So finally getting to the fun stuff of replacing.

First time spark plugs

1988.3 (still Lucas) v12 HE 5.3 North America (Canada)

I've read .025 should be the gap as .035 gap was for the early v12 with lower compression

So pulled the first one and checked the gap.

It's. 035.....

So what do the gods say? Consider I am removing all emissions stuff of that matters and will be doing cold/ram air intakes etc.

.025 or .035?

 

.025


Cheers
DD

 

Gap them at 0.025". Thee use a small gap because the coil can only fire so much, and small plug gap helps prevent high rpm misfires. That is also why it has the weird 2 coil system.

 

.035” is probably what they were from the factory (the spark plug factory, not Coventry). I would expect the last person who changed them didn’t bother to adjust them. Or they could have opened up as they wore, but that’s a lot.

 

Leave them at the gap the manufacturer (ngk I hope) set.

 

No. Don't. The spark plug manufacture doesn't know the specific application that the plug will be used in. So they set it to something, but they always need to be checked and adjusted before installation, in any car. The V12 is pickier about it than most, mostly because of the high compression ratio. Set the gap to Jaguar's spec (0.025").

 

If you do not gap your plugs on the V12 HE to 25 thou, you run the considerable risk of frying your ignition system.

 

We really need to think outside the square a tad here. Turn the JAG hat around a bit.

Jaguar did NOT make a spark plug specifically for the V12 HE, or get anyone else to make one either, they used and adjusted what was on the market already, hence the gap specified.

That same plug is used down here in our Aussie GM V8, from the late 1960's, and they specified 0,035, and later at 0.040", Fords also used it, at 0,030".

Many, many bits on any car are NOT specific to that car, they are used elsewhere, by other manufacturers.Why re-invent the wheel, when the one readily available, with a tickle here, a twist there, does exactly what you want.

Our B Bank top radiator hose, same as Mazda 929, who came first, who cares, there is an option.
Belts, generic for sure.
Heater return hose AT the radiator, same as Mitsubishi Magna By-Pass hose, probably no good for you guys up there.
So on and so on.

I cannot remember the last time I went OE for anything for any of my Jags, and that includes the newer pair. Its out there, just a small amount of searching, and you have it.

 

Good morning Grant, everyone.
I was having a look at the JDC Hunter region website, and the well known Jaguar modifier and enthusiast Ian Hissey has a few tips for spark plugs, as follows. I haven't tried the modification yet but am willing to give it a go, in the spirit of nothing ventured nothing gained, and of course Ian's reputation as someone who has recommended by experieince. Anyway for consideration as enclosed on the PDF

 

I have just read the piece you posted. Most interesting; but I do have two points:
The writer mentions a V12 "distributor engine". In fact (as I feel sure you know) there are three such: V12 carburettor engine; V12 flathead fuel injected engine; and V12 HE fuel injected engine. I cannot comment about any version except the HE engine.
A spark plug gap of 1.5mm in an HE engine with Lucas ignition will fry the system in no time. This is HUGELY greater than the 25 thou factory spec. Even the 31 thou the author recommends is considerably too large. I would very firmly counsel any owner from departing from the factory spec gap. The larger gap places a strain on the system that it cannot sustain.
As for the side gap idea, I am a bit sceptical. The HE engine is so constructed that an overall very weak mixture (that is too weak if evenly spread for the spark to ignite) is concentrated around the spark plug head, which then provides a zone rich enough to ignite. The flame thus generated is then able to propagate to the remaining very weak mixture in the chamber. I highly doubt that this process is in any way affected by the side-gapping idea. I further suspect the gap would go out of spec quite quickly.
Now I am always prepared to be wrong! But absent dyno data showing a definite change, I remain highly sceptical.

 

I am with Greg 100%, because he is buying the next round, on ya Greg.

Lots of theories out, none really wrong, but I am not convinced.

EVERY, and I mean EVERY V12, that I have had to sort for not running due to NO spark, be it HE or the earlier rockets, is 98% caused by wrong plug gap, as in too wide.

Even had this with the early XK 6cyl engine. Some I have had at 0.045 gap coz some "expert" somewhere claims all sorts of wonderful performance benefits.

I will NOT bag anyone, that is simply not an Aussie thing.

 

Just a little *generalities* on spark plug gaps, which leads to WHY the gap on a V12 is so small, given the available ignition system.

First, there are 12 cylinders, capable of over 6500RPM which means any basic ignition system has to handle

So 6500RPM / 60Sec = 108.3Revs Per Second / 2 = 54.15 spark events per cylinder X 12 = 649.8 spark events on a V12 per second. The coil(s) have to saturate and discharge that many times per second, and saturate with enough energy to jump a gap. The wider the gap the more energy required to jump it. If the compression is high, it makes it even HARDER to jump that gap. Add being lean and that adds an additional factor making it hard for spark to jump the gap.

You can see as those factors add up, the plug gap has to get narrower, OR the amount of energy available to each plug has to go up, and be available at high RPM. This is why 2 coils were used on the Lucas engine (to be able to saturate the coils with enough energy at 6500 RPM to keep up) and the coils were split into an A bank and B bank setup on the Marelli car. It was even spec'd at 0.25" for the later Nippondenso managed V12 with the EDIS coil packs. If one were going to be able to increase the gap, it would be by employing a Coil on Plug/Coil Near Plug system where each coil had near 720 degrees of available coil charging time to build up enough power to reliably jump a large gap at high RPM under the extra difficult HE engine specs.

~Paul K.

 

Listen,,,, I DONT know, so I have to ask... Are these number correct? Is that how they are or can be counted/calculated? 54 spark events per second per cylinder seems like a LOT...! Help me understand...

 

Jay,

They be spot on.

6500 rpm is waaaaaay out there for any engine, and a 12 cylinder is waaaaay out there anyway.

This is why we continually harp on about plug gap, HT lead integrity, EARTH point integrity, dizzy cap and rotor height (Lucas system), Rotor issues (Marelli system), and the list goes on.
That is why we also DRINK Jay, it soothes the nerves.

OK, not many owners ring the revs, Greg does, I do, maybe a FEW others. Most barely see 3000, for fear of breaking something, oops, DRINK SOME MORE and get into it.
I ran the Red beast in the NT (no speed limits back then), and I ran out of brave pills at 5800rpm, and have NO idea of the speedo, shiiit was zooming past the window at insane speeds, spouse was NOT impressed, and the damn thing still had us pinned to the seat, warm and fuzzies, YOU BET.

The V12 is one engine where "near enough is simple NOT good enough", but again, so few actually listen.

 

JJJ
A cylinder in a 4 stroke engine has a firing stroke every 2 revolutions of the crankshaft.
At 6000 RPM (ie 6000 revolutions of the crankshaft per minute) each cylinder will fire 3000 times (once per 2 revolutions).
Therefore as there are 12 cylinders in our engines, and each will require a spark 3000 times each minute, at 6000 RPM the ignition system has to generate a total number of sparks equalling:
3000 x 12 = 36,000 sparks per minute = 36,000/60 = 600 sparks per second = 600/12 = 50 sparks per plug per second.
The coils supply the sparks and so the coil switching system (the amplifier) has to switch on and off 600 times per second to generate 600 sparks per second at 6000 RPM. So the coils have to charge and discharge 600 times a second to supply the sparks.
Which means the coil only has 1/600th of a second to charge up enough to discharge an HT current sufficiently strong to jump the spark plug gap!
It is a miracle they run at all, isn't it!

 

I am literally BLOWN AWAY, and didn't have any idea of ALLLL or any of that...not at those rates anyways. Thanks for the explanation... I don't know what I'll do with it all (other than "know"), maybe WORRY a little more, lol...

My ignition amps are home on the "bench" (I wish it was a bench) waiting to be secured to the new fangled plates and bolted back in. Due to driving conditions here in the city I've kinda decided that Cherry Pie is going to be a early to late Spring,,, early Fall to late Fall kinda creeper... If it's only highway on a summer day, and not too hot, then too...

​​​​

 

I'll be there some day at high rpms. Especially in 1st gear 🙈

At least i have the drinking part is already covered🍺🍻

 

Since there are so many big brains here, I'll ask...

I have the Marelli and Marelli rotor. Should I be chopping down the steel post and doing the PVC fill routine on the underside of the rotor?

It's things like this that get me worrying some. I want to SEE the gaps in at least a few of my plugs to assure me that I did the right thing,,, when I did the gap thing. Cap is new. Rotor is the one I installed 3+yrs ago now - new at the time - but which brand I have NO idea.

Some of the CHEAP stuff I bought bringing the beast back to life was not top dollar (I'm like Grant here in OE ideas)... I mean, the car had no floors,,,, and and and and when I got her. Money WAS an object. I'm scratching my head, lol
She's happier today, though. Couldn't resist...

 

Hi Jay.
I chopped the post in my car when I got it. Just used a Dremel with a cutting disk and then filled the rotor arm. Works fine with no issues. Also replaced all the spark plugs and gapped to 0.25 again when I first got the car.
Rob.

 

I think its up to you. You already HAVE the most important thing. You KNOW about it. You'll be able to go.. hmm.... this isn't running right.... and know what to look for and what to fix. Its the owners that did not know who's car's burnt down.

The rest is kind of how do you want to handle it for your own nerves/maintenance beliefs.

~Paul K.