Anyone know the difference? NGK said the later was the correct replacement, but the ones I pulled out were the former.

I purchased the later at Advance for 40% off, the former seems to only be available online.

Also what do you think of the condition of the plugs?

I used a bit of aluminum anti-seize when replacing.

 

Plugs are good no worries, but the IFR ones (used stock by Jaguar) have a platinum tip in the ground electrode, which makes it also more expensive.

 

It also appears there is a gap difference, IFR is .040 & IX is .044... does that matter?

I spoke with NGK and they seemed to think it mattered also the platinum ground gives them the 100K mile life whereas the IX has a 50K mile life.

Unfortunately the IFR is not readily available locally but I did find a Bosch 9607 iridium & platinum, $36 for a set of 8 less $32 rebate = $.50 each! Bosch says these are .044 and are NOT adjustable but are within spec for the car.

Hmmmmm....

 

0.04 is a very small difference, and at least on the supercharged cars I have never experienced issues with 0.44 gap.

The IFR is very expensive, you could change the IX twice for that price and at least get a chance to read the plugs after 50Kmiles.

Have no experience with the Bosch plugs though.

 

I use the BKR plugs in both cars, they are cheaper and I would never leave plugs in for 100K anyway. The original plugs were the more costly ones. When I changed the plugs at 60 K in the XJ8, they were just beginning to corrode-seize in the heads so I'm glad I didn't wait 100K! I changed the plugs in the XKR at 50K as that is what is called for, and they were doing okay.

 

Well, I have to admit, I am a bit stuck on the removal of the LH coils on the super charged motor. No issue with the first one behind the dipstick, but the other 3 are blocked by several hoses and the furthest one is so close the the wall of the brake booster chamber I don't see how it can even come out.

The RH one were very easy, BTW.

Anyone perform this service?

 

Never mind, put a little more effort and was able to knock it out faster than expected.

I ended up going with Bosch.

By the way the LH plugs were wet with dark brown oil and a bit of black carbon.

 

Doesn't sound right. They all should look about the same.
Thinking this might be a telltale on why your mileage is suffering.
The dealer that did the throttle body (start of your mileage trouble) might want to look into this wet plug deal.
Just thinking out loud.

 

With the new plugs, the motor is running about 100 RPM higher at idle than it did before...

 

Good.
After a few cycles, the ecu should retrim the fueling and idle speed will go back to normal.
The rpm increase does show that the old plugs were on their last legs.
Does your car burn any oil?
I'm sure as young as it is, the plug wires and coils should be fine. Hopefully your mileage will improve.
Test it right away
Good luck.

 

Here's the other set...

 

Insulators I can see look pretty white. I'd like to see a little tan...

 

They are not white at all, plenty of tan deposits and black carbon