Nobody on this board with a bonneville should be running the 4 range cooler plugs. I would choose the 605'* for 6-10 lbs of boost, 104s for 11-13 and 103s for 14+

I like Denso plugs, IT16 is the same heat range as the NGK TR55-ix iridium plug,
IT20 is colder and IT22 is colder still. the gap on the Denso plugs are .044 , i open my gap
out to .052 . the center electrode on the Denso iridium plugs are 0.4mm compared to 0.6mm on the NGK iridium plugs.
A rule of thumb is, one heat range colder per modification or one heat range colder for every 75–100hp you increase. In identical spark plug types, the difference from one full heat range to the next is the ability to remove 70°C to 100°C from the combustion chamber.

ROD

I really like the Iridium plugs, more for the longevity than anything. Better than the Platnums! IMO

I think I'm runnin' 104'*, and lately they've started missing under boost. Could the gap be "growing"? We put them in during WCBF05, and I think we gapped them at like .055". When I did the rockers I had them pulled, and they looked great. I didn't re-gap them or anything. Anyways, I got some 103'* that I'm about to put on, but I'm kinda runnin' in the dark here. Without seeing the plugs, anybody got any suggestions?

the gap on "old technology" plugs grow with mileage. the gap on Iridium plugs grow at a much slower pace, thats why you can drive 75k to 100k before you have to change them. the temp. range of the plug is the same "old or new technology", the ability to remove heat from the combustion chamber.

ROD

My TR6 IX iridium plugs went in when I dropped to the 2.9 pulley and water injection but I still run them with the 3.1
I gap them at .052" and have not had any problems. In fact I broke one regapping it from the factory setting of .040". Be careful not to touch the center electrode as it is very brittle.

They should be equivalent in heat range to your 104'* at 2 ranges colder.