Best 5.0 stock replacement spark plugs

I've started seeing some very light knock at part throttle. After reviewing some datalogs, a knowledgeable member recommended checking the condition of my plugs as weak spark could cause a subtle issue like I'm seeing. I'd rather just put a new set in if I'm taking them out to inspect. Is there a particular plug that works better than stock? Or are the original motorcrafts as good as anything else? Should the plugs be gapped or run out of the box? I know some of the iridium or specialty plugs can be easily damaged so sometimes it's best to run them as-is? Car is basic bolt-on and will stay that way...no FI, nitrous, or anything crazy. Thanks for the info.

I wondered this as well, I don't trust Ford to gap a plug as far as I can throw them after my Ecoboost had plugs varying over .020 between the cylinders. I bought new Motorcraft ones for that car when I put it back to stock and measured them all and they were spot on but the factory ones were all over the place.

NGK or Brisk plugs

NGK has two plugs for NA mustangs:
LTR6AI13 and LTR6IX-11

The AI13's come with a larger gap .052 and work well used in completely stock engine. In a engine with a CAI and tune they would work great as well. I would recommend these for light modification as they have a longer life expectancy, plus no need to modify gap from purchase but checking them to make sure nothing happened in shipping is a good idea.

The IX-11 come with a smaller gap .044 but have a stronger spark. These are gapped ideal for NA, but with a bit more modification. With light modification I would open the gap on them to .055 using actual feeler gauges or the spark gap tool that has wire loops and that would give better spark performance. They will foul easier and need to be replaced sooner than the former plug, so the slight increase in spark performance and life expectancy should be weighed in your choice.

Brisk is another common plug used in racing applications. I would compare them to the IX-11's.

The factory motor craft plugs are good, but are known to have gap size variation issues. If you go with these check the gaps and set them all between .050 and .055. I found mine stock ranging from .040 to .063. three of them under .045 and two greater than .060 right from the factory. needed less to say I wasn't getting to optimal spark timing values. You want to run the largest gap you can get away with with out causing spark blow out.

Any reason to switch from the oem if your na?

I'm currently using Autolite XP part # XP5363 I've tried NGK IX part # 6509. No issues with either of them and both were iridium. Regapped to .50 for both sets.

I haven't hit my time to change plugs yet but I will be going with copper based plugs. Copper has the best conductivity vs iridium and platinum. Unfortunately, because they perform better they do have not so great in longevity compared to the other 2 metals.

How difficult is it to change the plugs in a 2015 GT? Are any of them hard to get to?

Thanks. :cheers:

Simple.

A ratchet, two three inch extensions, and a spark plug socket and a 10 mm socket.
- remove coil covers
- remove ten mm bolt from coils
- pull coils out
- unscrew spark plugs.

The only tricky one is cylinder four. I just attach the plug socket to a short extension, put it in the hole, attach the other short extension and lower it the rest of the way on to the plug. Reverse process on the way out.

My go-to plugs for almost anything with an internal combustion engine are NGK or Autolite, have been for years.

Decades even, now that I think of it. Never an issue.

Autolite for NA applications have never let me down.

I picked up a set of OEM replacement motorcrafts because the local parts store had a set and they were reasonably priced. The plugs that came out looked fine and were all gapped pretty consistently. The plugs I put back in were tapped pretty consistently out of the box, too. Easiest vehicle I've ever put plugs in for sure. Took me about an hour taking my time.

Unfortunately it didn't fix what I hoped it would though. Oh well.

markmurfie said:

Simple.

A ratchet, two three inch extensions, and a spark plug socket and a 10 mm socket.
- remove coil covers
- remove ten mm bolt from coils
- pull coils out
- unscrew spark plugs.

The only tricky one is cylinder four. I just attach the plug socket to a short extension, put it in the hole, attach the other short extension and lower it the rest of the way on to the plug. Reverse process on the way out.

Click to expand...

Thanks, Mark. I've got about 38k miles on her now, and although the plugs these days are supposed to last a long time, I might go ahead and change them out.

Couldn't hurt, right? :cheers: