There are two kind of iridiums plugs made by denso. IK22 and IK20. Everyone who orders the IK20 then you all have the wrong plug. IK20 is the stock plug gapped down. IK22 = 6097 which are exact as stock and one heat range colder.
http://supra.vache.org has the break down of the difference between all the plugs. I should know since I spent an hour looking and cross referencing all the plugs but that was long ago

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The IK22 are by far the superior plugs among them all. My idle cleared up atleast 50% with that plug and I haven't felt a miss on my single turbo. I also put the same plug a BPU+++ car without a problem. PHR put the 3330 in the car and it idled like crap when i retarded my exhaust cam gear. 6097 idled great but like all copper plugs went out at ~3000 miles.
I have read from numerous sources that you need to run a colder plug when you spray because of how much the heat goes up. Sound-Performance uses ik22 in all their cars and they have hi RWHP with NOS. Also these plugs are designed to outlast platinum plugs. I will never put copper plugs in my car again. I am not fond of changing plugs at every oil change. Wear and tear on the head and the spark plug clips are just some reasons to just put out the extra 10 bucks a plug and bite the bullet.
IMHO,
Sid