9/27/09:

I forgot to write down what spark plugs I have been running, and it is way past time to replace them.  I have a sense of urgency because my tuning is stalled because of a spark breakup at high RPMs.  So I need to know what spark plugs to run with my supercharged engine.

Kenne Bell recommends running Denso T20EPRU plugs (which are one heat range cooler than stock) gapped to 0.035".  Stock Denso plugs for 96-01 Cobras are T16EPR-U15.  Stock Denso plugs for 03/04 Cobras are Iridium.

The NGK plug that is equivalent to the Denso T20EPRU plug (heat range 6) is the regular plug BPR6EFS (comes gapped at 0.032") or the V-Power TR6 (comes gapped at 0.040").  Stock NGK plugs for 96-04 Cobras are TR55. 

The Autolite plug that is equivalent to the Denso T20EPRU plug is the 2543 in half thread and the 103, 5143 or 5243 in the full thread. (Full thread plugs will fit in half thread heads.)  Stock Autolite Plugs for 96-01 Cobras are 764s.  Stock 03/04 Cobra Autolite plugs are 103s.  Stock 03/04 Mach 1 Autolite plugs are 104s.

Here is a link to the Autolite heat Range chart.  The full thread plugs for our cars are at location D14 and the half thread plugs are at location D15: http://www.autolitecatalog.com/RelatedInfo.aspx?b=A&f=Autolite/ASP2009AutoliteHeatRange.pdf

The Champion plug that is equivalent to the Denso T20EPRU plug is the RS14YC.  Stock Champion plugs for 96-04 Cobras are RS12YC.

Per NGK:

"The heat range numbers used by spark plug manufacturers are not universal, by that we mean, a 10 heat range in Champion is not the same as a 10 heat range in NGK nor the same in Autolite. Some manufacturers numbering systems are opposite the other, for domestic manufacturers (Champion, Autolite, Splitfire), the higher the number, the hotter the plug. For Japanese manufacturers (NGK, Denso), the higher the number, the colder the plug."