Best Plug for 90 s13 with header - Hotter or Colder

[Peabody]

Hello, I have a 90 S13 with a header, no cat, HKS exhaust, air intake. I forgot is it better to use a plug that is hotter or colder then stock with a header. In the past I was using NGK ZFR5EGP that was hotter or colder(I forgot). When I asked the guy at Napa he did not know what I was talking about changing to a hotter or colder plug. The person behind me that built a SR20 said to get Iridium plugs. I had no idea what that was so I said yes get them. I do not know if the person at Napa got me a stock plug or a hotter or colder one. I bet stock because he just found the Iridium for my car and got it. They are Champion Iridium 9202 plugs that cost $10 each! I did not know this until I went to the cashier. Does anybody know if these plugs are any good? They have a 7 year replacement warrenty and seem to have improved my power but my mileage is very bad in the city. But the added power could be just me thinking that to justify $40 for plugs. I do get a backfire from time to time on hard excelleration when I take my foot off the gas and just coast. I just want to know when you have a header & no cat should you get a hot or colder plug? If I got a stock plug should I try to return it for a different one? And are these Iridium plugs a waste of money?THANKS

[180fan]

ngk heat 5 for your engine. if you're relatively stock stick with the stock heat range. you can get iridiums, pricier but needs less attention. i'd still suggest getting coppers on a stock engine and replacing them more frequently to get the most mileage.

price does not always reflect quality (although it usually does). for example, hks spark plugs are way over priced for rebadged ngk plugs.