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The old backfiring thing

Discussion in 'FAST Support Forum' started by crash520, Jan 11, 2018.

  1. crash520

    crash520 Member

    Joined:
    Dec 16, 2016
    so I took the car for a long drive today and of course it’s backfiring a little from the exhaust coming off the interstate so cruising at 60 and snap the gas off I get a little popping, what was interesting was I have a stand alone Inovate AFR gauge and it pretty much matches my XFI dash readings, well the moment I let off the gas and while it’s backfiring the gauge is reading max lean, like 24, it never did this with the carburettor, and once it stops backfiring it settles back into the 14-15 range. The log below shows TPS, timing and RPM, my base timing is set at 20 and my idle parameter is at 5, is that why it only try’s to take out 5 at idle to retard timing as the rpm is above idle, not it flat lines at 15 deg, can I reduce this to say 2 deg and then go to my base timing tables and raise them some 5127F6BB-B4C7-44D9-8D39-BE64435450DE.jpeg
     
  2. Fastmanefi

    Fastmanefi Active Member

    Joined:
    Jun 4, 2011
    Location:
    Sonora, CA
    Kill the fuel when decelerating. This is a high vacuum condition that only happens during decel. You should log that so you know what area in the fuel map to decrease. It's common to run very lean during decel since you don't need any fuel at that time. You can also ADD timing in that area only.
     
  3. crash520

    crash520 Member

    Joined:
    Dec 16, 2016
    Rich, so basically the bottom line on the VE table (10% load) fro say 1000 to 5000 rpm from the numbers to 10-20?
     
  4. Fastmanefi

    Fastmanefi Active Member

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    Jun 4, 2011
    Location:
    Sonora, CA
    I wouldn't guess. Do a datalog to see where in the RPM/kPa range the poping occurs, then change only that area.
     
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  5. crash520

    crash520 Member

    Joined:
    Dec 16, 2016
    Is a setting of 10 too low for that area
     
  6. Fastmanefi

    Fastmanefi Active Member

    Joined:
    Jun 4, 2011
    Location:
    Sonora, CA
    I have used ZERO on Cobras that make a lot of noise when down shifting. You don't need fuel if the car is pushing the motor.
     
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  7. crash520

    crash520 Member

    Joined:
    Dec 16, 2016
    Rich, one of the reasons I suggested from 1000 to 5000 was coming off a 1/4 mile run, sure cruising the interstate isn’t at 5000 rpm, at that point I’m at 3100 rpm and
     
  8. crash520

    crash520 Member

    Joined:
    Dec 16, 2016
    Ok, so I made some changes, took it for a drive, significantly better, made some more changes on the side of the road by taking fuel out and pretty much have killed all and any form of backfire, car still runs great,,,,, did a couple of WOT hits to get some logs and while at 101% load, timing dropped to 30*, my base spark table is set to that so it didnt do anything wrong but in my mind I just find it odd to take timing out at WOT, I need to research that a little more. Car didnt show any signs of issue at WOT, took off like a scalded cat, TPS only ended up at around 90% by the log, I just wasnt pressing hard enough on the gas, but man it got going, much better than the factory carb.

    THIS IS FUN, Im enjoying the learning.
    Thanks guys
     
  9. Fastmanefi

    Fastmanefi Active Member

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    Jun 4, 2011
    Location:
    Sonora, CA
    Many engines pick up a little torque at higher RPMs with a few degrees of reduced timing. That's not unusual. What is unusual is to find this while driving the car. Easy on a dyno but hard to do just by reviewing logs. Nice work!
     
  10. crash520

    crash520 Member

    Joined:
    Dec 16, 2016
    upload_2018-1-15_6-29-32.png
    Spark
    Fuel Pres
    VE
    TPS
    shown at decel point, also showing a WOT
     

    Attached Files:

  11. cobrajet

    cobrajet New Member

    Joined:
    May 28, 2012
    Location:
    Colorado
    "I have used ZERO on Cobras that make a lot of noise when down shifting. You don't need fuel if the car is pushing the motor."

    Rich, speaking of Cobras with the side pipes, I also experience the same issue with the crossover exhaust in a GT-40. My table is also setup with 0% load, and I can cut off all the fuel in the 2000-3500 rpm range, however, the result can be a sudden lurch when fuel is added back in (>.8 TPS). This could be due to the personality of the stack injection, but it's hard to get a smooth transition when adding fuel back in with the slightest of throttle opening. Actually, the cackle/popping adds a little "snarl" to the car and most observers tell me they like it, so I haven't tried all that hard to get it perfect, however, I'm curious to know if I've missed something in that area of tuning. BTW, I've changed my timing from 30-55 deg adv and it doesn't seem to matter much. Any ideas?
     
  12. Fastmanefi

    Fastmanefi Active Member

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    The transition can be tricky, but I have been successful by changing the axis resolution in that area to give me more cells to work with and then some creative smoothing. The timing can help if you leave the fuel there, but once the fuel has been removed timing doesn't matter.
     
  13. cobrajet

    cobrajet New Member

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    May 28, 2012
    Location:
    Colorado
    True on the timing with no fuel.... FWIW, if you turn on AL in the 0-15-35% load range (2000-3500rpm), and let it do its thing, it will start populating with VE numbers as high as 65 in those low/no load cells. Thanks for your input!
     
  14. Fastmanefi

    Fastmanefi Active Member

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    Jun 4, 2011
    Location:
    Sonora, CA
    Yeah those large VEs are showing up all the time under no load conditions. I think it's a bug. I turn the +Limit to zero in those areas.
     
  15. cobrajet

    cobrajet New Member

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    May 28, 2012
    Location:
    Colorado
    Good workaround....thanks!
     
    Last edited: Jan 16, 2018 at 12:53 PM
  16. Fastmanefi

    Fastmanefi Active Member

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    Jun 4, 2011
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    Sonora, CA
    Gotta do what you gotta do...
     
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