Engine & Power · Buyer's Guide

Spark Plugs & Ignition

Spark plugs ignite the air-fuel mixture in each cylinder 40+ times per second. Better plugs mean more consistent combustion, smoother idle, and better throttle response. The HEMI has a unique dual-plug design (two per cylinder = 16 total), so plug changes take a bit longer than most engines. Modified engines often need colder-rated plugs to handle higher combustion temperatures.

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All Spark Plugs & Ignition (44)

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22. IGNITION SYSTEM UPGRADES

The HEMI's Unique Setup: Dual Spark Plugs Per Cylinder

The Gen III HEMI is famous for its dual-plug design — two spark plugs per cylinder, totaling 16 plugs in the V8. One plug sits centrally in the combustion chamber; the other is in an offset position. Both fire simultaneously for more complete combustion and the ability to run either plug as the primary if one fails.

Implication: When you buy spark plugs, you need 16 plugs (not 8). Budget and installation time are double what you'd expect for a typical V8.

Stock vs. Aftermarket Plugs

Type Gap (N/A) Gap (Boosted) Interval Best For
Stock OEM copper 0.044" N/A 30,000 mi Stock, economical
NGK Iridium IX 0.044" 0.028–0.032" 60,000+ mi Street, longevity
Denso Iridium 0.044" 0.028–0.032" 60,000+ mi Street, OEM quality
NGK Ruthenium 0.044" 0.028–0.032" 60,000+ mi Best overall
Colder plugs (e.g. NGK one step colder) Slightly tighter Tighter still Varies Boosted, cam builds

Why colder plugs for boosted builds: Combustion temperatures are higher under boost. A "hotter" plug (with more electrode exposed to combustion) can pre-ignite under these conditions — called "plug firing." A one-step-colder plug has more insulated electrode, dissipates heat faster, and prevents pre-ignition.

Gap tightening under boost: Boost pressure physically opposes the spark. Tighter gaps require less voltage to fire reliably under high cylinder pressure. A boosted engine at 10 PSI may need the gap reduced from 0.044" to 0.030–0.032" to prevent misfire.

Ignition Coils

The HEMI's stock coil packs are reasonably robust. The community consensus is that coil upgrades offer modest gains on N/A engines but become more meaningful on:

  • Cam builds (harder to ignite under aggressive timing changes)
  • Boosted builds (higher cylinder pressure requires more spark energy)
  • High-RPM builds (coils have less dwell time to recharge between firings)

aFe Power Ignition Coils: Up to 20% more spark energy over stock. Direct plug-in replacement.

MSD Blaster Coils: Popular choice; allows slightly larger plug gaps for better combustion.

Recommended Spark Plug Change Interval

  • Stock copper: every 30,000 miles
  • Iridium/platinum: every 60,000 miles (or sooner with cam/boost modifications)
  • After any significant tune change: inspect plugs to read combustion conditions

Site UX Recommendations

  • Prominently note "Set of 16 plugs required" on HEMI spark plug listings
  • Boosted-build note: "Running boosted? You need colder-range plugs"
  • Gap guide by application: stock / mild mods / supercharged
AutolitePlatinum Spark Plug AP5324
$4.49+
AutolitePlatinum Spark Plug AP66
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ChampionCopper Plus Spark Plug RN9YC
$4.99+