Hellcat Spark Plugs Buying Guide

TL;DR: A Hellcat uses two spark plugs per cylinder, so you buy sixteen, not eight, and you replace all sixteen at once. The engine is boosted, which makes heat range and gap far less forgiving than they are on a naturally aspirated Hemi — a plug that is too hot or gapped too wide shows up as misfire and knock retard under boost, not at idle. Buy the heat range the application calls for, gap to the figure in the service information for your build, and torque into aluminium heads on a cold engine. Most Hellcat plug jobs that go wrong go wrong on access and torque, not on brand choice.

Why a Hellcat needs sixteen spark plugs

Every Gen III Hemi, the supercharged 6.2 included, is a two-plug-per-cylinder engine. Eight cylinders, two plugs each, sixteen plugs in the box. The second plug is not a spare or a backup. Chrysler used a pair of plugs per chamber to get two flame fronts started in a wide, shallow combustion chamber, which shortens burn time, cleans up combustion stability and helps emissions. Firing both plugs is handled by eight coils: each coil sits directly over one plug and fires the opposite plug in the same cylinder through a short lead.

That has three practical consequences when you shop.

First, quantity. A "set of plugs" listed as eight is half a Hellcat job. Order sixteen identical plugs.

Second, symptoms are muted. Lose one of the two plugs in a cylinder and the engine usually still fires that cylinder on the surviving plug. You get a rough spot, a lazy idle, worse fuel economy and knock sensitivity under boost long before you get a hard, obvious dead-cylinder misfire. On a boosted engine, that quiet half-failure is the dangerous one.

Third, you are also touching the ignition leads and coils every time. If the short leads are hardened, cracked or oil-soaked, replace them while you are in there rather than doing the job twice. Coil and lead options for this family live in the ignition components collection, and platform-specific hardware is in the Hellcat 6.2 supercharged parts collection.

The Hellcat is a pushrod engine, so nothing about this job involves overhead-cam covers or cam-drive access. What it does involve is a large twin-screw supercharger sitting on top of the valley, which is exactly why access is the hard part.

Heat range: the one specification that matters most under boost

Heat range describes how fast a plug sheds combustion heat into the cylinder head. A hotter plug retains more heat at the tip and resists fouling at light load. A colder plug dumps heat faster and resists pre-ignition at high cylinder pressure.

A supercharged engine lives at high cylinder pressure by design. That pushes the correct choice toward the colder end of the range compared with an otherwise similar naturally aspirated engine. This is why a plug that works beautifully in a 5.7 or a 392 is not automatically right in a 6.2 supercharged car.

How to decide:

• Stock power, stock pulley, street driving. Use the heat range the factory specifies for your model year and trim. Do not "upgrade" to something colder because it sounds safer. Too cold on a mostly street car fouls tips, dirties the ceramic and produces its own misfires.
• Pulley, higher boost, tune, or track use. One step colder than stock is the conventional move, and it is the step most calibrators ask for once boost and timing go up. Confirm the specific step with whoever wrote your calibration — they know the timing table you are running.
• Race fuel, methanol injection, big power. Heat range becomes a calibration decision at that point, not a catalogue decision. Go with what the tuner specifies and read the plugs afterwards.

Whatever range you land on, buy all sixteen from the same range and the same part number. Mixed plugs across banks make a car that is impossible to diagnose.

Electrode material, gap and what actually wears out

Material. Fine-wire iridium and platinum plugs are the default for a reason: the small centre electrode fires reliably with less voltage, and the precious-metal tip resists erosion, so the gap stays where you set it for a long time. Copper-core plugs conduct heat well and are cheap, which is why some racers run them and change them constantly, but they erode fast and are a poor choice for a street Hellcat.

Gap. This is where boosted engines punish carelessness. A larger gap needs more voltage to jump, and cylinder pressure raises the voltage required further. Push the gap too wide on a boosted, well-timed engine and the spark blows out under load — a stumble or a flash of misfire only at high boost, clean everywhere else. Many boosted applications specify a tighter gap than the equivalent naturally aspirated engine for exactly this reason. Set every plug to the gap called out in the service information for your engine and calibration, verify each one with a wire gauge before it goes in, and never assume the out-of-box gap is correct.

Gap fine-wire plugs with care. Do not lever against the delicate centre electrode; adjust only the ground strap, in small increments, and re-measure.

What wears. The gap grows as the electrodes erode. Growth is gradual and the ignition system compensates by demanding more coil voltage, until one day it cannot, and you get the boost-only misfire described above. That is why replacement is interval-based rather than symptom-based.

Interval. Follow the maintenance schedule in your owner's manual for a stock car, and shorten it substantially if you run more boost, more timing or track sessions. Do not carry over an interval from a naturally aspirated engine.

Doing the job without damaging the heads

Gen III Hemi heads are aluminium, and aluminium threads do not forgive.

• Work on a cold engine. Removing or installing plugs in hot aluminium is how threads get pulled. Let the car sit overnight.
• Blow out the plug wells first. Compressed air on every well before a plug comes out keeps grit from falling into an open cylinder.
• Start every plug by hand. If it does not turn freely by hand for several turns, stop and find out why. A cross-thread caught early is a nuisance; a cross-thread torqued down is a head repair.
• Torque to the value in the service manual, with a torque wrench. Not by feel, not by "a quarter turn past snug." Modern plugs have a real specification, and the aluminium is what pays for guessing.
• Skip anti-seize unless the plug maker calls for it. Most current plugs use a plated shell designed to go in dry, and anti-seize acts as a lubricant that makes a torque wrench read high — meaning you over-tighten while thinking you are on spec.
• Access takes longer than you expect. On these cars, some plugs are reached far more easily with the front wheel and inner fender liner out of the way than from above, because the supercharger and its plumbing occupy the top of the engine. Plan for that rather than fighting it with extensions and a mirror. Budget a multi-hour job at shop labour rates, not the hour a small-block would take.
• Inspect what comes out. Even ceramic tips, uniform colour and even erosion mean your heat range and calibration are sane. Blistered ceramic, melted ground straps or speckling on the tip are pre-ignition or detonation signals — stop and address the calibration before you put a fresh set in and repeat the damage.

If you have the supercharger inlet or the plenum off for access, that is the moment to check the seals and gaskets you have disturbed. Replacements are in the gaskets and seals collection, and blower-side hardware is in the supercharger parts collection. Cross-generation Hemi parts are grouped in the Gen III Hemi collection.

What a good Hellcat plug purchase looks like

• Sixteen plugs, one part number, one heat range.
• Heat range matched to your actual boost and calibration, not to the car's badge.
• Fine-wire iridium or platinum for a street car; consumable copper only if you are racing and replacing them often.
• Gap verified individually against the specification for your engine.
• New leads or coils if the old ones are hardened, cracked or oil-contaminated.
• A torque wrench and a cold engine on the day you do the work.

Do that and the job lasts. Skip the heat range question or the gap check and you will be chasing a boost-only misfire that never shows up in the driveway.

Frequently Asked Questions

How many spark plugs does a Hellcat have? Sixteen. Every Gen III Hemi, including the supercharged 6.2, uses two plugs per cylinder fired by eight coils. Always buy and replace all sixteen as a set.

Do I need colder spark plugs in a Hellcat? Stock cars should run the factory-specified heat range. Once you add boost through a smaller pulley, add timing, or run track sessions, one step colder is the usual recommendation — confirm the exact step with whoever calibrated the car.

Can I use the same spark plugs as a 5.7 or 6.4 Hemi? No. Those engines share the two-plug architecture, but they are naturally aspirated and their heat range and gap requirements are different. Buy plugs specified for the supercharged 6.2.

Why does my Hellcat only misfire under boost? That pattern points at gap or heat range before it points at coils. A gap that has eroded wider, or a plug set too wide from the start, needs more voltage than the ignition can supply once cylinder pressure climbs, so the spark blows out only under load.

Should I put anti-seize on Hellcat spark plugs? Not unless the plug manufacturer specifically calls for it. Most current plugs have plated shells intended to install dry, and anti-seize skews torque readings so you over-tighten aluminium threads while believing you are on specification.

How often should Hellcat spark plugs be replaced? Use the interval in your maintenance schedule for a stock car, and shorten it meaningfully for any car running more boost, more timing or regular track use. Boosted engines erode electrodes faster than the naturally aspirated versions of the same platform.

Sources

• Chrysler Gen III Hemi two-plug-per-cylinder ignition architecture, as documented in factory service information for the platform.
• General spark plug engineering references on heat range selection, electrode materials and gap-versus-cylinder-pressure behaviour from major plug manufacturers' technical literature.
• Vehicle-specific torque values, gap specifications and service intervals: consult the factory service manual and the plug manufacturer's catalogue entry for your exact model year and calibration. Those figures were deliberately not reproduced here because they vary by year, trim and tune.