Why A/F 13.5:1 not 14.7:1?

Discussion in 'SRT Hellcat General Discussions' started by ciphros, May 21, 2016.

  1. ciphros

    ciphros Gold Member

    Joined:
    Mar 13, 2016
    Messages:
    623
    Likes Received:
    305
    Trophy Points:
    433
    Vehicle:
    Charger SRT Hellcat
    Just curious why the car runs rich even at steady state cruise? Assuming this is a designed safety for the motor, but curious if anyone has any info.

    Thanks!
     
    • Like Like x 1
  2. Tightlines

    Tightlines Gold Member

    Joined:
    Apr 8, 2016
    Messages:
    276
    Likes Received:
    134
    Trophy Points:
    213
    Vehicle:
    Challenger SRT Hellcat
    I noticed that as well. Here for the replies.
     
  3. usmc341

    usmc341 Gold Member

    Joined:
    May 27, 2015
    Messages:
    1,448
    Likes Received:
    407
    Trophy Points:
    218
    Location:
    Henderson, NV
    Vehicle:
    Challenger SRT Hellcat
    You said steady cruise. I think 14.7 is idle a/f. Any throttle input will lower it.
     
  4. ciphros

    ciphros Gold Member

    Joined:
    Mar 13, 2016
    Messages:
    623
    Likes Received:
    305
    Trophy Points:
    433
    Vehicle:
    Charger SRT Hellcat
    Steady cruise should be stoic.
     
  5. usmc341

    usmc341 Gold Member

    Joined:
    May 27, 2015
    Messages:
    1,448
    Likes Received:
    407
    Trophy Points:
    218
    Location:
    Henderson, NV
    Vehicle:
    Challenger SRT Hellcat
    Jus noticed mine is 13.5 at idle. Definitely different from other cars I've owned
     
  6. Jontall

    Jontall Silver Member

    Joined:
    Aug 19, 2015
    Messages:
    246
    Likes Received:
    42
    Trophy Points:
    138
    Vehicle:
    Challenger SRT Hellcat
    Because it's supercharged, it's tuned richer.
     
  7. ciphros

    ciphros Gold Member

    Joined:
    Mar 13, 2016
    Messages:
    623
    Likes Received:
    305
    Trophy Points:
    433
    Vehicle:
    Charger SRT Hellcat
    Still seems odd. I have had turbo cars that still run stoic at cruise, is this a "normal" thing for supercharged only?
     
  8. ciphros

    ciphros Gold Member

    Joined:
    Mar 13, 2016
    Messages:
    623
    Likes Received:
    305
    Trophy Points:
    433
    Vehicle:
    Charger SRT Hellcat
    Ok, thinking about this more, the only way I can make it make sense is due to the instantaneous response of the supercharger compared to a turbo increasing pressure over some small amount of time. Turbo motor would have time to adjust timing and a/f where supercharged may not. Anyone know if I am on the right track here?
     
  9. TNCHARGER

    TNCHARGER SRT Hellcat Supporter Gold Supporting Member

    Joined:
    Nov 19, 2015
    Messages:
    1,002
    Likes Received:
    360
    Trophy Points:
    258
    Vehicle:
    Challenger SRT Hellcat
    I have had several Supercharged engines (after market) and all of them idled and cruised at 14.7. Not sure why the HC is different but may be an emissions issue but that is just speculation..
     
    • Like Like x 1
  10. ciphros

    ciphros Gold Member

    Joined:
    Mar 13, 2016
    Messages:
    623
    Likes Received:
    305
    Trophy Points:
    433
    Vehicle:
    Charger SRT Hellcat
    Would think emissions would be worse non-stoich. I did just read something about some supercharged motors needing to run a bit richer to idle well. Who knows. Just seems like we are burning excess all the time, mpg could get a tiny bump with a leaner mix.
     
  11. TallCool1

    TallCool1 Gold Member Hellcat Car Club Gold Supporting Member Nat'l Hellcat Tech Advisor HCC Charter Member

    Joined:
    Jan 5, 2015
    Messages:
    2,392
    Likes Received:
    791
    Trophy Points:
    458
    Location:
    Pacific NW
    Vehicle:
    Challenger SRT Hellcat
    Forced induction engines increase cylinder pressure which can introduce knock. Stoic is optimum burn but isn't optimum performance and engines never completely burn 100% of fuel. 13.5...13.2 is the general range for safe peak torque at low rpm light load for normal gas . Then getting richer as you get closer to wot where af is changed to produce max torque over power (and vica versa, or both) depending on rpm, load and other factors.
     
    Last edited: May 21, 2016
    • Like Like x 1
    • Useful Useful x 1
  12. TNCHARGER

    TNCHARGER SRT Hellcat Supporter Gold Supporting Member

    Joined:
    Nov 19, 2015
    Messages:
    1,002
    Likes Received:
    360
    Trophy Points:
    258
    Vehicle:
    Challenger SRT Hellcat
    I have never seen a supercharged engine that needed to idle richer (not saying there aren't some out there). At idle and cruise the engine is basically running as NA so nothing to compensate for.
     
    • Agree Agree x 1
  13. ciphros

    ciphros Gold Member

    Joined:
    Mar 13, 2016
    Messages:
    623
    Likes Received:
    305
    Trophy Points:
    433
    Vehicle:
    Charger SRT Hellcat
    So this is a supercharger thing then, since even at low rpm it has some ability to increase pressure? This is not true for a turbo, so just trying to understand the differences.
     
  14. ciphros

    ciphros Gold Member

    Joined:
    Mar 13, 2016
    Messages:
    623
    Likes Received:
    305
    Trophy Points:
    433
    Vehicle:
    Charger SRT Hellcat
    This is exactly why I am confused.
     
  15. ciphros

    ciphros Gold Member

    Joined:
    Mar 13, 2016
    Messages:
    623
    Likes Received:
    305
    Trophy Points:
    433
    Vehicle:
    Charger SRT Hellcat
    I understand it completely. I ran 22 psi all day long in the evo. But, at cruise I was 14.7:1, when the go pedal goes down and boost climbs, 10.5:1. I even went so far as to raise cruise to 16:1 for a while, but founder that stoich with higher timing Advance worked as well or better, and gave me a little more off-boost grunt.
     

Share This Page