Installing long electrode spark plugs

[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:




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.

[wiley-moeracing]

Sorry, a little hard to follow you, you got 8 plugs for the 4.0 engine? should only need 6.? The difference is the heat range between the 2 plugs and when summer comes and the engine runs hotter you can have detonation occur with the hotter plug. now if you have headers on the 401 and want to run the same stock plugs with the smaller 5/8 head use champion rc12yc I believe, can be a pain to find sometimes but this is the plug Edelbrock suggest for their aluminum heads.

[SJTD]

He's saying both engines seem to run better with the long electrodes.

Only thing I'm not clear on is the heat range. In the 401 he says it's supposed to run 14 and now he's running 12.

Which is hotter? You seem to be implying 12 is.

This has me wondering. In my CJ with the 4.0 head I don't remember if I used plugs specified for the 258 or the 4.0. Or maybe they use the same plug.

[JeepsAndGuns]

I am using the spark plugs from a 4.0L I6 in my AMC V8 (401).
The long electrode spark plugs are what comes in the 4.0 from the factory. If you go to any parts store in the country and get spark plugs for a 4.0, you will get the long electrode plugs pictured above.

For champion spark plugs, the larger the number, the hotter the plug is. So heat range 14 (factory for my 79) is one heat range hotter than heat range 12 (later model AMC V8's and the 4.0 I6). So since the later engines already call for heat range 12, there should be no problem using them in the older engines.
So installing the 4.0 spark plugs is moving 1 heat range cooler.
If I did not make it clear, the whole thread is about installing spark plugs from a 4.0 I6, into a AMC V8. All spark plugs for the 4.0 use the long electrodes.

Stock AMC V8 spark plugs have a 13/16 head, and standard length electrodes.
Stock 4.0 I6 spark plugs have 5/8 head and have long electrodes.

[joe]

As mentioned above on US plugs the higher the number the hotter the plug. Not so with Euro plugs(NGK etc).
On a sidenote hotter plugs don't offer a hotter/better spark or better combustion/mo powa. It just refers to the heating/self cleaning cycle of the electrode. Sparkplug technology has been figured out for decades so unless you built an exotic motor for non-standard use factory recommended plugs work as good as it gets.

[wiley-moeracing]

Ok so you can use either plug in hotter or cooler range, you will notice an issue when you get into summer temps using a hotter plug. Use what is recommended by the manufacture for your vehicle. If your looking for plug clearance for headers use the champion rc12yc plugs in amc v8 applications.

[JeepsAndGuns]

Ok so you can use either plug in hotter or cooler range, you will notice an issue when you get into summer temps using a hotter plug. Use what is recommended by the manufacture for your vehicle. If your looking for plug clearance for headers use the champion rc12yc plugs in amc v8 applications.

Yes we just discussed this. The plugs I am talking about are 1 heat range cooler than the factory 79 plugs, but the same heat range as all the later model amc v8's. (looks like they switched from heat range 14 to heat range 12 around 1985)

If you notice the part number I gave (RC12LYC) it is the exact same spark plug as the one you posted (RC12YC) just just the addition of the "L" in the part number, denoting "long" electrodes.

[wiley-moeracing]

Yes, give them a try, if they don't work your only out the cost of the plugs. I would pull them after running a while and check the color/condition.

[Cecil14]

Yes, give them a try, if they don't work your only out the cost of the plugs. I would pull them after running a while and check the color/condition.

He's been running them over a year. The post is purely informational, saying they work.


aa

[Carnuck]

In the past, whenever the ping monster was an issue, I switched out to NGK 2288 plugs which are BKR6EK with dual grounds. VW and Volvo used them instead of Bosch plugs nowadays to deal with the same issues. My last set got 50,000 or so miles in my 304 running points and a Mopar electronic ignition coil.

[ZackN920]

How did this go so much into heat ranges for plugs?... I read his initial post and got exactly what he was saying. Since you all are goin on about heat ranges, I'm not running stock... I kept having fouling problems. I think I went one range hotter and haven't had problems since. What I keep having problems with are the contacts getting corroded in my distributor. Gotta clean them every oil change.
So, why aren't y'all using actual Champions? That's all I ever use other than in my Suburban, that gets AC/Delco's.



So, 4.0 plugs go in a little deeper. Never paid much attention even though I maintain 2 of them engines (my XJ and dad's WJ). Anymore seat of the pants power felt? How about mpg's? Any increase?


Hey Carnuck, how do ya gap them things? Or do you just run them out the box?

[Cecil14]

Hey Carnuck, how do ya gap them things? Or do you just run them out the box?

Newer style plugs are not supposed to be gapped outside of what they come in the box. Whether that is good or bad, I won't say. I have had very good luck running NGK iridium plugs (not changing gaps) in my motorcycles. Have not bothered with them in my truck, though.


aa

[Carnuck]

How did this go so much into heat ranges for plugs?... I read his initial post and got exactly what he was saying. Since you all are goin on about heat ranges, I'm not running stock... I kept having fouling problems. I think I went one range hotter and haven't had problems since. What I keep having problems with are the contacts getting corroded in my distributor. Gotta clean them every oil change.
So, why aren't y'all using actual Champions? That's all I ever use other than in my Suburban, that gets AC/Delco's.



So, 4.0 plugs go in a little deeper. Never paid much attention even though I maintain 2 of them engines (my XJ and dad's WJ). Anymore seat of the pants power felt? How about mpg's? Any increase?


Hey Carnuck, how do ya gap them things? Or do you just run them out the box?

I check the gap before installing to make sure they weren’t damaged in transit with an old school wire gap tool. They are side gap plugs so they aren’t too hard to do and they handle the increased spark voltage that I run with later style ignition coils.
While I’m in there, converting the cap for the larger style (AKA Team Rush upgrade) I get the cap and rotor with brass contacts to cut down on the deposits. I was told long ago to dab dielectric grease on the rotor tip but never tried it. Even the HEI upgrade can use good contacts for longevity.

[ZackN920]

Hmm, but you have to make sure those gaps are EXACT since there are 2 grounds for the spark to jump to. Or you end up with spark only going to 1 side.

Yea, my ignition is set up like that. MSD cap-adapt(has brass terminals), 50K volt TFI coil, 8mm wires, and stock (other than heat setting) Champion plugs. Don't remember gap, I think it is increased... probably at .040 iirc. Still get build up in the distributor. Ive tried that trick with the dielectric grease, didn't help and then I had to clean the residue out.

There is a looong thread somewhere here about my ignition and a intermittent cold start problem I was having. It has a bunch of information about my set up because all of us were trying to figure out my Jeep's odd problem, that I thought was a wetness in the distributor problem.

[wincher]

I have Champion N12Y in my1978 Chief with 360AMC


So what is now a good choice to change for?


RC12yc
Iridium
the long electrode type for the 4.0l engine?
NGK PBR5Es
any other?