Why some 8mm films looks excellent , some horrid grainy , why process varies a lot

[Mustafa Umut Sarac]

Above question.

[David Cunningham]

If it's a modern film such as E100D, it will look remarkably fine grained. If it's an older stock or Tri-X it will be extremely grainy.

[Phillip Mosness]

Yes, some stocks are naturally grainier than others, but much of the variation can be attributed to factors other than processing. Unless you mean variations between different rolls of the same stock that you've shot yourself.

If you're looking at other people's footage online, there could be a multitude of factors in play. Exposure, lenses, transfer, etc.