Anyway, here's my topic:
On this board there have been a number of posts that the high octane tunes deliver more power but don't do much (if anything) for mileage compared to the 87 tune. This seems oddly counter-intuitive to me. Back in the day you'd run the highest octane you could find and then start fiddling with spark advance. If you wanted balls-to-the-wall performance you'd find the max advance under WOT that doesn't knock. If you wanted mileage you might go even farther but just keep your foot light enough to avoid knock - no WOT, no tire chirping, etc.
There's lots of discussion on high octane tunes here about advancing spark, reading the knock sensor etc. But it always seems to be about maximizing power, chirping tires, 1/4 mile times and stuff like that.
So I guess the question is, does the power gain from more advance come from greater efficiency? ( Or, if not, then where does it come from?) And if so, wouldn't greater efficiency also translate into better mileage, just like in the old days, if the driver is trying to be maximize mileage?
...which leads me to the knock sensor. I used to have a Saab 9000 turbo. Supposedly the computer on that was always adjusting timing to suit the driving conditions based on knock detection - low load, part throttle and it would be way advanced for economy. You could put in any gas you wanted and not worry. But with 93 octane the thing was a rocket. So isn't the purpose to detect knock and back off timing a little to the point that the knock disappears? If so, then wouldn't the optimum mileage setting be the point where there's occassional knock detection/correction under load? In the old days if you knocked going up a hill you just backed off the gas a little. Reading between the lines it sounds like the GM computer isn't dynamically changing timing like the old Saab. But that's hard to believe....
Sorry this is long, but the more I learn the more I don't understand and it drives me nuts. I think this is fundamental stuff to understanding what I'm doing. Maybe it will help some other folks too. Thanks in >>advance<<
for any comments or clarifications!
