Spark Plug Gap and Heat Range Guide for Modified HEMIs
Choosing the right spark plug gap and heat range is critical when you add power. Here's what to know.
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.
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