Just kidding.

I would like to know if anyone has had a specific plug that makes a SUPER smooth idle? I don't care too much about power or economy, but I like to not be able to tell if it's running when I am at idle.

Should I go hotter/colder? platinum/copper? bigger gap/smaller?

Honest questions, I have to change out my plugs soon so I figured I would ask.

 

Aside from a plug being worn out or completely the wrong configuration or heat range, the brand/type/gap will make no difference with idle quality.

Look elsewhere.

 

I know that it is at least possible on a NA AJ27.

The problem is that it belongs to someone else and so
I have no idea of what plugs are in that particular engine.

I do know that it was always serviced at the dealer.

In general, I know that a slightly wider gap at the warmer
end of the allowable heat range is good for idle. That would
be a heat range 5 on the NGK and a gap of 0.40".

 

You'll never get a super smooth idle from a V8, as they aren't very well balanced. For really smooth idle, you need a straight 6, or a V12. A straight 6, or multiples thereof, is the only engine with perfect primary and secondary balance.

 

Short answer the ones Jaguar recommend - I've always ran NGK IFR5N10 (10 being the gap in millimeters) These happily go in 26/27/33 SC engines. Dunno about NA.
Lumpy idle could be coils not 100% fit or connected, they're well balanced units in my experience they shouldn't rock and roll at idle.

 

Thanks all, great answers! I'll look elsewhere and swap out the plugs to the recommended ones this weekend and report back.

 

I friend of mine buy it for me from Amazon.com ngk spark plugs
Here are all the model available
https://www.ngk.com/Automotive-NGK-c1411.aspx
I use BKR6EGP for my XJR with good results

 

I've been running some Bosch Iridium's that I picked up for about $11.00 a plug, haven't had an issue with them. Actually, I didn't have any issue with the OEM plugs that had over 100k mi either. Did the timing chain, so I figured I'd replace plugs too since I was in there.

 

I know this thread is specific to the X308 engine, but you might find it interesting that the 'best' plugs for the X300 AJ16 is the Champion RC12YC which is best found at your local Walmart garden center lawnmower section for about $2-3 a piece

This is a very basic plug, no iridium, platium, multi-condctor, etc ... Now you probably should replace it every 20k miles, but on the X300 it's a 15 minute job, and about $20.

Jaguar issued a TSB steering us that way for the X300.


Point being that the highest price and technology plug may not always be the best application and beware the marketing hype. Manufacturer recommendations and experience by owners may identify a plug that meets the need but doesn't break the wallet.

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