Installing long electrode spark plugs

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JeepsAndGuns
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Joined: Sun Jun 10, 2012 4:12 pm
Location: cullman, AL

Installing long electrode spark plugs

Post by JeepsAndGuns »

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:

Image


4.0 spark plug:

Image

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.
79 Cherokee Cheif. MPFI 401, T18/D20, hydroboost, otherwise stock.
93 YJ. 4.0, 5 Speed, SYE, hydroboost, WJ knuckles/brakes, 8.8 rear, OME 2.5in lift, warn M10000 winch. 31x10.50's
99 WJ limited. 4.0, auto, 2wd, stock.
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REDONE
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Re: Installing long electrode spark plugs

Post by REDONE »

Thanks for being the guinea pig and bringing the proof to the pudding! I might give that a shot next time I screw around with the 304!
79 J-10 (Honcho Mucho) KE0LSU
304/Performance Fuel Injection TBI/MTA1/SP2P/Magnum rockers
T18/D20/D44s&4.10s/33" Mud Claws
Grizzly Locker Rear
4" front spring drop, 5" rear shackle flip
Chevy style HEI (ECM controlled)
Dolphin "Shark" gauges in a fancy homemade oak bezel
3/4 resto, rotting faster than I've been fixing it.

Cheapthrills
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Re: Installing long electrode spark plugs

Post by Cheapthrills »

Interesting .. I might have to try this.


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toothofwar
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Re: Installing long electrode spark plugs

Post by toothofwar »

Just out of curiosity, is your 401 carb or tbi? Duraspark or hei?

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rocklaurence
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Re: Installing long electrode spark plugs

Post by rocklaurence »

I wonder, are you seeing any increase in soot build-up or fouling? What gap are you running with the new plugs?

Nikkormat
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Re: Installing long electrode spark plugs

Post by Nikkormat »

Awesome! Thanks for getting some in head shots of these! I've been working on this after swapping plugs in my 4.0 and experiencing similar problems to your experience running V8 plugs in your wrangler. XJ guys notoriously hate the NGK V-Power plugs (P/N FR51) for hard starting, poor idle, and decreased mileage. They all recommend running a Champion RC12LYC, but, I won't run Champion plugs in anything anymore after 17 plug failures in my 74 Cherokee. So I installed the NGK's assuming they were all blaming the plug for other mechanical issues; but I too experienced poor running with my new plugs. Here's my theory as to why...

Lets visually compare a Champion RC12LYC, an NGK FR51, and an NGK ZFR5N which is the updated plug for the 99 and newer coil on plug ignition. The ZFR5N is the OE Mopar specified plug for 99 and newer...

First the RC12LYC pictured earlier in this thread, then an NGK FR51, and then an NGK ZFR5N.
Image Image Image

Notice something different about the plug in the middle? JeepsAndGuns in head pictures show the end of the threads are about even with the inside of the combustion chamber. So that extra ring of metal protruding past the threads of the FR51 will be sticking out into the chamber disrupting all kinds of flame-kernels and stuff. At least that's my theory, we'll see if my idle and mileage improve when I swap to the newer ZFR5N.

XJ People also swear that Bosch Platinum plugs will make your girl friend leave you for another man and your dog run away. Notice that they also have the strange ring of metal extending past the threads of the plug...
Image
Gabe, "reformed" Jeep hoarder.
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Topic author
JeepsAndGuns
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Joined: Sun Jun 10, 2012 4:12 pm
Location: cullman, AL

Re: Installing long electrode spark plugs

Post by JeepsAndGuns »

I would run either the RC12LYC or the ZFR5N. I am personally running the ZFR5N in all my jeeps. 93 YJ, 94 XJ, 99 WJ, and the above mentioned 79 cherokee with the 401.

I too am not sure why they put that extra ring around the electrode on some of the plugs. It appears only the FR51 and the bosch are the only two like that. I have actually run the FR51 plugs in the past with no issues or changes in driveability. But the ZFR5N seems like a better plug, and is the current factory recommendation for spark plugs for the 4.0, so I switched to them.
Another option is the champion RC12ECC. That was factory for my 99 WJ. Not sure why the switched to recommending the ZFR5N over the champion. But I assume they had a reason.
looking up the breakdown on what the part numbers mean

RC12LYC
R: resistor
C: 14mm threads, 3/4in reach, 5/8 head
12: heat range 12
L: Skirted Shell Firing End Projected .200" Core Nose
Y: Standard Projected Core Nose
C: Copper Plus Design

RC12ECC
R: resistor
C: 14mm threads, 3/4in reach, 5/8 head
12: heat range 12
E: .290" Core Nose Projection
CC: Double Copper

So it would appear the RC12ECC is a double copper plug, where (I assume) the RC12LYC is a single copper. I also assume the ZFR5N is also a double copper plug.
79 Cherokee Cheif. MPFI 401, T18/D20, hydroboost, otherwise stock.
93 YJ. 4.0, 5 Speed, SYE, hydroboost, WJ knuckles/brakes, 8.8 rear, OME 2.5in lift, warn M10000 winch. 31x10.50's
99 WJ limited. 4.0, auto, 2wd, stock.

rocklaurence
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Re: Installing long electrode spark plugs

Post by rocklaurence »

Could it be that the extended metal ring retains heat because it isnt in contact with the head-via threads? Could that hot ring create detonation?
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