Another 87 octane/aluminum heads question

nm9stheham said

May I ask under what conditions the AFR is running in the 13's? If this is at 2500 RPM and cruise conditions, that is too rich for decent economy, carb'd or TBI. Folks get hung up on staying with stochiometric AFR all over the operating range, but that is only needed at more open throttle conditions. For moderate cruise at 60 mph on level ground, your AFR should be in the 15's; no problemo with the engine running OK. Heck, Chrysler Lean-Burn engines ran up in the 18-19 AFR range!

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The 13.2 is at part throttle cruise on the highway. I can clear the memory in the PCM, which also wipes the fuel trims, and it will jump up in the mid 14 range for a while, but after a few hundred miles it's back to 13.2. One thing I haven't done is recalibrate the wide band since the initial install, it's possible it's lying to me a bit, but given the 10 mpg it's not stretching the truth much.

The narrow band o2 that the PCM uses to derive an average fuel ratio doesn't give a specific reading on this 30 year old system, but it does switch rich/lean/center as it should at cruise, I've watched it on my Snap On scanner.