I drive a '99 GT. My question is which type of spark plugs should
I use. I've changed them once and I used Ford plugs. Is there
anything better?

 

i have denso iridiums, i believe nkg's are good as well, but i could be wrong. much more expensive for iridiums though. if i remember correctly my densos were 110$

 

autolite are good

 

I use autolite double platinums. I saw an article online on a 4.6L where they were dyno-ing some bolt-ons and one of the things they did was to change the plugs. I believe the car only had 5600 miles on it. They took out the original plugs and installed NGK...not the iridium and the car gain a few hp. They tried the iridiums and it dynoed the same. I'd either go with autolite or NGK. NGK's are big with the motorcycle and boating crowd....2 types of "vehicles" that are hard on spark plugs.

 

i heard the sutolite platinums are the best for 4.6's

 

I use NGK "V" plugs in mine. Steeda has hundreds of them on their shelves for all models. I've got a couple of links here of newer plug types that have hit the market recently you all might find interesting reading:

http://www.briskusa.com

http://www.e3sparkplugs.com

The e3's have recently been advertised in most of the popular auto mags. It won't be long before one of the major books does a test on their performance: www.automfg.com/articles/020503.html

 

I'd go with the Autolite Double Platnums, If you compare the size of the electrode to NGK's or Bosch the autolites are like 10 times larger. But if your really looking for performance plugs then denso's are about the best you can buy.