Taco, the SG can show vacuum to a tenth of a PSI.
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Thanks for the suggestions so far, some good points to think about that I wasn't expecting. I think it would take me some time to actually be able to put the information from a wide band to use though. Without remapping the ECU, how would you apply the feedback its giving you?
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Using a wideband to lean out your mixture involves setting the gauge or driving unit to produce a 0-5 linear AFR voltage. This voltage must be fed to a switching circuit that produces 0v when the input voltage is above a certain threshold and 0.9v when the input voltage is below a certain threshold. In effect, you're deciding at what ratio you tell the ECU it's running lean or rich. The result is you train the ECU around a ratio of your liking. Though I cruise as lean as 18:1:1, I find that 16.2:1 produces the best economy for my vehicle.
One caveat is that your scanguage cannot tell you which ratio produces the best economy. This is because it measures air, not fuel. When leaning out, it will show your mileage getting worse even though it's getting better, because you're using more air, but less fuel. After a tank of lean-burn, you'll find that your fillup numbers and the SG's estimate are off. It's only after a complete tank of running a certain mixture, that you can halfway trust the SG. A far more accurate way of measuring fuel is an injector based solution. This will tell you the truth for any mixture in any situation.