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Originally Posted by metromizer
My '96 (3) Cylinder TB injected engine is basically stock with (2) 4-wire O2 sensors (maybe the ECU averages between the two?).
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The dual-O2 sensors is part of the OBD-II scheme to diagnose the proper functioning of the catalytic converter. One of the O2 sensors is upstream to detect pre-cat exhaust oxygen content and should switch back/forth past stoichiometric to let the cat function properly. The downstream O2 sensor is to detect that the cat is functioning properly and should not switch, but read constant...cant' remember which voltage. I have heard that some folks "fool" this sensor by putting it in an empty coke bottle with just air inside it to stop a MIL lamp with a bad cat.
I don't know much about Metros, but if you don't have a knock sensor that will automatically pull back timing if your engine pings you might get some gains by doing what you propose. If any timing gets pulled out to fight ping you'll probably not see much benefit. You also don't want to run lean-burn for too long because you may cause damage to your catalytic converter. The cats require the switching between lean/rich to properly function as both an oxidation and reduction catalyst. The cat needs a lean mixture to oxidize HC and CO, but then needs a rich mixture to reduce NOx. Without running rich, your cat can't reduce NOx so not only do you emit more NOx, but the cat heats up and can become brittle and crumble from only oxidizing HC/CO which produces heat.