That's a good question. I don't know the answer.

The general consensus about colder plugs and tuning is that you "should" go one or two heat ranges cooler when upping boost, adding timing, etc...

But it's interesting for me that the AJ126S is already running a 7, when the AJ133S is running a 6. But this kind of makes sense when you look at how much power the V6 makes vs. the V8, when you start comparing hp/litre, hp/cylinder, cylinder sizes, etc...

I'd try to find a compatible plug with heat range 8 first, then worry about it.

There are actually two plugs listed for the AJ126S, it varies by VIN range.

AJ812988, up to K05405 in the F-Type, up to U04717 in the X250 XF
C2Z28434 from K05406 in the F-Type, from U04718 in the X250 XF

AJ812988 is the SILZKAR7C10S

C2Z28434 is SILZKAR7E8S according to what's under the label of the OEM box.



So it would appear that the C2Z28434 is actually a 0.8mm gapped plug?

And JLR changed the spec of the plug not too long after introducing the engine... there must be a reason why...

But there are some part numbers to cross check for a heat range 8 plug...
EDIT, here this looks about right https://www.ngk.com/product.aspx?zpid=9718

 

I don't have any feeler gauges at the moment but once I get a set I will measure them, carefully. I don't yet have the car back from service, hopefully in the next week and a half.

From what I have been told, the higher the boost, the smaller the gap as it becomes more difficult for a good spark to form. One of the tuners builds custom Ford motors with Eaton blowers and runs gaps and of.020-.023

 

Update: Wanted a bit colder plugs and smaller gap: BKR8EIX , 0.8mm .

 

Okay picked up a set of gauges and measured the gap on 2 of the plugs. The result was the same on both, .711mm or .028 inches, not the .030 or .8mm as listed in the specs.

 

Makes sense, the 1.0mm were a narrower gap straight out of the box, so the 0.8mm at 0.711mm is not surprising.

Thanks for checking.

 

Installed the new NGK 5794 IFR7F-8DS plugs today. Not too difficult and much easier than the SuperV8. I couple of the plugs required a bit of force to break free, with the new ones I applied a bit of aluminum based anti-seize. I took pictures of one of the removed plugs, overall at 54K miles, they appear to be in very good condition and were probably serviceable for quite a bit longer.

Car started up fine with no issues and seems to run perhaps just a little smoother, but really I had no issues before so it is hard to tell. It will be interesting to see how the car performs on a few spirited drives, high speed, high revs, etc.

 

Are those NGKs not ones which already have an anti-seize coating and which NGK say not to add anything?

 

They are both NGKs. As mentioned prior, I did use a bit of aluminum based anti-seize. I have never used anti-seize before but I also have never had to change plugs which I have already changed. I was not aware use of such a product was prohibited but I did not over torque them, tightened them by hand then just a bit more, perhaps an 1/8th to 1/4 turn with the wrench - hope they are tight enough.

 

Should note the original factory plugs measured .889mm or .035 inches after 54K miles of use, the new ones as mentioned above are at .711mm or .028 inches.

 

I just have cheap copper NGKs that are one step colder, if I remember correctly gaped to .035

 

Cooper is ok of course (maybe you need to gap it smaller), but it also wares quicker then the iridium ones, so you need to change them more often to ensure a good spark and less stress on the coils.

Attached is an interesting spark plug test on a lightning engine done within a lab (at least that was what was written next to it).

 

Interesting, although I also read somewhere that cooper were actually a better conductor and they were less prone to detonation, the downside was only that they didnt last long

 

What I learned is that it’s not just conductivity (iridium plugs also have copper cores), it’s also the sharpness of the electrodes and the size of the main electrode. As copper is softer it wears quicker reducing the sharp edges (i.e. rounding them off), making it harder for the plug to fire. Especially for a boosted setup it will become gradually harder to fire (higher voltage requirement), which can reduce the lifetime of coil but more important you risk more misfires. I don’t have the graph of how quickly copper wears so when it needs to be replaced though.

Considering all what’s written about iridium being able to fire much better under harder conditions (leaner mixtures, higher boost) lasting much longer as well, I can’t see any reason for going back to a copper plug.

 

The sharpness is critical because it is what assists the initial flow of electricity (same for any high voltage use).

It's one of the reasons why there are warnings never to touch an iridium plug electrode with anything metal (especially not feeler gauges etc).

 

FYI, from NGKs own website, the truth about "copper" plugs...

5.“Copper plugs”

“Copper spark plugs” is a term mistakenly used for a standard material spark plug. A standard material spark plug traditionally uses a nickel-alloy outer material fused to a copper core. Almost all spark plugs use a copper core center to conduct the electricity, jump the gap, and promote heat dissipation. However, as an outer electrode material, copper would not be a good choice, as it is soft and has a low melting point (resulting in a plug that would last minutes, not miles). Nearly all NGK spark plugs, including precious metals iridium and platinum, have a copper core. When one talks in terms of nickel alloys, platinum and iridium, one is referring to its durability, or how long a spark plug will last before it needs to be replaced. However, when one talks about copper, he or she is referring to its ability to conduct electricity that is needed to fire across the gap and ignite the air-fuel mixture.

 

The comments on the self cleaning temperature- and reaching it- being a factor are along the right lines along with some materials having a wider operable heat range- ie reaching the self cleaning temperature quicker but still being cold enough plugs to not cause fouling. That's the other end of the equation: Not only is the heat range of plugs decided upon by looking at knock limits at WOT and the like (boosted engines are typically knock limited even less than WOT) but the vehicle is put through a 'city type' drive cycle and tested for fouling. The cycles on file for the Aston Martin and the Jaguar may be different and also bear in mind that the transmission and gearing will have some effect on this.


Think about what kind of driving you predominantly do and make your decision. I'm finding here in the USA I do ALOT more dual carriageway and Motorway driving- steady state cruising and very little chockablock traffic. London and the UK was bad for that in general as was Southern California. A colder plug will tend to mitigate knock better but may increase the propensity for plug fouling. Can this be mitigated with iridium or platinum?


Cambo what do the Lincoln LS cars use- and whats the equivalent heat ranges in Denso and NGK language? The reason I ask is because the Lincoln LS (close your snobbish ears- Aston drivers) uses the same coil packs as the Lincoln LS8s....(and early X200 S types)...

 

If you're the Sean I think you are- How are all your yank tanks?

 

I had a quick look at the sparkplugs offered for the Lincoln LS 3.9, seem to be the same NGK 5 heat range as the Jag plug, and so far as I can tell the "10" gap which would also be the same.

There is a Motorcraft plug offered for the Lincolns (no surprise really).

I notice the same problem with plugs on offer in the aftermarket for the Lincolns, they have the gap too wide at "11" which is a 1.1mm gap, but the factory spec is a "10" or 1.0mm gap, which in reality would be 0.9-1.0mm, or ~0.9mm straight out of the box.

 

My 4.2 XKR is showing about 92k miles now so almost due for new plugs (according to the service schedule). It's not tuned in any way and although I have no plans to do so right now I'm not ruling it out. Wondering if I should go for a grade or two cooler plug for future proofing? Any benefit to this on a stock engine and tune?

 

Unless you live in a hot climate, there's probably no benefit running colder plugs on a stock car.