^^^ idk what the Zetec/ATX is limited at.
FYI - There "is" an aftermarket manufacturer with a 'new' cast for the SPI SOHC head that supposedly addresses the dropped valveseat problem and makes it a non-issue. A thread somewhere here in this section during the last year was started on this topic.
Member of the Dropped Valve Seat Club.
Replaced #4 Piston, Rings, Rod+Bearings, AERA Re-Manufactured Cylinder Head and a light Hone. Then CLEANED the hell out of everything.