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Old 10-11-2023, 01:36 PM   #3 (permalink)
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Any fuel being wasted is likely to improve emissions. For instance, my MX-5 will retard timing significantly during the first 15 or so seconds after a cold start, to blow still-burning fuel into the catalyst, to light it off sooner.

RAC Yes, all blowdown to the mighty catalytic converters…yes a major part of the PCM is decaded to feeding and caring for the catalytic converters

There really isn't any magic to engine tuning. You have your air fuel ratio (which you can see with an OBD reader), which could be worth 0-5% with a very lean tune. You have your ignition timing, which, if too retarded, would burn vales, and if too advanced, would cause knock -there might be another 2-5% on the table there at WOT, but likely zero at cruise. You have EGR, which can either be from a valve or by advancing cam timing on VVT engines - there's maybe 1-2% to be gained there. And, that's it. Everything else is mechanical.

Lean burn doesn't allow normal catalytic converters to work properly. That said, it's generally worth less than 5% extra fuel economy. You're not going to turn 18mpg into 28mpg with a tune. Maybe 20.

RAC Two answers, I have done a 5MPG improvement in a 2000 Mercury Grand Marques that got 30MPG at 65MPH at 1700RPMs by going to 16.4:1. Part two of one, GM/Chevy did these in the 85/91 Camaro with the TPI and had an automatic Lean Burn Setting in the PCM which were said to give 5 to 10MPG more when turned on.

2) I said 22 MPG not 28MPG, that will only be possible with a second overdrive allowing the truck to cruise at 75 at 1500RPMS.

The main issue with running a lean burn tune on an engine not designed for it is that it slows the flame front down significantly. You have to advance ignition timing a lot, which ends up causing a lot of the combustion pressure to build before the piston reaches the top, creating negative work, negating most of the gains you'd otherwise get. Factory lean burn engines have some head and piston and valve design tricks to help speed up combustion of lean mixtures.

RAC Again I added 5MPG to a 2000 Mercury GM JUST by dialing the A/F ratios to the peek of 16.4 and allowing the PCM to control all else…yes it lost power but no the exhaust temps hardly changed.

IF this SUV acted normally as did my 2000 4.6 and my 03 Crown Vic with a 4.6 without this kick down of MPGs at 60MPH then she should get 22 MPG at 75 MPH...not great but I hate being cheated out of 5 MPG by a trick in the PCM.

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