Which glowplugs are the best for an '88 7.3 IDI?
 

Get the Motorcraft ZD9's, dont even look at the other plugs.

Could you tell me how or why they're superior?

The other brands have a habit of burning out within a few months and swelling inside the cylinder. When you try to remove them, they break off and fall into the pre-chamber. If you get lucky, you can fish it out with a vacumn and compressed air, otherwise you get to remove a cylinder head. I've read enough posts and seen enough pictures to realize the minimal cost difference was not worth the risk. I actually took one of my old autolite plugs and hooked it to a battery for a few seconds and watched it swell. I got lucky that several other plugs were burnt out and wouldn't let the controller warm it up all the way.
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Other pics I have seen show the entire tip had expanded, or people that are just plain missing the last 1/2" of the plug. Some people run autolites or bosch plugs with no problems, but I haven't read of anybody having a problem with the motorcraft plugs. It looks like the price went down on the MC plugs as well, I paid $14.99 each at O'reilly's a few months back.

I just bought the ZD9 glow plugs for my 88 and it starts great never turns more than full revolution before it pops off even when its really cold. Im glad i spent the extra money bc i figured since the originals have been in there 180,000 miles these should be good for the life of the truck. Got lucky the orignals came out like they were finger tight.