Re: Best plugs for a 1999 4.6 ?
Don't know if this thread is still active, but thought I would offer this from my experience with owning a 1999 Crown Vic.
I tried both single and double platinum plugs and found that the single platinum plugs ran better and gave me better fuel economy than the double platinum plugs. I have been told by more than one automotive pro that if the car calls for nickel or single platinum plugs, use them for best performance and fuel economy, rather than double platinum, iridium, etc., because that is what the ignition system was designed to fire. Double platinum, iridium, etc. will last longer but they have higher resistance and require a higher energy ignition to fire them, so they may missfire and give lower fuel economy on a car such as a 1999 Crown Vic that was not designed for them.