I’m just wondering if cross firing can be an issue on these cars. When I put together HT leads for my V8s, I go to a fair bit of trouble to avoid the possibility of cross firing due to the proximity of leads that could be problematic. On my 140, I have a cardboard tube fixed to two head bolts that have the HT leads packed tightly together inside. This could easily be a source of cross firing - has anybody done anything different?
That’s the standard Jaguar set up, I never had any hint of cross firing oin my 3.8 ‘S’
The conduit has been used into the 70s, they would have rationalized if it was problematic. There were tufnol blocks that kept the wires at a distance, for racing, they look good. Arguably the prettiest solution is the wires coming in from the back of the valley, but that approach didn’t last long at jaguar. With good wires induction or leakage should not be a problem.
Thanks both. I’m thinking of using Moroso spiral wound wires (with the 123 distributor) which I’ve used in several V8 engines over the last 20 years with great success. I’d guess these would be less prone to cross-firing than the copper variety. I’m wondering if cross firing is an issue at all with a straight-six firing order? It would be good to keep the leads looking relatively stock.
It might theoretically happen but in the very worst case it would be about 130° advanced, so unlikely to ignite anything I think?
I have all original parts and even with the old plug wires (69…75) there was some blue light around the wires in total darkness and it ran smoothly. Now I have stranded copper wire as before, just new. No blue light, runs the same.
That was on XK120 from engine W1001 to W6696 with the horizontal outlet distributor cap. The plug wires ran under the carbs and up into the back of the valley, so you don’t have them draped over the cam cover. They used the same fiberboard conduit cover, reversed and attached to two studs on the left hand side, or in some cases no conduit at all, just rubber o-rings capturing the wires together. The coil was also mounted on the block below the front carb.
The reason why they changed to the vertical outlet distributor cap is unclear, possibly related to the Mark VII which never had the horizontal, or the addition of a heater to the XK120 FHC.
Mine is W3962 so in theory I should have the horizontal, but I have learned through bitter experience that putting the coil under the carb is a bad idea when you’re stuck late at night in a McDonalds parking lot trying to diagnose a dead ignition system.
Apparently in the early days the parallel wires look was a popular idea among hot rodders.
and even on the D-Type, though this may have been for changing plugs in a hurry during a race.
Anyway, cross firing has never been a problem for me.
Roger:
I would echo the comments of Rob and others, after some 60 plus years of running an XK120 I have never experienced a cross firing problem with the wires in the conduit.
EXACTLY…
I must say, I find all this comment looking for a solution to a cross-firing problem that simply does not exist to be somewhat bemusing…
If there was any problem at all with the XK engines HT leads layout through the card conduit then Jaguar would have fixed it before the first XK engine was dyno tested and fitted to a car… But no, they subsequently built hundreds of thousands of XK engines with this HT lead layout with no apparent problems at all…
If anyone has a problem now, maybe they have not replaced their original HT leads, or the connections properly, or are not using satisfactory HT leads…
And I think you will find the racing XK engine idea of a holder to separate all the six leads is a convenient method for easy maintenance in racing environment, rather than an issue with cross firing…
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…which has answered my question, thanks all!
This is the first XK engine I have owned, so having followed advice on some other marques to keep certain HT leads separated for this reason, I now know that it is not an issue on the XK.
On my '50 OTS, I run my plug wires through a long bakelite/ phenolic tube underneath the carbs to the back of the engine where they then come up and are then routed forward through the cylinder head valley. Never had issues with cross firing. The tube keeps engine oil and possibly fuel from carbs off of the wires and makes for a cleaner look – not that one can see it from above. For maintenance/ service in that area, I’ve gotten used to removing the right front wheel and inner splash shield on that side. A set up that’s not for everybody, but it seems to work for me. Plus, I can easily get in there with a rag and wipe that side of the engine clean. Yeah, I’m a bit fussy that way.