So I thought I would share a mod I did and get some opinions and see if anyone else is interested in trying the mod too.
But first, a little back story on how this came about.
So I have 3 other jeeps and they all have the 4.0. I have always wondered why the spark plugs in the 4.0 has a lot longer electrode than all other "normal" spark plugs. So one weekend, I was bored, and had some spark plugs identical to the ones in a 4.0, but had the shorter/standard length electrodes. So I install them in my wrangler to see if it makes any difference.
I notice right away, it took maybe a tiny bit longer to start and the idle did not seem as smooth. I take it for a test drive and it just doesn't seem like it runs as good and a tiny bit lower on power. I swap the original spark plugs back in and it is back to running normal.
So now my gears start turning. Ever since I installed headers on the 401 in my cherokee, the spark plugs have been a pain to remove, as the socket rubs the header flange. I had been wanting to swap to some spark plugs with a 5/8 head to give more clearance.
So the light bulb turns on and I think, what if I use the 4.0 spark plug, I wonder if they would have the same (but opposite) effect on the 401 that the short electrode plugs had on the 4.0.
The only thing I was concerned about was piston clearance. Well I had a spare set of cylinder heads so I do a test fit with a 4.0 spark plug.
Stock spark plug:
4.0 spark plug:
I use a straight edge and it does not stick down past the surface of the head. So even with flat top pistons, I do not think there would be any clearance issues. If you have a engine with shaved heads and/or dome top pistons there may be a issue. But I think any stock engine with factory dished pistons will have no issues.
Now I think about heat range. The stock 79 heat range is champion 14. The 4.0 is heat range 12. Now the funny thing, is on the later model wags, it looks like they changed to heat range 12. So I think the 4.0 heat range should be fine.
For champion spark plugs, the larger the number, the hotter the plugs. So the spark plugs from the 4.0 are 1 heat range cooler than the factory plugs for 79. So there should be no issues running the 4.0 plugs in regards to heat ranges.
The 4.0 spark plug is champion RC12LYC. I use the NGK equivalent ZFR5N.
So I go to the local parts store, pick up 8 new spark plugs for a 4.0 and swap them in.
Right away I notice it started up just a tiny bit quicker and the idle is noticeably smoother. A quick drive and the engine seems to have just a little bit more pep and all around just seems to run much smoother.
So now I wonder why. The only thing I can think of, is the longer electrode places the point of ignition closer to the center of the combustion chamber and the piston.
I have been running these plugs for a year now and have had zero issues. I wanted to run them for a while before I posted up my findings to make sure no issues did pop up.
I am curious what other think and wonder if anyone else might be interested in trying this swap too and see if they have the same results.