Ethan - According to my GS4/3 shop manual, the plugs recommended for the 300 are either DENSO #PK16TR11's or NGK #BKR5EKPB11's gapped at .043. The 400 needs DENSO SK20R11's or NGK IFR6A11's with the same gap. Muhammad said these are the best to use even in his supercharged engines so I'd stay with them. U can get them from your dealership or Pep Boyz; Auto Zone or any good parts place.
For those folks running NOS I recomend to stay away from factory Platinum and iridium and switch to a cheaper NGK or autolite plug. For folks running MSDs or other ignition setups I also recommend normal non-platinum plugs. Not worth the expense.
Ethan - Those part numbers came directly from my GS shop manual so I'd imagine they're good plugs, even though I'd personally move up to the iridium for my GS ( that design came in the car ). Might call Mo at Swift Racing for a suggestion or I know we have some guys on here that have heavily modded SC's - have U tried the SC Forum? If nothing else, just keep running what U have in there already.
Hey Mind - why run a cheaper plug with nitrous when the ignition/fuel delivery system is so critical to those engines? I've always experimented with heat ranges & found that, for heavy NOS use, a 1 or 2 heat range colder plug works best but DOES require more frequent cleaning as they have a tendency to foul. Unless it's a cost or availability issue, what do U know that I need to know - help me out, my friend!!
As anybody running NOS can tell you...it is better to always be checking your plugs, it is the best way to know if you are going lean in one cylinder. I have known so many folks running NOS in so many cars that NONE, I say none, spend extra on EXOTIC blah blah blah plugs. Just get good brand name plugs (which are cheaper but not bad) with the correct heat range (I never said anything about changing that) and go from there. Iridium and Platinum plugs are left for stock systems or almost stock systems because they just LAST longer, they are not better (as far as performance is concerned) than the other sparkplugs!!
So if you are fouling plugs, eating up electrodes and running NOS why have something that is supposed to last with the STOCK system? My logic says to run normal plugs and change them more often than normal and in this case more often than cars running NON-platinum plugs. I have heard of spark plug changes at oil change intervals for heavy NOS applications to be on the safe side. I recently switch to non-platinums ($$$) but car runs perfect. I am running NGKs BCPR6ES in my 1UZ-FE engine for now.
Run a search in the internet for this topic ...running platinums and NOS and you'll switch to regular plugs quickly. <<<---correction (don't listen to me)
I just reread your post so if cost and availability is not an issue then I guess stick with platinums but keep on checking those plugs!
Mind - Thanks for the links! I understand your point completely & agree that feedback from the plugs is the best way to determine what's going on in the cylinders. I guess it does come down to costs, at least until U find the plug that works best for ya. I only changed my 'Vette plugs once every 6 months but I wasn't going crazy with the nitrous & had gone thru several heat ranges to find the one that gave me the best compromise between everyday driving & occasional spraying. Will check on those links to try & learn some more about the different pros & cons of electrode materials - Thanks again!!