Question on Spark plugs 98 mustang GT
 

Two questions

1.) what spark plugs are recommeded?

2.) Do they give any increase in performance?

NGK tr-5 is the stock heat range, i believe.
copper plugs deliver more power potential than platinum plugs though they will need replacing more often.

with boosted or nitrous applications, a cooler heat range is desireable.

typically, in a stock application spark plugs will give no significant performance enhancement unless the old plugs are fouled/old/etc.

Give this a read: Spark Plugs 101
It was written by a fellow Tbird owner with a supercharged 4.6L, so the info holds true for most modular Mustang owners also.

if you ever do the head swap you willo need a colder plugs as i just found out the stock autolight 763 are pretty good for stock but head swappeds def need 103 for plugs

Motorcraft AWSFA-32-C

Here's what I just installed into my 1998 GT.
NGK Iridium IX - Part No. TR55IX-7164

Note these are threaded all the way.
They work great. My gas mileage increased about 1-2 mpg.
Didn't really feel a power increase but it does seem to rev a little better after 4500rpm.

I got mine at -
http://www.clubplug.net
Ask for Frank (1-866-772-7584)

Good luck.
Ryan

7.12 a plug eh, how about them autolite, DW does the commerical:D