When I went to change the spark plugs in my 1997 XJR, I wound up on a bit of a journey to find the proper currently available plug to replace what Jaguar originally specified. Here is what I learned:

• Champion RC89PYC - original specification for the engine. The "P" in the name of this plug indicates platinum. Somewhere along that way, Champion discontinued this plug.
• Champion RC12YCC - appears to be a common replacement for the RC89PYC. This is what was in my engine when I bought the car. It's not platinum. I cannot find any manufacture information that shows this to be a proper replacement for the RC89PYC, although it's a popular choice with many XJ owners. The double copper indicated by "CC" in the name, is no longer available and Champion indicates that the RC12YC is the proper replacement for the "CC" version.
• Champion 344 (aka RC9YC) is listed as an OEM equivalent for my engine. This is a copper plug. I have this installed currently and the engine ran significantly smoother than with the RC12YCC that they replaced. I think it had more to do with the condition of the plugs than a material difference in the plug specifications.
• Champion 3344 (aka RC9PYC) is listed an OEM equivalent for my engine. This is a platinum plug that I believe is the truly proper replacement for the originally specified RC89PYC.

The plugs I have in there now seem to work well. As I looked more into the difference between copper and platinum plugs, I wondered if using copper where Jaguar calls for platinum could cause me a problem longer term. Copper plugs require more voltage to make a spark and they are not as hot as a platinum plug. Could this stress the coils? I am doubtful as I believe the coil cannot increase voltage supplied to the spark plug based on anything about the spark plug. I think it supplies the voltage it is designed to send and the ECU may de-tune the engine a bit to accommodate the difference in spark. Regarding the heat, the lower heat with the copper seems like if may be an issue relative to carbon build up. I suspect this may be a real issue based on the RC12YCC plugs I recently removed.

I clearly have too much time on my hands as I consider these things. However, I am curious if anyone else has looked into this and may have info to share.

Does anyone else agree that the RC9PYC is the correct plug for this engine and is the equivalent of what Jaguar originally specified for these engines?

 

There is a balance in the current flowing through the coils and plug gap resisting the current and the heat generation related to the coils life and the quantity of spark

The different lower intake gas compression between the supercharged and engine load effects this

Platinum plugs are higher resistance , I didn't find that out until a loaded down dynameter on a different car, external carbon tracking

Here at the Center for the Institute of Ponderance I have been wondering about the 3 prong plugs

 

I am running the champion rc12yc and the car mostly runs well.
I checked my plugs just yesterday due to other issues and they look good.

The car idles smooth for the most part but occasionally it will sort of jolt a little,p. Generally not noticeable on the tacho but you can feel or see a shudder in the block. Only ever happens at idle and is very minor,

I have wondered if changing to a different plug might alleviate this.

 

There were many previous discussions on spark plugs, and I think the main points the majority agreed are
(a) the AJI6 is very sensitive to spark plugs and there isn't much advantage using platinum plugs;
(b) the purists would stick with the factory's recommendation, i.e. use Champion RC12YC as RC12YCC are no longer available; the older bulletin quoted RC9YC which are still used among non-supercharged engines; and
(c) the pragmatists prefer NGK BKR6E or BKR7E plugs (the 6's series is a hotter plug) as NGKs are probably more readily available elsewhere.

I am with the latter group because NGK plugs are easy to find in Hong Kong, and I had a horrible experience with a Champion plug disintegrating inside the well and destroying my coil-on-plug (unfortunately my then new, and hard-to-find coil pack of the Diamond brand manufactured in Japan (see pictures)

 

Wow, I hate seeing that. My point is that the original spec from Jaguar for the 1997 XJR is a platinum plug, not copper. The RC12YC is copper. I am not sure what was specified for 95-96 XJR, platinum or copper. For the record, I have had excellent results with NGK products in other cars and motorcycles. My inclination is to always stick with as close to OEM as I can get.

Consequently, that led me on this somewhat maddening pursuit of what is the OEM equivalent for a 97 XJR. At this point, I believe that to be platinum and Champion since that is what Jaguar specified. As far as I can tell, the closest spark plug to the original and no longer available RC89PYC is the RC9PYC (aka 3344).

Here is what Champion shows as being compatible, with the copper RC12YC. The last column indicates VIN.

Here is what they show for platinum RC9PYC. The last column indicates VIN.

They show similar information for the iridium, double platinum, etc for both types of plugs. It's confusing and as contradictory as can be. Here is a link to the Champion website in case anyone else has too much time on their hands and would like to jump into this: Champion Spark Plugs

In summary, the copper RC12YC does not show as compatible with the 95-97 XJR. I am going to replace the copper RC9YC that I recently installed with the RC9PYC. I will update everyone on anything notable that I find. It would be great to hear from others on their experiences along with what others have already mentioned. It is hard to know what is truly correct here.

 

Might try the ngks in mine