Hi all

Plugs are ready to be installed, gapped and side electrode position marked on top of the spark plugs.

Some months ago I figured out that the side electrode attachment point should be facing outwards compared to engine center, i.e. to the right and a bit forward on A1-2-3 and so on.
Now I'm in doubt if it's correct.

Any quick tips out there, or should I start looking at valve and spark plug hole positions to convince myself it's correct... ?

 

I've concluded it must be the other way round: the spark gap must point outwards and the side electrode attachment point must therefore point towards the engine valley.

 

Leo
Spark plug positioning in the head as far as the "clock" position of the electrode within the cylinder bore is concerned, is an complete and utter waste of time.
Greg

 

There's sufficient to worry about with a V12 Jaguar without adding spark plug orientation to the equation.

Graham

 

It's done, and it feels right. The engine might not care, but it was kinda fun...

I have the V more or less stripped, so access was good. I would never do it with the V full of components

 

The plugs are tapered seats in later V12's (not sure if it started at the 6.0L) and can not be indexed. I have this issue with my 6.0L.

 

My plugs are tapered as well.

I did the indexing by mounting all plugs in same cylinder, noting their angular position and afterwards swopped the around until all is angled as good as possible.

 

I forgot to mention.

You could side gap the plugs, there are loads of howto's on this on youtube.

 

Side gap ? Meaning that you let the side of the gap point towards the combustion zone/inlet valve ?

What I have done is to place the side electrode attachment point (=backside) towards the valley allowing a ± 45° tolerance as I didn't have 60 plugs to choose from...
The swirl will then sweep the spark gap sideways and always see the spark and never the backside of the side electrode.

 

I did mine. Got 11 of 12 to index well. I'm now of the opinion that this is without any benefit.

First Jaguar makes no mention of it.

Second, Jaguar makes no provision for it. The plugs have no shims and orientation of Jaguar suggested plugs electrodes and threads is consistent. Therefore there is no method for varying the index.

Thirdly, the Bernoulli effect ensures that you cannot shroud the spark with the electrode. If fact, it's possible that electrode towards swirl may be ideal. Certainly, it's impossible to avoid ignition in any orientation.

 

I have never actually checked but...

1. are the spark plugs EXACTLY THE SAME when it comes to the thread? Do they start in the same area and end the same?
2. are the threads in the cylinder heads all identical as in start and finish?

I have heard of people doing this on engines with high specific outputs (super tuned turbos etc.) but that "lump" under the bonnet in V12 form is too old to really benefit from this... If it were supercharged, I'd maybe look into again but then, where do you want to point the spark at and will it actually work there properly?

 

To 1 and 2: it's a NO and another NO.

11 out of 12 plugs ended up within 180° angle when mounted in 2A though, so there might be a tendency that exceeds what could be expected from a random position.
Regarding the head thread start is random, at least on my '89 engine.

I have ensured that the spark is not covered by the side electrode without aiming for a specific angle. That was simply not possible without having a much larger amount of spark plugs to choose from.
I used to work with gokart in the early '90s. 100 cc high performance twostroke. Yes, they respond to indexing

 

All the NGKs I bought ( 24 ) were identical. Precisely.

My 86 plug holes are all bored identically. The threads all begin at the same orientation.

How would there be any difference in combustion based on the index of the plug? Either ignition begins in the spark gap or it does not. If it does not the the cylinder doesn't fire. If it does then the cylinder fires.

Is there some way indexing the plug alters the flame propagation? I'm not arguing that there is no possibility, just asking.

 

Here my original, raw and not photoshopped notes...

Top line is the angular position of my 12 NGK plugs placed in 2A, indicated as time. Most positions is between 7 and 1 o'clock

Below is where these plugs ended up and how the position was. Number 12 was mounted in 4 cylinders with 4 results.

12 o'clock = side electrode mounting point towards valley, i.e. Gap pointing outwards

Conclusion: I do not have the same experience as JigJag...

 

Simple answer is yes - but.

It does depend a lot on chamber shape, the HE chamber has the plug quite high ) closer to the piston than say the GM LS7 chamber, so indexing is less likely to have any benefit. Whereas in an LS7 chamber the plug is more central and closer to the top of the chamber and could shroud the spark if the plug is facing the top.

The idea is that the flame front should start in the middle of the piston and radiate out evenly across the piston crown, doing so evens out the pressure on the piston crown causing less piston rock in the bore.

The HE chamber has the plug offset to the exhaust valve side so the flame propagation is already uneven on the piston crown causing side loading of the piston. In a stock engine indexing plugs will more than likely have no effect on engine performance.

 

Did your plugs vary the relative position of the thread starts and electrode or did only your plug holes vary their start position?

 

Both:
Same plug in different plug holes results in different positions. Hole thread start must therefore change.
Different plugs in one plug hole results in different positions. Plug thread start must therefore change. This changed less than I would expect from a completely random distribution, but still a lot.