I searched my owners manual but could not find what spark plug the factory recommends. I also checked a chain autoparts website and found about 10 different options. Finally, I searched the forum and found only a few comments about plugs, but nothing really specific about what people use. I suspect this will open up a can of worms like discussions about synthetic vs Dino oil, or the proper transmission fluid, but I think those discussions are (generally) healthy.

So, can anyone tell me what the factory uses, and what you have successfully or unsuccessfully used in the xk8. I will say that with some cars I have owned in the past, I honestly have had issues using certain brands instead of others, even though the ones that didn't work well for me were spec'ed correctly. 2 Fords I owned needed Autolite, as the cars didn't run well with NGK. But other cars I have owned in the past were ok with anything.

Just looking for real world proven successes etc. Thank you in advance.

 

Mine had NGK iridiums in it. That's what is specified in the spec manual in our FAQ thread.

 

To be honest with you, the reason most cars run good with certain plugs is that they are tailor made for the car. What I mean by that is that when the plug is tightened down, the electrode points to the center of the cylinder, you might have read about it in another term, it is called indexing, where you buy 20-30 plugs and mark the elctrode on the plug itself with a magic marker, then tighten the plug, if it the mark points torward the center you leave it, else you put in another plug and see if that works. Delco used to make plugs specifcally for the car or head was tapped just for the plug, that is a rumor that I have heard over and over, autolites may be the same way. I have used NGK's in my vette, T5R and both of my daughters cars with good look. I was about to put them in my XK, but may not now.
One thing I do know, if you over tighten or leave to loose, a car will not run good, especially if the plug is loose.

 

Just installed NGK Iridium- BKR5E1X-11 at 96K miles, original NGK's were in it from factory. Original Plugs were NGK - PFR5G-13E and at 96K were in pretty good shape, with no indication of fouling or oil. Got an immediate 3-4 MPG and a little more kick in the pants. Had looked at the Forums and talked to NGK Tech Support (they were no help at all) about the -11, which is a 0.044" Gap. Seems that the some persons were saying to use a .035 - .039 Gap. But couldn't get a decisive answer anywhere. So went with the -11 Gap. Only thing that I had wanted to do was maybe change the Heat Range down from a 5 to a 6 because of the High Engine Temps. But again no info on the Forums and/or help out of NGK, so went the the 5 Heat Range from the original Plugs. All in all, a good plug, increased mileage and response. Price before Sales Tax was in the $57 range for the set of 8, in stock (no shipping charges) at NAPA.