'91 #473 Plugs

[jss06c6]

After reading Roadster's write up on pulling and replacing plugs, I thought it would be good to do the same to #473.. Just turned 40K miles and have done a lot of tuning, but never put in a fresh set of plugs. I've ordered the NGK's, which should be here tomorrow or Tuesday. Pulled the old ones out to look at them. Not too bad, but interesting that #5 & #6 area a bit brown on the insulator, while the others are pretty clean, especially 1,2,3,4...

Having trouble again posting pictures.. I'll try as independent post..

Not too much trouble pulling #7 & #8, just used two universals and a 5/8" plug socket.. Mine all had never seize on them, so not hard to remove. Torque value was not consistent on all 8 however.. I'll correct that with the new plugs when they arrive.

[jss06c6]

no luck posting pictures. 800x600 jpg.. should not be an issue. I encounter this frequently on this forum.. not always, but more often than not..

Will try later..

[jss06c6]

Go figure. 5th time worked.. =D>

[jss06c6]

Gents,

I note that the AC-Delco's I took out have a compression gasket rather than a taper.. The NGK's have a taper seat.. Is there another Iridium with a compression gasket??

Steve

[Roadster]

Gents,

I note that the AC-Delco's I took out have a compression gasket rather than a taper.. The NGK's have a taper seat.. Is there another Iridium with a compression gasket??

Steve

Steve, here is a link to the showing a plug with the washer. Don't know what heat range it is, but it is listed for the 91 ZR-1. They also show the tapered seat plugs for the 91.
http://www.autopartswarehouse.com/sku/Chevrolet/Corvette/NGK/Spark_Plug/1991/ZR-1/NG5464.html
I think either style plug will work in any year LT5 head, IIRC what I have read..

My plugs look similar to ours, but you got more mileage out of yours....lol

[jss06c6]

Thanks! Looking at the plug seat in the head, it appears to be cut for compression gasket.. the ones you sent in the link look like the ones I removed. Hmmm.. don't want to install the wrong profile..

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[Roadster]

Thanks! Looking at the plug seat in the head, it appears to be cut for compression gasket.. the ones you sent in the link look like the ones I removed. Hmmm.. don't want to install the wrong profile..

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Here is a post from efnfast from 2 years ago with an email from
Marc Haibeck on this very issue....

"This was an e-mail from Marc H."

"For high load operation I recommend a spark plug that is one AC heat range cooler than the OE plug. The best spark plug is the NGK TR7IX. Part number 3690.

It has a tapered seat. All LT5's are machined for both tapered and flat seat plugs. Carefully open the gap to .050"

Best of luck.

Marc"

These are the plugs I am using in the 94, after today's ride, I am very satisfied with them. Although it is mentioned above, Hib also suggested this "colder" plug in recent posts. I took his suggestion, then found the above post at a later time. Glad I went with this plug....

[jss06c6]

Golden!

Thanks!!

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[Roadster]

Golden!

Thanks!!

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You are welcome sir!!!:)

[jss06c6]

Gents,

Completed the job with no issues. I used a 7" piece of vacuum hose to thread the new plugs into the heads. Then slipped a 5/8" plug socket (with the rubber retainer removed) into each plug seat to tighten to around 15 lbft. Only one you need to be careful with is good old #8. I held the plug socket against he valve cover and popped on the 3/8" drive universal joint (actually used two but really only needed one) then eased the socket down onto the plug. Worked like a charm. When recovering the socket, just reversed the process and held the socket against the valve cover, removed the universal and recovered the socket.

Fired up the engine. All good. Will take it out for a WOT pull when the weather clears here in Houston..

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[XfireZ51]

You should look at the ground straps to consider what heat range works for ur motor. I used NGK 7 (too cold), and used NGK 6 for quite a while, but have settled on NGK 5s.

[jss06c6]

I'll do so Dominic.. will get it out this weekend..

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[jss06c6]

Finally got the car out. Runs great. Might be imagination, but seems smoother. Pulls beautifully throughout the RPM band at WOT. Never skipped a beat. Very happy so far.

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[XfireZ51]

Finally got the car out. Runs great. Might be imagination, but seems smoother. Pulls beautifully throughout the RPM band at WOT. Never skipped a beat. Very happy so far.

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So which plugs did u settle on?

[jss06c6]

Still have the NGK 7's in. Will pull one later today to look at timing..

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[jss06c6]

Some pics of #160816082

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[XfireZ51]

Overall, looks good altho from what I can see of ground strap, I would say to try
The NGK 6s. Looks like it could use a little bit hotter plug. You want the color change on the strap to occur in mid "radius" of the strap. How many miles on the 7s?

[jss06c6]

Less than 10 miles..

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[Hib Halverson]

Back in the days when I owned "Barney" (1995 VIN 00140) I did a lot of testing with different NGK and Denso spark plugs.

The NGK 7 heat range works very well for street high-perfomrance. It is not a plug for cars which are never driven hard or are show cars only.

In the early '00s I switched to the Denso IT-22, because, at the time Denso was the only plug out there with an iridium tip and a cut-back side electrode. I ran the IT-22 after that and sold the car with a fresh set of them installed. I also converted all my other engines over to Denso...either the IT-20 or IT-22. Also, in the case of the LS7, I've done a lot of street testing with the IT-24 and it works well but I went back to the -22 on the street because the -22 has a projected tip where as the -24 does not.

For racing and chassis dyno work I used either the NGK 8 heat range or the Denso IT-24.

All LT5 cylinder heads where machined with a "universal spark plug seat" which accepts either flat seat or conical seat spark plugs, however, to use a conical seat plug in a pre-93 cylinder head, you need to do a one-time over torque of each plug to form the conical seat.

Torque to 22-25-lbs ft, then back off the plug and retighten to your normal torque setting.

The factory heat range is way hot for any type of sporting driving. This is because just before the car was released for sale the heat range spec was changed from the AC 1 heat range to the AC 2 heat range because of concerns about pre-delivery plug fouling.