I believe it has something to do with fresh air making it up to the valve having more oxygen than more normal exhaust air. This causes more carbon deposits. the theory that back pressure or long exhaust tubes will prevent "fresh" air from making contact with the valves. There is some discussion if this is actually true or not.
2001 ZX3 Zetec - Rebuild "The TICK" (
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2004 ZX5 SVT - Restoration "Code Name Rocket" (http://www.focusfanatics.com/forum/showthread.php?t=722698&referrerid=62897)