Greetings!
I'm Dave Buckshaw (of DHP) and by day I'm a technical trainer for Autolite spark plugs. I would suggest using our 104 heat range plug as it is 15% cooler than the 605's and the 3800 really likes a colder plug when you start bringing up the boost. I run 104's in my GTP motor swapped Montana with a freeflowing exhaust, rockers and a 3.4" pulley. A colder plug helps rid the plug electrodes and combustion chamber of detonation causing heat faster and this is important especially on a non intercooled application that has very air inlet temp deltas, (changes), and runs close to the knock window already.
The only "worry" to using a cold plug could be some low tempurature misfires. Too hot of plug can MELT parts and destroy an engine! The 104 however is a factory spec'd heat range for many OE engine applications including older 3800's and I don't find it too cold. That being said, if you are worried at all about cold operating misfires I'd run our XP Iridium plugs, (XP104), but since they require about 10% less voltage to fire than a standard electrode spark plug there's MORE available voltage at the electrode and this helps prevent misfires all over the RPM range. How else can you really improve the output of the OE DIS system for a small cost? Coils? MSD?
You may have heard not to use a "Double Platinum" type plug in a GTP application. The issues that many have experienced with DP's are because DP plugs have precious metal pads welded to the electrodes and the resistance increases a bit due to the pads and high RPM misfires can result with a modded car that has increased boost. An Iridium plug however radically necks down electrodes to a .6mm point and than adds it's precious metal pads to the electrodes. The huge benefit here is that my personal experience in our lab doing some extreme pressure bomb testing is that the Iridium XP's required about the same voltage as our racing plugs with cut back center electrodes! That's pretty darn good IMHO!
Keep in mind that a standard copper core plug will suffer significant gap growth and erosion on 1/2 of the plugs on a DIS ignition in a rather short period of time as 1/2 of your plugs initiate the "arc" from the sidewire to the centerwire and the other 1/2 fire conventionally from center wire down. A standard sidewire firing reverse like this may only last 18,000-20,000 miles before that erosion cause the gap to get large enough to cause issues on the "big end" under boost. Autolite DP's also have a necked down center to help combat the increased resistance of the platium pads but it isn't nearly as radical as the XP's design.
Mostly I see guys running .045-.050 gaps with DP's and even copper cores and on the Delco DIS with aggressive boost I myself like to run .035. The Grand National guys will even run .028-.030 gap when their boost goes up. I'm also a Grand National guy so I know all too well about misfires!
So for a street car I'd be using on IR plug without any worry and take advantages of of its ability to launch the flame kernal quicker than a standard plug, (which helps convert more of your chemical energy to kinetic energy), and its ability to fire leaner mixtures that can result from tuning issues or transient conditions not account for in the PCM due to mods. As a side note GM uses IR plugs in the LS7 and the ZR1 V8's. When I did development work on the LSX V8 at another company we made 810 hp on a 2300 Eaton Blower on 428 CID of LS V8 on finewires!
I've had no issues with finewires at even 20+ psi of boost on a turbocharged 3800. What's nice is the suffer virtually ZERO gap growth or Erosion for a long period of time and that means you'll have one less thing to worry about. For an all out race car or a car where you change your plugs very frequently, (for fun?) I'd run standard copper cores, (104's) or our AR racing plugs if you have a max effort car.
Forgive my long windedness here. The 3800 V6 is my engine of passion and choice and I want to be sure you guys are getting some decent information from a technical point of view. It's good to see solid information from your fellow board members as well. This community has been through most of the bad combinations and setups already!
Dave
, they only had 30,000 miles on them. I thought that this was excelerated wear, dont know why but I bout $hit a brick when I saw them.