3.5 setup... what is the best spark plug? Can't figure out heat range...

**MM704** · 02-24-2015

So I have a 2000 GTP with 117k miles. It has a 3.5 inch supercharger pulley, PLog, 3 inch downpipe, CAI, 180* thermostat, and Overkill tune. I'm currently running AC Delco 41-104 plugs (from CTS-V, ZL1, ZR1) gaped at .045, which was personally recommended by Will Overkill for my setup. My KR numbers are very low, usually under 2 at WOT.

This set up worked fine for the first few months during the fall but once winter came I started having some RPM hesitating/chugging issues with light throttle while driving on the highway, particularly uphill. I put in a beefier set of spark plug wires (AC Delco wires > cheap wires) and that definitely lessened the problem but it's still there. I'm willing to bet my spark plugs are too cold.

My question is what spark plugs should I be running with this particular setup? Based on what I've read Autolite 605's with a .050 gap is the most consistent answer, but I'd rather stick with AC Delco Iridiums if possible because they last the longest and GM engines seem to respond to them very well. Would 41-993 (from 4.3 Vortec/Blazer) be the heat range I'm looking for? I'm assuming I want 1 heat range colder than the stock 41-101's.

Please let me know what you think. Thanks a bunch!

**ADrugDealer** · 02-24-2015

Frankly most guys here will say 1 of two things:

Stock for 99% of things
AutoLite 104/105s for 1% of other things

I currently run 104s in my Turbo GP and have been considering stepping up one more heat range.

**99GT2dr** · 02-24-2015

I have almost the same setup as you right now, but with an ssic, I'm running 104's @ .055. No issues under any conditions, 104's are good for a pretty wide range of mods.

**Impala131** · 02-24-2015

Go with the 605's... I have switched back and forth between 104's and 605's many times on 3.5/3.4 setups, and have never been able to add timing or increase AFR based on swapping plugs on those pulley sizes. Heck, 606's might be more appropriate for a 3.5, but I haven't tested that extensively.

**Scottydoggs** · 02-24-2015

being every car takes mods differently, try the 104's gap em a few times, 55, 60 53, 50 see what feels better. 0.45 seems small. try 0.50 for your plugs now. if that dont work swap plugs. copper core plugs seem to work best too.

no kidding i had my plugs in and out like 10 times over a weeks time playing the gap game. 104's gaped at .50 feel the best in my car.