Iridium Plugs

[jss06c6]

This has been discussed in other posts, but wanted to add some pics and history. My '91 is ported, full SW exhaust (headers to tips) and tuned.

I installed NGK TR7ix plugs gapped at .050" about 2,500 miles ago. Had dead coil, so replaced it. Wires were good. Had a slight, but noticeable stumble at low rpm acceleration. Decided to pull plugs. What I found shocked me. Only 2 of the 8 plugs remained at .050. Others were about .058. However #7 and #8 were .068! As others have indicated, batch fire and iridium don't seem to like one another. Moved to NGK 4177 (TR6) copper core. Gapped at .045".

Engine is back to 100%. Picture of #7 attached..20201030_183430.jpg

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[Paul Workman]

Originally Posted by jss06c6

This has been discussed in other posts, but wanted to add some pics and history. My '91 is ported, full SW exhaust (headers to tips) and tuned.

I installed NGK TR7ix plugs gapped at .050" about 2,500 miles ago. Had dead coil, so replaced it. Wires were good. Had a slight, but noticeable stumble at low rpm acceleration. Decided to pull plugs. What I found shocked me. Only 2 of the 8 plugs remained at .050. Others were about .058. However #7 and #8 were .068! As others have indicated, batch fire and iridium don't seem to like one another. Moved to NGK 4177 (TR6) copper core. Gapped at .045".

Engine is back to 100%. Picture of #7 attached..Attachment 13329

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Interesting "head scratchier".

Pete Polatsidis put me onto AC/Delco 41-602 copper plugs some 8 yrs ago and I've never had a reason to switch (to another plug)! And, the fact that they're only about $2.00 ea. means I can install new plugs every season* (less than 3k miles or <4.8km) and gapped at 0.040" (1.016mm).

*Pete ran his ported 5.7L LT5 into the low 11s on those plugs too. What's not to like about inexpensive plugs and cheap enough one can run on essentially "new" plugs all the time (i.e., one isn't tempted to put up with deteriorated performance while trying to wring out the last 3-4k miles (~5-8km) on pricey rare-earth plugs...Just sayin!)

[efnfast]

Just checked my plugs (same plug as above), they had about 5,000 miles on them. I would say they were all within the margin of error.

[jss06c6]

Originally Posted by efnfast

Just checked my plugs (same plug as above), they had about 5,000 miles on them. I would say they were all within the margin of error.

AC Delco or NGK TR7ix?

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[efnfast]

NGK TR7ix. Sorry, I guess there were two "as above" plugs.

[XfireZ51]

So what heat range are you all running? I?ve read that Iridiums run a bit hotter than equivalent copper plugs. I?m trying out BKR7Es right now which SOTP seems to like. Also appears to have helped w random knock.

[jss06c6]

Originally Posted by XfireZ51

So what heat range are you all running? I?ve read that Iridiums run a bit hotter than equivalent copper plugs. I?m trying out BKR7Es right now which SOTP seems to like. Also appears to have helped w random knock.

Moved from TR7 to TR6.. the 7's were Iridium, the 6's I just put in are copper core. Dropped gap from .050 to .045. Engine running great again. After having my Bosch injectors cleaned and flow tested, I'm having to lean out VE table since they are flowing about 4% more fuel. The balance between the heads is much better! I matched primary and secondary to even out total flow across all 8 cylinders.

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[jss06c6]

Originally Posted by efnfast

Just checked my plugs (same plug as above), they had about 5,000 miles on them. I would say they were all within the margin of error.

Very interesting!

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[XfireZ51]

Steve,

I'm using the the FIC injectors which are supposed to be balanced to within
+- 1%. However, did not have them flow checked.

[Marc Haibeck]

Denso has recently released a spark plug that is specifically designed for wasted spark ignitions. It has a platinum tipped negative electrode. This addresses the erosion issue associated with the negative polarity spark that half of the cylinders have on wasted spark systems. In addition the negative electrode has a pointy tip that provides an excellent electric field.

The model number is IT20TT. Some parts catalogs only use the part number 4714. It is the same heat range as the OE spark plug and has a tapered seat which is a universal fit for all LT5s.

This is an expensive high mileage spark plug, good for at least 40k miles. I agree with Paul that the dirt cheep ACDelco 41-602 works great for 10k miles.