Spark Plugs Again

[MIZR1]

I did a search on spark plugs to see what recommendations you experts have on plugs and gap for a 91 ZR-1 with no mods other than Borla and a K&N filter, but it seems that the latest posts were a couple of years old. So, as time and technology marches on, I was wondering if y'all could share your recommendations on plugs and gap. I was looking at NGK iridium plugs but don't want to spend the extra money if they are really just a gimmick. Any info will be appreciated.

 

[Hib Halverson]

I've used the following in my LT5 with good results.

In the past, NGK BKR6EP-11 or AC Rapidfire #9

Currently, the Denso IT-22.
Of those three, the Denso seems about the best.

Iridium plugs are not a scam. Evidence of that is that Denso iridium-tipped plugs have been OE on Corvettes since 03. I can speak to the NCK iridiums, because I haven't used them, but I'll guess that they work ok.

 

[MIZR1]

Thanks for the reply, Hib. A couple of follow up questions. I looked up my 91 ZR-1 on the Denso site and it identified an IT16 gapped at .06. Any idea why their recommendation is so different from yours? Second, the Denso plugs are very expensive. Is there any reason to suspect the NGK iridium would substantially underperform the Denso? Finally, if I get the NGK's, what gap would you recommend for a '91? Thanks in advance for all your assistance.

 

[Hib Halverson]

Thanks for the reply, Hib. A couple of follow up questions. I looked up my 91 ZR-1 on the Denso site and it identified an IT16 gapped at .06. Any idea why their recommendation is so different from yours?

Great question!
A lot of the aftermarket's plug recomendations for LT5s are based not on actual testing but on engineering decisions which build on what AC Delco originally made for the car. For instance, Brand XXX spark plug company, looks at old ACD catalogs and finds it listed the FR2LS as the stock plug. They look at their spark plug designs and decide that their part number ZZ!!** plug will work because it has the same heat range and design so they recommend it. Their engineers might add some different design features but the heat range and physical dimensions of the plug usualy mimic what GM starts with.

Add to that the fact that the FR2LS was not the original LT5 plug...that was the FR1LS...and was changed one range hotter just before the start of MY90 production due to GM's fear of LT5s fouling plugs at dealers after pre-delivery starts without warm-ups.

The bottom line is that virtually all spark plug recommendations for LT5s are, IMO, too hot unless the duty cycle is exclusively easy street driving and includes starts without warm ups. Problem is, there is a fairly significant faction of the ZR1 owner community that don't drive their cars like that. They drive them hard.

I've always run a plug at least one range and usually two ranges colder than stock. I ran the NGK BKR6EP-11, one range colder, when I was testing aftermarket double-platinums. I ran the AC RF#9 for along time. Now I use the Denso IT22 which is two ranges colder. It works very well but again, I don't drive my car easy and seldom, if ever, subject it to starts wiithout warm-ups.

In retrospect, the IT22 is perhaps a bit cold for anything but real aggressive driving.

If you're not a constant leadfoot, maybe the IT20 would be a better choice. It's still one range colder and will work well in a more typical, street high-performance duty cycle.

However, if your ZR-1 is a show car, if it's driven easy on the street or sees a lot of starts without warm-ups, I'd use the IT18.

Second, the Denso plugs are very expensive.

Unfortunately, that is the case. I think that's because of the cost of making them. Denso's approach to iridium is based, first, in engineering---several years ago, Denso's technical staff submitted a very interesting SAE Paper about iridium use in plugs...and only, secondly, based in marketing. I should add that the O.E.s are jumping on the iridium bandwagon and more often than not, that tends to lend credibility to a technology, especially one that's a bit costly.

Is there any reason to suspect the NGK iridium would substantially underperform the Denso? Finally, if I get the NGK's, what gap would you recommend for a '91? Thanks in advance for all your assistance.

I can't answer that as I have not used the NGK iridium spark plugs. My gut feeling is no, it would not "substantially underperform" the Denso. If they underperform, it will be incrementally.

 

[MIZR1]

Wow, Hib, thanks for putting so much effort into a very informative reply. I really appreciate the help.

 

[Hib Halverson]

You're welcome.
Glad to be of service.

 

[tomtom72]

I second the thanks!

I would like to also thank Mr. H for the response. I truly learned something that I likely would have missed completely! That's one of the nice aspects of CAC, the information exchange is priceless! to Hib!

 

[srthom13]

Mr H, Thanks for the info and as usual very informative. I see you have a 71 BB. What plugs would you recommend and/or use in your BB? And at what heat range and gap? It has been my experience with the BB that it will usually foul plugs within 3500 miles. A visual of the plugs reveal no obvious signs of deteriation - so what actually is wrong with the plug to make it foul? This fouling was consistant with all name brands. I have not used Denso plugs, therefore I can not comment on how or when they will foul. Even if you were able to get 10,000 miles out of the Denso is it worth the expense. Is their performance, not longevity, that much better and noticeable than the regular name brand plugs? Also, I was under the impression that for most normal and semi agressive driving it was better to go one heat range higher. In my case it would mean using the AC45 as opposed to the manual listed AC43. Thanks, Steve

 

[Hib Halverson]

If you have a BBC which fouls plugs in 3500 miles, if this is not a case of using a way too cold plug on the street, you have a problem in your engine that should be addressed. Ordinarily, I'd say you've got a problem with oil use or overly rich air/fuel ratio, but you say there's no obvious signs of deterioration. Is the igntion system working properly?

In my 71, I currently use the AC Rapid Fire #1, but, at the next spark plug change; I'm going to the Denso ITF22, which is the plug for iron heads.

Your idea that going one range higher for aggressive driving is incorrect. Reality is just the opposite. If you run the car hard or race it, you need to go at least one range lower, ie: the AC 42 heat range. Unfortunately, there is no uniform system among plug makers. With AC the numbers get lower as the plug gets colder. With Denso, the numbers get higher as the plug gets colder. Same with NGK.

 

[koolaid117]

Your idea that going one range higher for aggressive driving is incorrect. Reality is just the opposite. If you run the car hard or race it, you need to go at least one range lower, ie: the AC 42 heat range. Unfortunately, there is no uniform system among plug makers. With AC the numbers get lower as the plug gets colder. With Denso, the numbers get higher as the plug gets colder. Same with NGK.

Where do you find the heat range on a spark plug and why is it important or not? The more I find out about this car the more I realize I just don't know.

My driving consists mostly of running around town and short trips, less than 10 miles at a time.

Jim

 

[tomtom72]

Heat range indicators

Hi Jim, I think the heat range is coded in the plug number. I asked the parts guy to translate the heat range from the info in the parts book. As Mr. H says there is no uniform industry standard. Each plug maker has their own way of denoting the heat range. ACDelco the lower the number the colder the plug.

I just asked the parts guy at the local chevy dealer what replaced the FR2LS and went with that one. I don't do that much hard driving, except for cleaning the 2*. Most of my driving is highway for 50 miles or better at a clip.
Tom

 

[91VETTEZR1]

i am using the new 'IRIDIUM " plugs by denso or ngk. it's a new metal they are using and is suppose to be state of the art. my car runs great with them