I have a '91 305 z28 5-speed. It is stock for the most part. What is the best spark plug to use.

 

A new one

Seriously: Unless you're running something that's been seriously modded and is running low ET's then any "non-performance" spark plug is fine. I like to use AC Delco since they're what our cars rolled out of the factory using.

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--Steve S--
1984 Trans Am 305 LG4, 5 speed Daily Driver, Flowmaster 80 Series
Holley 600 cfm & vacuum advance
350 ready to get in and run as soon as i find a WC T5.

 

Anything made by AC/Delco. I use the Rapid Fires and like them a lot, I also know a lot of people who use the AC platniums and have good luck.

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1987 Chevy Camaro IROC-Z
L98 TPI 350 (5.7L)
TH 700R-4 Transmission
Borg Warner 7.75" 9 Bolt Rear End with 2.77:1 Gears.

Current Mods: LT4 HOT Cam, Comp Cams 1.52:1 Roller Rocker Arms, LPE High Flow TPI Baseplate, Edelbrock TES 1 5/8" Headers, Catco 3" High-Flow Catalytic Converter, Hooker 3" Aerochamber Cat-Back System, Transgo Shift Kit, Performance Resource Chip, Accel Ignition, K&N Filters, JET TPI Air Foil, All Free Mods, Falken ZIEX Z-Rated Tires.

Best ET: 14.32 @ 97.7mph
(corrected for elevation)

 

The cheapest AC delco plugs you can get your hands on that are new. Dont waste your money on anything else.