Spark Plugs and Ignition Coils: The HEMI Ignition Upgrade Guide

# Spark Plugs and Ignition Coils: The HEMI Ignition Upgrade Guide

The ignition system is how the engine turns compressed air and fuel into power — a precisely timed spark that initiates combustion. The HEMI's unique dual-spark-plug design makes this system both more capable and more maintenance-intensive than most V8s. Here's what you need to know.

---

The HEMI's Dual-Spark Plug Design

Every HEMI V8 cylinder has two spark plugs — one centrally located, one offset to the side. Both fire simultaneously (actually nanoseconds apart). The two-plug system provides:

• More complete combustion — two ignition points mean the flame front reaches more of the combustion chamber
• Redundancy — the PCM can operate on one plug per cylinder if the other fails (limp-home mode)
• Cleaner burn — reduces unburned hydrocarbons at the exhaust

The practical implication: When you change spark plugs on a HEMI V8, you need 16 plugs, not 8. Budget and plan accordingly. A spark plug job takes noticeably longer than a typical V8 and costs more in parts and labor.

---

When to Change Spark Plugs

| Plug Type | Interval |

|---|---|

| Stock OEM copper plugs | Every 30,000 miles |

| Iridium or platinum | Every 60,000 miles |

| After a cam swap or tune change | Inspect immediately — new combustion conditions |

| After a supercharger install | Replace with colder plugs immediately |

| If experiencing misfires | Inspect regardless of mileage |

What worn spark plugs cause:

• Misfires at idle or under load
• Rough idle
• Reduced fuel efficiency
• PCM-commanded timing retard (to manage the rougher combustion)
• CEL with misfire codes (P0300–P0308)

---

Understanding Spark Plug Heat Range

"Heat range" refers to how quickly a spark plug transfers heat away from its tip to the cylinder head. This is one of the most misunderstood aspects of plug selection.

• Hotter plug: More electrode material exposed to combustion; runs hotter; good for low-load, low-RPM engines that might foul plugs in stop-and-go traffic
• Colder plug: More electrode is insulated; dissipates heat faster; prevents pre-ignition (plug firing before the spark) under high load and high temperature

For boosted builds, you need colder plugs. Under boost, combustion temperatures are dramatically higher. A stock heat-range plug can become hot enough to glow and cause pre-ignition — detonation that is not spark-triggered. This damages pistons.

Standard rule: Step one heat range colder when adding a supercharger or turbocharger. For aggressive boost levels (10+ PSI), go two steps colder.

---

Spark Plug Gaps: The Boosted Build Rule

The spark gap is the distance between the centre electrode and ground electrode — where the spark jumps.

Wider gap = bigger spark = better combustion efficiency (more complete burn)

Narrower gap = easier for the spark to fire reliably under pressure

Under boost, the increased cylinder pressure physically resists the spark jumping the gap. Too wide a gap under boost = misfire.

| Engine State | Recommended Gap |

|---|---|

| Stock / N/A bolt-ons | 0.044" (1.1mm) |

| Mild cam (N/A) | 0.040–0.044" |

| Supercharged (mild boost, 6–8 PSI) | 0.032–0.036" |

| Supercharged (moderate boost, 8–12 PSI) | 0.028–0.032" |

| High boost (12+ PSI) | 0.024–0.028" |

When your tune is written, your tuner will specify the correct gap for your setup. Follow their recommendation.

---

Top Spark Plug Recommendations

NGK Iridium IX

The community-favourite aftermarket plug for the HEMI. Iridium electrode provides excellent longevity and consistent spark across the full RPM range. Direct drop-in for all HEMI engines.

• Best for: N/A builds, mild cams, daily driver
• Heat range for boosted builds: Go one step colder (e.g., NGK 94201 instead of standard HEMI plug)
• Interval: 60,000+ miles

NGK Ruthenium HX

Newer material, considered a step above iridium in both longevity and combustion efficiency. Growing community adoption.

• Best for: Any build — best overall plug technology available for the HEMI
• Price premium: ~15–20% over Iridium IX

Denso Iridium

Another premium iridium option. OEM supplier for several manufacturers. Good quality, competitive pricing.

• Best for: N/A builds, OEM replacement quality

MSD Iridium Performance

Available in multiple heat ranges, making it easier to select the correct one for a boosted build. MSD's name recognition comes from their ignition system expertise.

• Best for: Boosted builds where heat range selection matters

---

Ignition Coil Upgrades

The ignition coil takes low voltage from the battery and transforms it into the high-voltage pulse (20,000–45,000 volts) needed to fire the spark plug. Each cylinder has its own coil pack on the HEMI.

Do You Need Upgraded Coils?

On a stock or mild N/A build: The community consensus is that stock HEMI coil packs are quite robust. For typical bolt-on builds (CAI, exhaust, tune), aftermarket coils provide minimal measurable improvement over well-functioning stock coils.

When coil upgrades make more sense:

• Cam builds: The new cam timing and increased RPM range stress coils as they have less dwell time (time to charge between firings) at high RPM
• Boosted builds: Higher cylinder pressure requires more energy to fire reliably. Stronger coils allow you to run a slightly wider plug gap for a bigger spark
• High-mileage engines: Stock coils can weaken over time; replacements with better units restore ignition performance
• Misfire diagnosis: If coils test weak on a misfire, replace with upgraded units

Top Ignition Coil Options

aFe Power IGNITION Control Series

Up to 20% more spark energy over stock. Direct plug-in replacement — no wiring changes. Popular choice for cam builds and boosted applications.

MSD Blaster Series

MSD is a well-respected ignition brand. Their coil upgrades for the HEMI provide increased output and allow running slightly wider plug gaps for a more powerful spark. Good match with MSD plugs.

Mopar OEM Coil Packs

If coils are failing or weak, OEM replacements are a reliable, cost-effective option. Not a performance upgrade but a quality stock replacement.

---

Installing Spark Plugs on the HEMI: What to Know

Access: On the 6.4L, the rear bank of plugs (cylinders 2, 4, 6, 8 on the passenger side) are more difficult to access than the front bank. The coil packs sit on top of the plugs and need to be removed first. Budget extra time for the rear bank.

Torque: Spark plugs are typically torqued to 13 lb-ft (±2 lb-ft) on the HEMI. Use a torque wrench — over-tightening can crack the plug or damage threads in the aluminium head.

Anti-seize: Many experienced builders apply a small amount of anti-seize compound to plug threads before installation — prevents galling in the aluminium cylinder heads. Check the plug manufacturer's recommendation first (some iridium plugs advise against it).

Gap check: Always verify the gap before installation, even on new plugs. Shipping vibration can close or open gaps slightly.

When to do this at a shop: If you're not comfortable working in tight engine bays, this is a reasonable shop job. Budget 2–3 hours of labor.

---

Reading Your Plugs After a Tune

A spark plug "read" (inspecting the plug tip after a hot WOT run and immediate shutdown) tells you a lot about your combustion conditions:

• Light tan / grey colour: Perfect. Correct air-fuel ratio, correct heat range.
• White / very light grey: Lean condition. Fuel map needs enrichment.
• Black / sooty: Rich condition. Too much fuel, or oil fouling.
• Black and wet: Oil fouling — potential ring or valve seal issue.
• Melted or eroded electrode: Too hot — detonation, wrong heat range, or severe lean condition.

An experienced tuner will ask you to do a plug read as part of calibrating a new build. It's one of the most reliable low-tech diagnostic tools available.