Bored with no work to do on my 88 XJS V12, so i indexed my spark plugs! (I didn’t feel like removing throttle tower, so i was not able to do 5A, 4B, 5B.) Two plugs had electrode pointing right at exhaust valve. Two were borderline, and the other five were fine. I had to buy four new plugs.
Well, i can tell the idle is better. My idle was fine before, no missing. But i was getting that intermittent mild stumble our cars in the mid80s are known for.
Throttle towers are a pain. A few years back I took mine out of my 82and cut off various sections, which are difficult to see when assembled, that let me get to the spark plugs much more easily without removing the to wee.
Merry Christmas all
Here are some pictures of the V12 throttle pedestals that I had modified at a local machine shop. Removing and replacing those rear bolts is now much easier. I made this modification to my wife’s 1990 XJ-S convertible and my 1990 V12 Vanden Plas.
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
9
Greg, you shoulda recorded the sound at the exhaust tips before and after, including with $100 bill.
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Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
10
Why would you even consider using copper washers of varying thickness? Wouldn’t that vary the compression ratio? I thought spark plug indexing was always done by swapping plugs around to get the electrodes lined up right.
1 Like
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
11
Y’know, the skeptics are gonna claim you replaced a bad plug.
Indexing plug washers is a standard practice, in “normal”, club racing engines.
The big boys likely use specially-made plugs.
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Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
13
Back in the day, racing involved those weird spark plugs with the gap down inside the recess and a fixed gap, the ground electrode was permanently installed in a drilled hole in one side. Do racers still use those? Anyone know why they were popular with the racing crowd?
I actually don’t have much of an issue getting out those four bolts that hold down the pedestal.
What i don’t like is all the other junk that comes with it like coil, and then always having to readjust both throttle links and throttle linkage to pedal/TPS.
Removing the pedestal messes up those little tweaks of precision that sometimes takes me two or three tries to get perfect.
Not sure of the design you reference: on 2-strokes, racers would sometimes use surface discharge plugs, to lessen the chance of a ‘whisker’ forming between the ground and center electrode.
EDIT: think I figured out the plug you’re referencing: they could certainly help in engines where piston crown-to-head/valve clearances were very tight, which was the main reason I would index plugs.
Having nothing better to do on Christmas eve, I just calculated it for a 6 liter V12 with 10:1 CR and 14 mm X 1.5 mm thread pitch plugs. Maximum “indexing” (1/2 turn) will change CR by 0.2%. This is much smaller than the % variation in compression among cylinders, as indicated for example on a compression gauge. So changing the compression by this tiny amount is just as likely to move a given cylinder closer to the average CR of all cylinders as it is to make it more of an outlier. IMHO.
Bill,
Thanks, but I can’t take any credit for the idea. I read about it here on Jag-Lovers and took the throttle pedestals into a local machine shop when I removed them for other work that I did on the engines.
Yes but the perfectly machined work looks better than free handed die grinding. I can’t count the number of times I have torn apart things to get to a problem. Only after all the things are reassembled do I see neat mods like this.
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
20
The plug I’m talking about doesn’t show up on that list. It’s closest to what they call a “Flat Ground Electrode” plug, but the ones I recall had the ground electrode pointing toward the side of the center electrode rather than over top of it.
