4V Heads - Different spark plugs
I have a very interesting situation with 2 XYGT's, one being automatic and the other manual. Both uses 351C 4V engines but spark plugs for the automatic is the bigger type F9YC and the manual use the smaller type V12YC.
Why is there a difference in spark plugs sizes and plug ratings (hotter or colder??) Is it perhaps a different type head design ie closed or open chambers and maybe different compresion ratios?
Can someone pls help explain reasons for this?