Reasons engine gets richer approaching torque peak? | FerrariChat

Reasons engine gets richer approaching torque peak?

Discussion in 'Technical Q&A' started by luckydynes, Jan 3, 2009.

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  1. luckydynes

    luckydynes F1 Rookie

    Jan 25, 2004
    3,930
    CA and OR
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    pit bull
    #1 luckydynes, Jan 3, 2009
    Hey all,

    Been thinking about how I had to alter my EFI map to get a nice flat a/f ratio at WOT and I'm wondering if it's a symptom of not enough spark energy??? I'm running MSD DIS4 in waste spark so energy is being split between two cylinders and spec's don't seem like the energy is that high but don't know what's necesary with naturally aspirated high compression ... also mixing avgas and someone mentioned the stuff doesn't burn as easy ????


    Is there any logic to me having to drop the injector open time around tq peak 'cause wideband a/f ratio was showing a rich "hump" on both banks? ... thinking about it more efficient combustion efficiency seems like should read leaner if injector times are the same ... higher BSFC, etc.???? I've been running with this map for over 1 year now and during some pulls starting in 1st gear I'll get a miss in 4th gear right around 6k rpm. Started thinking it was dirty fuel filters but it seems like once I get things HOT it goes away. Colder air temps made me think it was combustion temps/spark but ???? ... don't have my wideband hooked up right now to see what's going on with a/f during miss ... was thinking if it was plugged fuel filters mixture would be going lean and would get a backfire???? Haven't had the wideband hooked up for a year now so things might have changed. I want to hook the data into my ECU to have proper correlation with the EFI map vs. seperate software with the stand alone wideband.

    Cheers,

    Sean
     
  2. Gary48

    Gary48 Guest

    Dec 30, 2003
    940
    #2 Gary48, Jan 3, 2009
    Not sure if this will apply to your application but here goes anyway. If your fuel map indicates much richer at near optimum RPM then its possible that your engine intake and exaust is beyond optimum efficiency and just cant take any more inputs and tends to become richer because of that.
    A phenomenom associated with this condition is called "sonic choke" where the reversionary characteristics of a given intake system and a given high RPM come together to create an actual choke condition caused by ultra high sound frequency's which bounce back and counter flow. This can happen at lower RPM in an engine that has smaller ports and hence higher velocity. "Sonic choke" has been associated with conditions mimicking the speed of sound. The normal remedy for sonic choke would be to increase air flow thereby reducing velocity and extending the RPM range upwards before the condition arises again.
    Things you can do to extend RPM range before sonic choke again appears is bigger intake and exaust ports, bigger exaust headers, higher lift and greater duration camshafts to name a few.
     
  3. luckydynes

    luckydynes F1 Rookie

    Jan 25, 2004
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    pit bull
    #3 luckydynes, Jan 4, 2009
    Interesting, thanks for replying ... my post might of been a little confusing ... I had to lean the map at peak tq (5500) and then richen up again (6000) to keep a/f ratio around 12:1. My wideband showed a/f going rich at tq peak so leaned the map.

    "Sonic choke" makes some sense and one could hypothesize I'm seeing this at/near peak tq 'cause fuel is being atomized/mixed more thouroughly because of the "sonic choke" thus more efficient combustion/more power?

    I think I want to try hotter plugs for the occasional miss at 6k during hard pulls ... never happens in 1st and 2nd and rarely 3rd ... thinking it happens more in 4th 'cause the motor isn't winding up as quiick so it stays in this "poor combustion" area of the map for a bit longer ... might be too rich but need to hook up wideband again before I play with any mapping. Thought about hotter plugs 'cause I'm not seeing the plating burnt off the first thread .... running a mix of avgas and colder intake temps (40-50 deg f) might be making things difficult in that respect. The colder air intake temps will automatically richen the map quite a bit too ... theoretically the correct amount based on ideal gas law. Just a side thought on that, based on ideal gas law a 40-50 deg f difference in air intake temp makes for 10% more hp??? ... didn't open the physics book but that seeme right from memory. Might be time for another dyno run now winter is here :).


    Cheers,

    Sean
     

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