On older ignition systems, plug conductivity is something to keep in mind. Newer systems, or systems that have had upgrades installed (MSD/Mallory boxes, high output coils, etc) have enough power to handle the increased resistance that is inherent in the newer plugs. Its also one of the reasons the newer systems come spec'd with a wider gap. Consider that a 91 5.8 calls for .042 to .046, while a 2002 calls for .052 to .056. While this amount may seem minimal, when you add in the increased resistance of a exotic metal plug, and the increased resistance of the compression, with a stock ignition (especially one that is showing some age) it can add up to possible misfire. The "6litre" tuneup that keeps getting mention in various threads is actually very simple in the reason it works. Basically it is making sure all the components are new, thereby removing any aged/worn components. Add in the higher output coil, and the system can handle jumping the increased gap, thereby exposing more of the spark kernel to the air/fuel mix. This in turn helps promote a more complete and efficient burn, increasing power and mpg. The short of it being, if you are running a stock/original system, stick with the copper plugs, however, if the system has been upgraded to better wires, coil, etc. going to a platinum or such plug won't hurt things, and you will gain added life between plug changes.

 

I changed mine to motorcraft platinum rated same as copper. Haven't had any problems and getting little better performance and mpg. Started troubleshooting and did find bad control module. a/c voltage to low. Motorcraft plugs i went to are asf32p or new part # is sp-501

 

I find it odd that 96 trucks actually specify the platinum plugs, while 95 and older doesn't.

 

Right. Suppose more research was done and they figured they were better. Like I said, I went to motorcraft website and the platinums asf32p was listed for my truck. They've been working fine so far. She's running like a dream now. Advanced my base timing to 13*. Seams like i'm lighter on the foot and get the response and power i need. Hopefully that relays to better mpg.