quote:
Premium fuel may exacerbate combustion chamber deposits because premium fuel has a different BTU rating than regular gas (lower BTUs than regular gas, actually). So, unless your engine has timing that's advanced enough or enough compression to make sure combustion is complete at TDC, using premium fuel MAY, and I stress MAY, lead to higher rates of deposit formation from the deposition of soot.
There's an old addage, too, that says that the higher octane content of premium leads to slower flame propagation, which would add soot as well, but that just goes along with premium's lower BTU rating. I don't know if I totally buy the old addage, because if it burned too slow we'd see a lot more burnt exhaust valves, but oh well-
You have to remember, though, that there's a multitude of factors that interplay in an engine when deposits develop. Oil control, water jacket temperature, head quench area design, total engine timing, ambient air temp and humidity, EFI fuel mix settings or carb jetting, spark strength and plug indexing, idle mixtures-all these things can effect how deposits form on pistons and in cylinders. I think that trying to find the cause of "more soot" on a piston crown is like trying to find the reason for federal government debt-lots of reasons, just pick one!
