With the advent of lead-free gasoline and converters All tail pipes now run black. The old white or light gray tail pipe colors when run at highway speeds with light throttle settings were caused by the lead deposits from the old leaded gasolines.
Nowadays when you pull a head of an engine run with lead free gas for even 100,000 miles the chambers are unbelievablely clean and free of carbon deposits, it seems more like candle soot. This is why spark plugs seem to last forever (of course the tips are now tungstun and certainly help longivity)but even their coloring make them look nearly new. As a diehard presmog addict, I have to conceed to the presence of these visible effects of leaded gasoline (damn it!).
