What AFR is too lean for boost?
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  1. #1
    50bhp 3G Impreza amill's Avatar
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    What AFR is too lean for boost?

    I do a decent amount of data logs. My car with the Cobb OTS stg 2 map would consistently be around 11.15 under boost. My local tuner did some data logs and other monitoring and said everything seemed good. AFR was very good. My TP stage 2 map I'm using now runs around 11.14 under boost, which seems better. However, I was on nasioc and reading horror stories about cracked ring lands due to detonation and lean afr. I was just wondering what is considered "rich" afr under boost and what is considered borderline safe on the lean side during boost?

    2010 WRX Limited Hatch 287awhp/328lbft (before CAI, w/ Cobb OTS Stage 2 map)
    TP Stage 2 Map
    Turbo XS DP
    SPT Catback
    Turbo XS CAI
    Turbo XS RBV
    STI Short Shift Kit
    Perrin 22mm Adjustable Front Sway Bar
    Whiteline 20mm Adjustable Rear Sway Bar

  2. #2
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    eminehart's Avatar
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    Portland, OR
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    Re: What AFR is too lean for boost?

    WOT Afr's cannot be determined from datalogging with the AP a wideband is needed to observe WOT afr's
    from my guide to datalogging:
    Quote Originally Posted by Eric's Guige To Datalogging and Understanding Datalogs
    [b]Air/Fuel Ratio (AFR)[AP] or A/F Sensor #1(AFR)[RR]

    This displays the current AFR as measured from the front wideband oxygen sensor. It is normal to see readings around 14.7 AFR during idle and cruise. The AFR's will also climb to the 20's on deceleration because the injectors are shut off. The sensor is only accurate for determining AFR's during light throttle, cruise and idling. Because of it's location on the exhaust manifold, the exhaust gases become compressed as the turbo is spooled. The compressed exhaust gases cause the the sensor to read richer then actual AFR. The ECU does limit output of the oxygen sensor. Generally it will read down to the low 11's.

    You need a wideband placed post turbo to measure actual AFR during WOT. If the stock oxygen sensor is reading AFR's above this lower limit at high boost levels it most likely running dangerously lean.
    The Capped limit of the AFR sensor in the ecu is 11.02 however different Atmospheric Pressure changes will alter what value the capped limit actually is. that is why you are seeing slightly different AFR's

    Eric Minehart

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