Submitted by IowaEagle
Still under construction
One of the most inexpensive ways to improve your 258 is to give it some improved spark which will result in some modest performance gains. Much easier to perform, basically all it amounts to is a tune-up, and lot cheaper and easier than doing the HEI upgrade. According to those who have done both the results are about the same. The TFI upgrade is considered by many the second thing you should for your Eagle's 4.2 engine after your replace the plastic valve cover. It has been reported the TFI conversion will add 800 to 1,000 additional useable RPM and a slight improvement in your MPG. I know it did for me. And the best part it uses off the shelf parts for virtually a plug and play installation.
Why should you do this? Your stock AMC coil puts out about a measley 18,000 volts. To run much more efficiently you will benefit from a hotter coil that puts out 35,000 - 40,000+ volts. However just replacing the coil will not do much for you since with the small original equipment distributor cap that is on your car presently the hotter spark will arc between the contacts within it. So you will need some other easy to find parts to finish up the job.
The parts you will need are from Ford applications with the 300 Inline-6 engine. Since AMC used a Ford distributor and ignition box these parts interchange without any modifications whatsoever. And the best thing is you don't have to remove your present distributor to do this.
I purchased my parts from Advance Auto so I will be using their part numbers -- your part's person can easily cross these numbers over to the brands they sell.
You will need for the distributor a new larger diameter cap, an adapter for the cap to fit on your current distributor and the rotor for the new cap.