MIZR1 said:
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plug recomendations for LT5s are based not on actual testing but on engineering decisions which build on what AC Delco originally made for the car.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.