104s are too cool for a N/A engine. AL605s are one heat range colder for the N/A engines. 104s are two heat ranges colder than stock. AL606 is the stock equivalent Autolite plug.

Plugs aren't the first place to go for knock. Tuning will help out immensely, though as I'm finding out it isn't always easy going.

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2004 Impala LS - got some mods
NAIOA Moderator

Making you think twice before picking on Impalas

Explain exactly how a 104 plug is 'too cool' for a N/A engine? Especially one that is modded.

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2.8 Pulley, Intense IC, P&P Gen V, Machined Intake, Intense PCM, 65lb/hr Injectors, N* Throttle Body, 1 3/4" Headers, 3" Exhaust, Intense Shift Kit, 3.69:1 Gears, 4000 Stall, N20

It's two heat ranges colder than the stock plug. When should you EVER jump two heat ranges just for an intake and some exhaust? Most L36/L26 owners I've seen running the 104s or NGK equivalents end up fouling or suffering mileage. 605s are right on the money and that goes for some of the most modded N/A builds you can think of.

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2004 Impala LS - got some mods
NAIOA Moderator

Making you think twice before picking on Impalas

^^^ correct.

To the OP. No one offers a better mail order custom tuned PCM than Overkill. He is a member here. :: Overkill Motorsports ::

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04 Indy SS build #972
XP, TOG's, Lightning injectors, 2.8 pulley, 105 CC springs, Wizaired, 3.29 gears, tripleedgeperformance ported blower, machined LIM, 180* tstat, RS12YC@.053, FP rewire, Alt rewire, stage 1 V boost, PRJ wires, Stage 2 IC