But you can't get any hotter than a 5, it is literally the hottest plug available.

I've had a quick look at other factory supercharged engines, they are running 6's or 7's as OEM

GM LSA = 6
Ford Miami (AUS) = 6
AMG (E55) = 6
Hellcat = 7

And it seems most people are going at least one step cooler from there once they start adding boost, tunes, etc...

Anyhow...

I've bought a set of 6's and 7's to try in the XJR, SteveK has bought a set of 7's for his XK.

So we are about to make a test on both naturally aspirated and supercharged Jag engine.

Watch this space.

 

The main reason could be to meet new Euro3 emission requirements from Y2K. Running as hot as possible plugs lowers HCC and CO. AJ27 has better knock control than AJ26, so Jag was able to take a small risk with hotter plugs.

 

That seems like an excellent , or at least more logical explanation .
Although did the Aston not need to meet the same standards ?


Cambo i am keen to hear what you find .
So far that was some good investigative research .

Shame to swap mine out thare only about 15000ks old.
But I see exactly what your on to .

 

It wasn't my idea, SteveK brought it to my attention

 

Nice spotting stevek.
I would have over looked that .
But it seems as though we're late to the party , with two people here already that fitted colder plugs to there's , I havnt seen this discussed before . so I'm thinking its old news that I somehow missed . Thanks all for bringing this to my attention .

 

The two guys in this thread who are running colder plugs are also running Twinscrew superchargers, funny that...

 

I worked at Jag as a engine management cal engineer when these engines were being developed (still do infact).


The reason for the "5" heat range plug was to meet sign off for plug foul in extreme cold temperatures. The decision to keep NA and SC engine the same was to prevent NA plugs going into a SC engine by mistake at the factory.


Hot plugs do not cause detonation, they cause pre ignition. Which is very hard to detect, and very destructive. The standard tune on 4.2 SC's (and the NA's for that matter) is tuned to not get the spark plugs to hot. There is an area of the ignition map that is deliberately retarded for this, it is set by using instrumented spark plugs. And in this area the plugs will get hot enough to cause engine failure levels of pre ignition way before any spark induced detonation occurs.
Obviously, anybody trying to tune one of these engines in the after market will not be able to measure this. So fitting colder plugs is absolutely the right way to go. And as others have said, the Aston 4.3 plug is a direct fit.


Cheers
Sean

 

Awesome, thank-you Sean.

 

I have searched but not able to find documentation where these parts are interchangeable. Perhaps it is only the gap difference of .040 v .044?

EDIT: Did you look at this NGK 5114 IFR7L-11, specs appear to be the same as the one you found other than the ground electrode is OE spec platinum instead of nickel.

By the way, I recall reading somewhere that you are not supposed to re-gap iridium plugs as the tips could be damaged, how would you compensate for the gap difference between OE spec of .039 and these replacements with a .044 gap?


Some interesting info on heat range from NGK:

Heat Range

The term Heat Range refers to the speed with which a plug can transfer heat from the combustion chamber to the engine head. Whether the plug is to be installed in a boat, lawnmower or race car, it has been found the optimum combustion chamber temperature for gasoline engines is between 500°C–850°C. Within that range it is cool enough to avoid pre-ignition and plug tip overheating (which can cause engine damage), while still hot enough to burn off combustion deposits that cause fouling.

The spark plug design determines its ability to remove heat from the combustion chamber. The primary method used to do this is by altering the internal length of the core nose. In addition, the alloy compositions in the electrodes can be changed. This means you may not be able to visually tell a difference between heat ranges.

- When a spark plug is referred to as a “cold plug”, it is one that transfers heat rapidly from the firing tip into the engine head, keeping the firing tip cooler.

- A “hot plug” has a much slower rate of heat transfer, which keeps the firing tip hotter.

An unaltered engine will run within the optimum operating range straight from the manufacturer, but if you make modifications such as adding a turbo or supercharger, increasing compression, timing changes, use of alternate fuels, or sustained use of nitrous oxide, these can alter the plug tip temperature, necessitating a colder plug.

A good rule of thumb: use one heat range colder for every 75–100hp added.

In identical spark plug types, the difference from one full heat range to the next is the ability to remove 70°C to 100°C from the combustion chamber.

The heat range numbering system used by spark plug manufacturers is not universal.

For example, a 10 heat range in Champion is not the same as a 10 heat range in NGK nor the same in Autolite.

Some manufacturers numbering systems are opposite the other - for Champion, Autolite and Bosch, the higher the number, the hotter the plug. For NGK, Denso and Pulstar, the higher the number, the colder the plug.

It is not recommended that you make spark plug changes at the same time as another engine modification, such as injection, carburetion or timing changes. Performing too many modifications or tune-ups at once will lead to misleading and inaccurate conclusions if any issues occur (an exception would be when the alternate plugs came as part of a single pre-calibrated upgrade kit).

When making spark plug heat range changes, it is better to err on the side of too cold a plug. Running too cold a plug can only cause it to foul out, whereas running too hot a plug can cause severe engine damage.

 

I'm actually running even a 9 range (denso IK01-27, their hottest one), but that requires some cleaning after some K of driving, so therefor I think the 7 would not be to far off from ideal on a 4.2 boosted (and more advanced ignition) setup.

 

....and you have working knock control all revs

 

You almost make me feel guilty ;-), but yes, it is a very nice insurance and I wouldn't be surprised if that has already saved me on some occasions.


I only now see there was a 2nd page, some good info added here!

The reason for me to go to the IK01 was not because of the colder heat range but more for the plug design with my extreme journey. I really wished they had it in 7 as that would have been my preferred choice, but it was the hottest they had in that type of plug, its not something I recommend for others!

 

Oops, typo, I it should have been 24 Denso heat range, and that is an 8 compared to NGK.

 

I can't believe it...





The Aston guys are gonna go nuts when they see this...

 

Wow and thanks for sharing. Off to order now.

 

OK bit of an update from my side.

I put the IFR7N-10's in the XJR a few days ago.

Firstly I want to talk about the plugs that came out, a couple of years back I put in a set of Denso SK16PR-A11 plugs, I was told at the time they were OEM equivalent, in hindsight I now know they aren't.

Denso 16 = NGK 5, so yes, same heat range as the stock IFR5N-10's

The problem is the gap.

The factory gap is specified as 0.9mm to 1.0mm (0.35" to 0.39")

The brand new IFR7N-10's were measured at ~0.9mm straight out of the box.

The Denso SK16PR-A11's I pulled out of the engine measured at ~1.15mm on average. So the gap was waaaaaay too big.

I had also bought a set of BKR7EIX-11 since they were recommended in this thread. They measured ~1.0mm out of the box. So basically at the upper limit of the specified range from the beginning. Put a few miles on them and the gap will grow. Actually the -11 in the part number is the giveaway. The gap is spec'd as 1.1mm, but when they are new they are 1.0mm, this allows for 0.1mm wear before the plug needs to be replaced.

And that makes sense compared to the IFR7N-10's as well, they measured 0.9mm new, and with wear up to 1.0mm would be within spec. Same as Jag's own spec of 0.9-1.0mm.

Too big a gap is bad. Especially in a forced induction engine. It also has the effect of retarding the ignition timing, puts an increased load on the ignition coils, and so on. Too big a gap is probably costing some power...

Something else to consider is the wear on the plugs that came out of the engine. Let's assume that they were originally ~1.0mm gap, same like the other -11's I measured. So the gap grew from ~1.0mm to ~1.15mm in about 15'000kms. That seems like a lot to me, considering that Iridium plugs are meant to be long-life and the actual service interval from Jag on plugs is 160'000kms / 100'000miles.

I don't like assuming, but let's assume that the modified tune in the car; leaner with more spark advance, has accellerated the wear on the sparkplugs. This was also exacerbated by putting in plugs with too big a gap from the beginning (1.0mm vs 0.9mm) and the heat range 5 was too hot for the conditions/tune.

In summary;

If you have any performance modifications on your AJ-V8; pulley, tune, etc then you really must look at the sparkplugs.

If you're running stock OEM plugs, consider the service period greatly reduced.
If you're running aftermarket "replacement" plugs pull them out and check the gaps!
If you're running a tune, pulley, etc then look at the plugs with a colder heat range AND the correct gap.

Spark plugs are important!

Now... I have not been to a dyno, or to the track, since changing the plugs, so i cannot give you any objective numbers, or confirm if there has been any gains in performance since swapping out the plugs.

However there is one thing which was really obvious after changing the plugs. The engine is now running MUCH smoother than before. What do I mean smoother? Well at idle before you could feel the engine shaking, lots of vibrations felt in the car. Now it's much smoother and you can barely feel the engine running. Same when cruising along, there is less vibration through the car than before. That in itself tells me it was not right before...

 

And this all got me thinking about the 5.0L engines, what's the story with the plugs on them?

And I found something interesting!

If we look at the X150 XKR with the 5.0L, there were different plugs specified for the regular 5.0L XKR, and the XKR-S.

XKR from B32753 to B45394 = AJ812146 = NGK ILKAR6C-10
XKR from B45395 onward = C2P23089 = ILKR6C-10

XKR-S from B32753 onward = C2P23089 = ILKR6C-10

The C2P23089 is the same plug used in the F-Type V8.

What i see from the various technical manuals is that the 5.0L plug ILKR6C-10 gap is defined as 1mm, no range given. This ILKR6C-10 is shown in the XKR manual, also the F-Type manual. Safe to assume that the gap range is going to be 0.9mm to 1.0mm, and they are probably 0.9mm straight out of the box.

Funny enough the 3.0L AJ126S is spec'd as SILZKAR7C-10S, gap 0.9-1.0 mm

So yeah! The AJ126S runs a colder plug (7) than the AJ133S (6).

It was mentioned in a thread here on the forum about someone putting a tune in an F-Type, and the tuner recommended going to a colder plug. I suppose that's going from a 6 to a 7. Hope they got the gap right!

Here is a 7 plug that looks to be the right one for the 5.0L https://www.ngk.com/product.aspx?zpid=9824
And apparently there is an 8 version which was used on the AMG SLS as an OEM part...

 

As I have mentioned in other threads, I replaced the plugs on the SuperV8 at around 60k miles. With new plugs the motor definitely ran smoother so 100k is not really a good rule to go by.

After further research and speaking with Motorsport professionals in my market, running a smaller pulley and a performance based tune requires plugs with a colder range AND a smaller than OE gap on forced induction engines. On some with higher boost they are gaping the iridium plugs as much as .015 smaller than OE spec. In light of this I settled upon the NGK 5794 with a heat range of 7 and a gap of .031-32. They'll be going in along with a few other modifications as soon as the car comes back from some other service work, hopefully in the next couple of weeks.

 

I looked into plugs with a smaller gap for the 4.2L as well.

One that came up as a recommendation was the IFR7X8G which has a specified gap 0.8mm, that's most probably going to be a 0.7mm gap straight out of the box, maybe 0.75mm. But it's also a 7 heat range.

The NGK 5794 = IFR7F-8DS which is different again... what the differences are I don't know off the top of my head... but from the part number it's also an 0.8mm spec gap.

Did you measure the gaps on these plugs straight out of the box by any chance?

I was told that too small a gap is also going to cause problems, although not as bad as too big...

 

Cambo,
You say my 2015 F-Type S already runs SILZKAR7C-10S plugs, ie heat range 7.
Are they plenty good enough still now that I have the VAP tune on?
What about if/when I buy and fit the larger crank pulley and then put the associated higher tune on, or should I then consider going to heat range 8 plugs?