
    My Aquamist HFS-5 install: http://forums.evolutionm.net/showthread.php?t=361654

  4. #4
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    Re: Intakes, Fuel Trims, and AFR

    Quote Originally Posted by EvoJoe
    Great reading Naji. When we spoke about getting a "fuel economy tune" you would set the cruise settings of my injectors to target a 15.2:1 ratio? The Evo VIII, IX, and X ecu is a pretty good work of art when you start to understand it more.
    Yes, that is it, but it involves tricking the ECU into letting us run 15.2:1 AFR. I will have to modify some tables for that. And I think it is wise to have a wideband in the car to monitor the AFR.

  5. #5

    Re: Intakes, Fuel Trims, and AFR

    Quote Originally Posted by Looney Tuning
    Quote Originally Posted by EvoJoe
    Great reading Naji. When we spoke about getting a "fuel economy tune" you would set the cruise settings of my injectors to target a 15.2:1 ratio? The Evo VIII, IX, and X ecu is a pretty good work of art when you start to understand it more.
    Yes, that is it, but it involves tricking the ECU into letting us run 15.2:1 AFR. I will have to modify some tables for that. And I think it is wise to have a wideband in the car to monitor the AFR.
    Well. I think I'll have to get a wideband soon and see what we can do.
    My Aquamist HFS-5 install: http://forums.evolutionm.net/showthread.php?t=361654

  6. #6

    Re: Intakes, Fuel Trims, and AFR

    let me see if i get this... so in order to scale he maf for a bigger inlet pipe (ie aem) youd want to adjust everything -5% in the high octane fuel map?

    although this is why i never plan on touching my car, cause i hate numbers and percentages...
    i like debugging things.

    im just gonna put my injectors in the car and pour gas on the cars behind me.
    toku ni

  7. #7

    Re: Intakes, Fuel Trims, and AFR

    nice write up Naji.
    KT Motoring

  8. #8
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    Re: Intakes, Fuel Trims, and AFR

    Quote Originally Posted by toku-one
    let me see if i get this... so in order to scale he maf for a bigger inlet pipe (ie aem) youd want to adjust everything -5% in the high octane fuel map?
    No...We do not have the MAF scaling table in ECUflash. I read that some have it and are testing it and there will be a new release with new maps.

    At the moment, one way to adjust the trims is to adjust the injectors latency table. The numbers are in miliseconds and if adjusted properly will keep the trims within +/-5%. That will prevent the trims from drifting and making the AFR either too rich or too lean.

  9. #9
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    Re: Intakes, Fuel Trims, and AFR

    Quote Originally Posted by toku-one

    although this is why i never plan on touching my car, cause i hate numbers and percentages...
    i like debugging things.
    I guess the lesson that you can learn from this write-up is to ask your tuner if he is going to fix the drifting fuel trim on your Evo. If he tells you that the intkae has no impact on the trims and will not change the AFR over time, then go elsewhere. I can tell you for certain that the most popular CARB legal intake on these forums drifts the trims positive like no other I have logged. I logged +11.9% on Gear Grinder's Evo X. I was able to bring down the number to a consistent +/- 5-6%. I have logged the same on another Evo X with that intake.

    You now have mode 23 and can log the trims on your car. If your car has an intake, then go ahead and log LTFT-idle (lo) and LTFT-cruise (mid). You have to let the car idle for multiple cycles of 4 min and see where the trim ends. Then drive at a steady speed on the freeway and log the LTFT-cruise. Again, you have to drive for at least 20-30 min to let the trims build and settle.

    Report back to this thread with your trims. I would love to know what they are

  10. #10

    Re: Intakes, Fuel Trims, and AFR

    i think i already have a bunch of info to provide. ill just have to dig it up over the weekend when i get a chance.

    but i can provide these for now (different days):
    ltft idle 6.05 and cruise 11.5 - this would have been after an hour drive
    ltft idle 2.92 and cruise 12.5 - this would have been after 5 minutes.
    toku ni

  11. #11
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    Re: Intakes, Fuel Trims, and AFR

    Quote Originally Posted by toku-one
    i think i already have a bunch of info to provide. ill just have to dig it up over the weekend when i get a chance.

    but i can provide these for now (different days):
    ltft idle 6.05 and cruise 11.5 - this would have been after an hour drive
    ltft idle 2.92 and cruise 12.5 - this would have been after 5 minutes.
    The cruise trim is just as I expected it would be with that famous CARB legal intake. The +12.5% cruise is maxxed out. If it continues like that it might trigger a system too lean CEL. Basically, the ECU cannot add anymore fuel to closed loop operation to keep the AFR at 14.7:1.

    But it is the one hour drive that I would look at. There the trim is +11.5%. This is almost the same number I got after a long log of GG's car. I got +11.9%.

    When you go to get tuned INSIST that the tuner lowers the trim to a STABLE +/-5-6%. If the tuner says that he did, then double check afterwards by logging the trims.

  12. #12

    Re: Intakes, Fuel Trims, and AFR

    i got the p0171 for the first time in months the other night cruising in the freeway in 5th (like every other time i had ever gotten it) so my car doesnt do it very often. id say in total since installed it in dec that ive gotten maybe 10 p0171's.

    i was supposed to get a map for the aem and i was gonna throw in my 800cc's and scale them to 696 so i can just cruise with them for the next few weeks... but as it looks now, ill probably just end up scaling the injectors and leaving the rest as is.
    toku ni

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    Re: Intakes, Fuel Trims, and AFR

    Quote Originally Posted by toku-one
    i got the p0171 for the first time in months the other night cruising in the freeway in 5th (like every other time i had ever gotten it) so my car doesnt do it very often. id say in total since installed it in dec that ive gotten maybe 10 p0171's.
    Yep, that is the system too lean CEL. Your ECU is detecting a lean AFR at cruise and cannot add any more fuel to bring the AFR to 14.7:1, so you get the CEL. That is an easy fix. Have your tuner fix it, when you get a tune.

  14. #14
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    Re: Intakes, Fuel Trims, and AFR

    Quote Originally Posted by toku-one
    i was supposed to get a map for the aem and i was gonna throw in my 800cc's and scale them to 696 so i can just cruise with them for the next few weeks... but as it looks now, ill probably just end up scaling the injectors and leaving the rest as is.
    Changing the scaling on your stock injectors will change the AFR during WOT operation. Adjusting the latency does not. the fix is to adjust the latency and NOT the scaling. If you want to do the scaling, then you MUST have an AFR meter to monitor the AFR.

  15. #15

    Re: Intakes, Fuel Trims, and AFR

    im just scaling when i throw in the bigger 800cc injectors

    thats acceptable right?
    toku ni

  16. #16
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    Re: Intakes, Fuel Trims, and AFR

    Quote Originally Posted by toku-one
    im just scaling when i throw in the bigger 800cc injectors

    thats acceptable right?
    No...The proper way to do big injectors is this:

    1. Set the Injector scaling in the table ~15% lower than the size of the injector. So if you are installing 800 cc injectors, then the scaling should be around 680-690.

    2. Find the injector latency for the DW800 and place them in the injector latency table. I think HB Speed posted the numbers on Evom. Here they are:

    5.904
    5.400
    2.800
    1.944
    1.464
    1.080
    0.816

    3. Now comes the hard part. Log the idle trims and see how far they drift from +/-5%. Adjust the trims to bring them back into line.

    4. Log the cruise trims at a steady speed and se how far they drift from +/-5%. Adjust the trims to bring them back into line.

    5. With a wideband check your AFR and adjust it to 11:1 during WOT operation. Or if you want the car to be slow, adjust it to 10.5:1. Anything below 11:1 makes the Evo X sluggish.


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    Re: Intakes, Fuel Trims, and AFR

    Someone on Evom posted a very interesting theory in my similar thread. The reason why some intakes drift the trims positive and other do not has to do with the placement of the MAF sensor on the intake tube. The closer the MAF is to the filter element, the more the LTFT will drift positive. The further away is the MAF is from the filter element, the less the trims will drift positive. He posted pictures of some intakes and I added others to the list. I placed a green square around the MAF. Here we go:

    AEM intake. Confirmed positive drift of +12.5%. Todd confirmed a p0171 (system too lean) with this intake. Note the MAF placement.


    HKS intake. Confirmed drift of up to +10.2%. Could have been higher than this, because I was logging the trim while tuning for 3-4 hours. MAF placed very close to the filter.


    Injen intake. Logged a max of +6% drift on this intake. This is really good. The MAF is placed the furthest away from the filter in all the intakes pictured. Furthermore, this intake lowered the IATs the most during WOT operation in my testing. IMO, it is the BEST intake for the Evo X.


    AGP Intake. This is a true CAI. The filter is located down by the front left fog light. Logged a max of +9.75% LTFT cruise drift with this intkae. Some have reported a p0171, system too lean, code with this intake.


    Agency Power. No data on this intake. The MAF is too close to the filter for comfort. If anyone has this installed, I would love to log the trims on your car for free. If they are drifting, I will fix them for free. I will do anything for data*


    AMS. The MAF is too close to the filter for comfort. I logged this intake and the trims drift way positive. I do not remember if it triggers a p0171, system too lean code.


    ETS. The MAF is too close to the filter for comfort. I logged the trims on this intake and they drift way positive. I do not remember if it triggers a p0171, but the trims are too high for comfort.


    UR intake.. The filter on this intake is too close to the MAF. The fuel trims were very high at 12.5% for both idle and cruise. The filter triggers a p0171.

  18. #18

    Re: Intakes, Fuel Trims, and AFR

    i wish i had a means of fabrication so that i could try something out.
    id like to move the maf back on the aem (similar to the injen) to see if this corrects the problem.
    seems like if you could put it on the rubber element right before the bend (near the stock dv location) it would be far enough away.

    the one thing that i have to say about it is that its all relative to the filter. the maf is close to its original stock location on all those the intakes in question... so its really a combination of the maf being too close to a cone filter... maybe it has something to do with difference in the air "turbulence"... as im not engineer i really have no idea. but it seems like that would be something interesting to test out, if one had the means.
    toku ni

  19. #19

    Re: Intakes, Fuel Trims, and AFR

    Quote Originally Posted by Looney Tuning
    Quote Originally Posted by toku-one
    im just scaling when i throw in the bigger 800cc injectors

    thats acceptable right?

    No...The proper way to do big injectors is this:

    1. Set the Injector scaling in the table ~15% lower than the size of the injector. So if you are installing 800 cc injectors, then the scaling should be around 680-690.

    2. Find the injector latency for the DW800 and place them in the injector latency table. I think HB Speed posted the numbers on Evom. Here they are:

    5.904
    5.400
    2.800
    1.944
    1.464
    1.080
    0.816

    3. Now comes the hard part. Log the idle trims and see how far they drift from +/-5%. Adjust the trims to bring them back into line.

    4. Log the cruise trims at a steady speed and se how far they drift from +/-5%. Adjust the trims to bring them back into line.

    5. With a wideband check your AFR and adjust it to 11:1 during WOT operation. Or if you want the car to be slow, adjust it to 10.5:1. Anything below 11:1 makes the Evo X sluggish.

    the question i have is if i did 1 and 2 without 3-5 (seeing as how i have no tune at all at the moment, and am not looking for gains, just for the car to stay close to how it is now so that im not just pouring gas into the motor) would i still see the 10.6-9.4 afr between 2700 and 7000.
    toku ni

  20. #20
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    Re: Intakes, Fuel Trims, and AFR

    Quote Originally Posted by toku-one
    the question i have is if i did 1 and 2 without 3-5 (seeing as how i have no tune at all at the moment, and am not looking for gains, just for the car to stay close to how it is now so that im not just pouring gas into the motor) would i still see the 10.6-9.4 afr between 2700 and 7000.
    I would not do 1 and 2 only. That is not wise. There is no guarantee that your trims will be in check.

    Injector scaling involves a lot of logging and tweeking. The numbers that are provided here are just the starting point. You have to tweak them to fit your car.

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