Spark Plug Indexing V-12 HE

Just stealing a quote here from Kirby’s mighty fine book, “ to avoid is with the back side of the ground electrode aimed toward the nearest end of the head; in other words, you don’t want the electrode on the spark plugs in the front three cylinders on the forward side and you don’t want the electrodes on the spark plugs on the back three cylinders on the aft side”
I was wondering if anyone does this for any noticeable performance. It sounds like a great idea that I’ve never considered but if I’m changing plugs this week why not give it a go.?
I was just curious if any JL’s members have tried it both ways and noticed a tangible difference.

I’ve tried it…placebo effect, it helps right away. But in reality, no performance or idle difference ive noticed.

Only thing i believe it ‘could’ help, maybe plugs wear a little better over time?

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Thanks and I’m guessing from the lack of response to this post , I’m assuming (maybe incorrectly) that a lot of JL’s posters are not concerning themselves or dong this?

Check the archives, it’s been talked about a lot back in the day. Nobody has been able to prove a difference.

I only did it because I was changing all 12 plugs, and thought I’d have fun trying.

No: on a streeter that is naturally aspirated, my research has shown doing so only makes a difference on the mind dyno.


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And FYI, when I tried it I used a total of 18 plugs. Unless you want to play musical plugs and see if all twelve plugs just happen to line up perfectly with a specific cylinder, i found it easier to have a large selection of plugs to try, instead of trying to start swapping with already installed and indexed ones that may or may not work.

One uses indexing washers, of varying thicknesses.

Going down the nerdy rabbit hole here a little bit. I spent a summer in 1985 selling Ford Tempos at my buddies Dad’s Ford dealership. They took a 2.3 L engine and gave it a HSC engine. ( high swirl combustion ). I think the idea being a spherical cylinder head for better air fuel combustion equals more power , economy and maybe emissions.
Maybe the May head in the XJ-S similar idea and or maybe the Mopar hemi , similar idea.
At the end of the day with all that swirling action of air fuel mixture going on , going back to my original question, would it affect the placement and orientation of the ground on the spark plug? Seems like the answer is maybe not.

Nope. Can’t do that with tapered seat plugs, like the V12 uses.

Ah… that would complicate matters!!!

On an HSC head?


True. I did the indexing and noticed no change whatsoever

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Not if one has any sense, one doesn’t. First off, these are taper seal plugs. Second, using washers of different thicknesses would alter the depth the plug extends into the combustion chamber, probably screwing things up more than any hoped-for gains from indexing.

On any regular engine, indexing is a fool’s errand. I only mentioned the theoretical possibility of benefits in regards to the H.E. in particular due to its swirl combustion chamber.

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Well, thanks for that…

I must not have any sense, or a clue, since the couple hundred race engines–almost all run on dynos–I did it on seemed to run just fine.

May I assume you have done quite a few plug indexing jobs, and found out it “probably screwed up things worse than the indexing!”

I thought plug indexing was to prevent gap being closed by contact with pistons or valves due to components “growing” at high rpm. Silly me.
Hemi heads are generally not high swirl; advantages are ports have no sharp bends and valves are not shrouded. Disadvantages are a lot of surface area in the combustion chamber, flame travel is blocked by piston dome, and no quench area results in high HC emissions.
None of which applies to the V12

By and large, that is correct.

On highly turbo- or supercharged engines, it does make a difference in flame front propagation.

My 1200 Datsun had extremely tight combustion chamber tolerances: my V/p clearance on the intakes was on the order of 0.040", and I had to index the plugs so the ground electrode was “up,” otherwise it would contact the piston crown.

I agree with Kirbert, he said

He didnt mention race engines.

And I already stated that point.

Do remember that for the average HE engine no one even cc’d the combustion volume or even checked the inside of the distributor cap for wear, so there are plenty of genuine maintenance tasks higher up the food chain waiting.

kind regards

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Very good point. I believe a lot of issues are related to distributor wear, aftermarket parts that are not made to original specs, and general poor maintenance.
An example is the vacuum advance capsule; there are at least 2 different on the HE- if you get an aftermarket one like I did, you will find it has more travel than the original. Who knows about movement vs vacuum. Also stretched centrifugal advance springs…what about the nylon stop piece “thingy”. Broken, missing, or improperly installed. I would be surprised if 10% of distributors are working correctly, and if they are, those advance curves were for fuel of that period, not the E15 we have now in the US.
Probably a market for a better system…