Quote:
Originally Posted by SentraSE-R
So, I recall from the old days that under load, you had to retard your timing, or go to high octane fuel, to avoid detonation and engine damage. It stands to reason that under light load, coasting on the level or slightly downhill, timing could advance significantly, and give us our best mileage.
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Now you see my confusion above. But I've been thinking about it. If you are an aggressive driver, high numbers are probably bad, because they suggest that timing has been retarded because of detonation.
But, if we start with the premise that you are trying to drive to maximize fuel economy, there is a factor I missed. The comment about the test track above made it click. A lot of times, you aren't even in 'light load', but overrun, essentially coasting with the injectors closed and O2 in the exhaust.
In another thread I posted some logs to show the trouble with OBD-II MPG calculations. If you look at the very first log, before I turned a bunch of traces off, you can see that timing advanced significantly when all the other indicators are for overrun. So, in that sense, 'high' is good. But, at the two ends of the log, when I was going up hills, higher would not have been good, but bad, so I'm still not sure if 'higher is better' always works.
-jjf