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!)

[Karl]

Another vote for AC/Delco 41-602.

I swapped mine earlier this year and gapped new plugs at .050. and the car seems to like it even better than the .035 gap.

Can't beat the price and they are good for 10k miles.

[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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[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.

[jss06c6]

Originally Posted by 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.

Marc, thanks very much for this tip!!

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[Hib Halverson]

Originally Posted by 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.

Actually, the IT20TT has been around for a number of years and, like Marc says it's a great choice to waste-spark ignition systems because it's a "iridium/platinum" plug.

The Denso 20 heat range is about the same as the NGK 6 and the old ACD 1 heat range. It's a good choice for LT5s with two exceptions: 1) it's too cold if the car is a show car which seldom sees aggressive driving and does experience frequent starts-without-warm-up and/or short trips. If that's the duty cycle, I'd use the IT16TT and 2) it's too hot for serious track use. For that you need the Denso 22 heat range and unfortunately there is no Denso IT22TT. If you're NGKm you need the 7 heat range.

When I owned Barney, I ran Denso IT22s (iridium tip) all the time and never had a problem other than more rapid errosion of the side electrode on the half of the cylinders which had negative polarity. I just changed those four plugs more often.

Lastly, someone said earlier that iridium plugs "run hotter". Please, post some facts which support that statement.