Spark Plug Gap and Heat Range Guide for Modified HEMIs

Why Spark Plugs Matter on a Modified HEMI

The stock spark plugs are calibrated for stock boost levels, compression, and fuel. When you add a supercharger, tune for E85, or raise compression, the stock plugs can cause:

• Misfires under boost (detonation from gap erosion)
• Pre-ignition (wrong heat range causing hot spots)
• Fouling (too cold a plug at idle-heavy use)

Heat Range Explained

Heat range describes how quickly a plug transfers heat from the firing tip to the cylinder head.

• Hotter plug: Slower heat transfer — stays hotter. Better for stock/mild builds, eliminates fouling.
• Colder plug: Faster heat transfer — stays cooler. Reduces pre-ignition risk under boost or high compression.

For forced induction or high compression builds, drop 1–2 heat range steps colder than stock.

Recommended Plugs for HEMI Applications

| Application | Plug | Gap |

|---|---|---|

| Stock 5.7L | NGK 6510 (stock) | 0.043" |

| Stock 6.4L | NGK 6510 | 0.043" |

| Mild tune, 93 oct | NGK 6510 or Denso 3297 | 0.040" |

| Supercharged (stock boost) | NGK 97506 (1 step colder) | 0.035" |

| Hellcat stock | NGK TR6 | 0.035" |

| High boost / E85 | NGK TR55 or TR7 | 0.028"–0.032" |

Gap Reduction on Boosted Engines

Tighter gaps resist blowout under boost. Stock-style engines gap at 0.040–0.045". As boost increases:

• 6–8 psi: 0.038–0.040"
• 10–12 psi: 0.032–0.036"
• 14+ psi: 0.028–0.032"

Always gap plugs with a proper feeler gauge — wire gauges lack accuracy for tight gaps.

Installation Tips

• Torque to spec: 13 ft-lb on aluminum heads (stock HEMI). Over-torquing strips threads.
• Anti-seize: Opinions vary. NGK says no anti-seize on their plugs (factor in plating). If you use it, reduce torque 30%.
• Change interval: Every 30,000 miles stock; every 15,000–20,000 miles on a tuned/boosted build.