NGK Iridium plugs Heat Range 6 after 25k miles

[northernlights]

In Oct 2012 I installed NGK 6418 Iridium plugs (BKR6EIX) in my 1994 850 Turbo. This is what they look like as of today, 25k miles later, in decending order of cylinder 1 to 5. I replaced them with an identical new set.











I only run 93 octane of no brand in particular. The deposits are heavier than I expected them to be, and notice how the corners of the ground strap (as best seen on #4) have eroded away a bit, but otherwise the plugs look good. Because the deposits are consistently on one side of each plug I suspect it is the side pointing at the intake valves as that part would run a bit cooler from fuel impingement.

These are a NGK heat range 6, which they recommend for nearly all Volvo turbo 5 cylinders, and almost all Subaru turbos for that matter. Given the fact that they do not appear to be cooked I suspect going to a cooler 7 heat range is unnecessary in anything remotely close to a stock engine. Keep that in mind when someone tries to convince you that you need colder plugs after any modification.

On this car, major modifications as of now are a 628 (850R) ECU and a Simons Sport exhaust.

I mention the heat range aspect because cold plugs invariably will run a bit rougher under light loads, which is where we drive most of the time. As setup, this engine has a consistently smooth idle and is good for about 24 mpg in 'mixed' driving. The Simons exhaust gave me almost a 1.5 mpg increase. I did not expect this, but it was a nice surprise.

(ADDENDUM: In all fairness, I also did replace the compressor bypass valve at about the same time, but I would be surprised if that was as much of a contributor as the exhaust.)

[matthew1]

Interesting post, thanks NL.

Did you check the gap on those? Did it hold?

FWIW, Lucky recommends closing the gap to .025" for chipped/tuned cars.

[northernlights]

A quick check on the old plugs shows the gaps to be between 0.031-0.034, so they are close. The NGK spec is 0.8mm (about 0.0315). Due to the fragile nature of the center electrode on the Iridiums (I believe it is only 0.6 mm diameter) I leave them alone before installation and assume they are right!

[erikv11]

Good stuff, educate me please ... these plugs seem expensive from a quick search, and are only lasting 25k, is there strong motivation to go with them over the cheaper Bosch platinum-iridiums that Volvo brands up and sells as OEM?

Not criticizing at all, just want to know your thoughts on this!

[jimmy57]

If they used sturdier ground electrodes they'd have a great plug.

[northernlights]

They are definitely more expensive than standard plugs, but hell, a Grande Latte probably costs the same as one plug, and it only lasts 15 minutes!

The car was running fine when I replaced the NGKs, so the 25k is more of a reference point. When I originally installed them, I wasn't sure what heat range I wanted to use because the NGK '6' is perceived by many to be a little warm.

So after 25k miles, I thought it was time to check. I bought another set of NGK 6418's, plus a set of Champion 9801 Iridiums (RC8WYPB3) as back-ups in case the NGKs looked bad when I pulled them. The Champion '8' comes up on cross-reference charts as a hair cooler than the NGK '6', and it's close to the now unobtainable Champion RC7GYC.

The NGK's didn't look bad, so in went the new set.

This is what I used for reference for heat ranges - it's from Denso.



There are all kinds of rules-of-thumb about when one needs to go colder, but most of them are very general and based on carbureted V8s. I don't think it's enough to divide by cylinders to try to correct the ROT's - I think you need some kind of specific output value like hp per cylinder * liter.

I looked at some older turbo cars for inspiration of what the manufacturers felt comfortable selling with a warranty.

The WRXs seem to consistantly use NGK '6' while the Mitsu EVOs use NGK '7' heat ranges. The old WRXs were rated at 227 hp with a 2.0 liter four, or about 114 hp/cyl*liter. The earlier USA market EVO's were 271 hp for the same displacement and number of cylinders or about 136 hp/cyl*liter.

Take a B5234 at 240 hp (for the 850R), wave your hands a little, apply the same ratios, and you can calculate 104 hp/cyl*liter. This is closer to the WRX (a '6') than the EVO (a '7'). At the same specific output as the WRX, the B5234 could produce about 260 hp for that NGK '6' plug, and about 310 hp to be at EVO NGK '7' levels. Those seem pretty reasonable numbers to me, especially since most of the mild tunes (like from IPD) do not advise colder plugs out of the gate. Maybe somewhere in the middle it's time to consider a colder plug?

While this may seem like a stretch, think about what happens at the limits. Take a 2.3 liter 4 cylinder and a 2.3 liter 5 cylinder. If they make the same power, then they are probably generating about the same amount of heat. But with 5 cylinders instead of 4, each individual plug is exposed to only 20% of the total heat of combustion instead of 25%. That probably makes the plug's life easier in the 5 cylinder engine.

[cn90]

I use the same plug NGK 6418 Iridium plugs (BKR6EIX) in my 1998 BMW 528i, the car runs very smooth, better than stock Platinum plugs.

My 2007 Honda Odyssey van comes from factory with Iridium plugs and the owners manual says "replace the spark plugs every 105K".

So, I think when you use Iridium plugs, you don't need to replace them every 25K. I think you can keep it for 90K-110K or so. This way you save money for more Grande Latte lol...

[cn90]

From what I understand, Denso Iridium is 0.4mm tip, and NKG Iridium is 0.7mm tip.
In my BMW, I use NGK Iridium and the engine runs very smoothly.

I am thinking using NGK Iridium for my Volvo at the next spark plug change.

Below is info published by Denso showing Iridium requiring less Voltage for spark to happen:

gi1.jpg (22.12 KiB) Viewed 4870 times

Graph applies to Denso Ultra-fine wire Iridium spark plugs

[cn90]

Some good readings on spark plug, see attached pdf by NGK.

Some comments:

1. NGK says don't use anti-seize for fear of dmagae to thread.
I use it because I don't want to deal with galling, seizing issues. Zero problems with antiseize for me.
In an experiment found on the web, someone took an old Ford engine and torque the heck out of it. At 100 ft-lb, the spark plug broke where the porcelain joins the spark plug threaded portion.
Yet the cylinder head threads remained intact.
So at 16-20 ft-lb range, you will not damage the cylinder head threads.
In contrast, the #1 cause for thread damage is from cross-threading!

2. Even for Iridium plug, one can adjust the gap, just be careful not to touch the Iridium tip.
Go very very slowly when measuring the gap.
However, in my experience with my 07 Honda Odyssey van, when factory Iridium spark plug was removed at 50K, the gap was still within spec! In contrast, "copper" plug will have a bigger gap at 50K.

3. Copper vs Platinum vs Iridium: all spark plugs have a copper core for conducting current.
The so-called "copper" plugs have tip made from Nickle Alloy, not copper!
Platinum has Platinum tip.
Iridium has Iridium tip.

[epukerob]

Regarding Bosch Platinum plugs, I don't like 'em. I still experience occasional high-speed misfire with them, setting that pesky P0300, P0302, etc. I started removing and inspecting them and found the little screw-on caps which the plug wire snaps onto will vibrate loose, and I notice also some manner of black residue forming there. Clean them up, re-tighten the little post caps, and the cars runs like a champ... for awhile that is. So, I will go thru this procedure again tomorrow with a cold engine ... I bet I know what I will find!