NGK Spark Plug Selection Guide

Pontiac Engines — Aluminum & Cast Iron Heads

Pick your section by head material. Each track is self-contained — fitment, the full plug ladder across every application (street, strip, all-out race), and both fuel systems (carbureted and EFI).

A plug doesn’t make heat — it removes it. Heat range is how fast it sheds combustion heat into the cooling system. On NGK, the lower the number, the hotter the plug (a 6 is hotter than a 9). Too hot → pre-ignition and detonation. Too cold → fouling. Run the coldest plug that still stays clean — more power, compression, boost, or nitrous means a colder plug (higher NGK number).

Gap (Butler guideline):  Points → .035″  ·  Electronic / MSD → .040″–.045″  ·  CDI/MSD box → follow the box’s instructions.

Butler’s first questions on any plug call: what heads? Street or race? Naturally aspirated or power adder? Those three set heat range and tip before anything else — and plug choice goes with tuning (timing curve, total timing, carb), stepping one range at a time off plug reads.

Where most Butler high-performance builds land, and where the full plug range lives. Fitment is shared across every application: 3/4″ reach · 14mm thread · gasket seat · 5/8″ hex. You only change heat range and tip style as the build gets more serious.

1. Colder (HR6 → HR7) — more detonation margin for the higher-output build.
2. Extended tip → recessed tip — the recessed electrode pulls back into the shell, clearing domed pistons and removing a pre-ignition hot spot on high-compression combos. Extended (projected) tips sit deeper in the chamber and self-clean better at street speeds, which is why the street plugs use them.
3. Resistor → non-resistor — see EFI note below.

Carbureted, points: gap .035″. Resistor not critical — 6962 or the race plugs both work.
Carbureted, MSD/electronic: gap .040″–.045″. Resistor plug or resistor wires recommended.
EFI (FAST, Holley, etc.): resistor plugs strongly preferred — RFI from non-resistor plugs can disrupt crank/cam sensors and the ECU. The 6962 is already a resistor plug (plug-and-play on EFI). The 4091 and the rest of the R5671A race series are non-resistor — if you run them on EFI, use resistor plug wires to compensate. Gap per your ignition (commonly .040″–.045″, tighter on high-cylinder-pressure builds).

Gap notes: more ignition energy + lower cylinder pressure → open the gap (.040–.045). More cylinder pressure (high compression, boost, nitrous) → tighten to prevent spark blowout. The 4091 ships pre-gapped tight (~.020″) for race use — open it to your target for most naturally-aspirated street/strip combos.

Factory iron heads are the constraint — seat type and reach are fixed by the casting. Pontiac changed the spark plug seat around 1972, so read fitment first.

Same rule as everywhere else: colder plug (higher number) as compression climbs. The YR-5 is the one resistor plug in the group, which makes it the natural pick on an EFI ’72+ iron-head car. Butler’s own guideline for these heads: pump-gas street −5 to −7, high-compression race −8 to −9.

Street / mild performance: the R5670-6 (HR6) is the plug — factory iron heads, pump gas, typical compression. This is the right combination.
Strip / higher output: factory iron heads top out fairly low on power and compression before they become the limiting factor. The practical move for serious power is switching to aluminum heads (Part 1) — which is exactly why Butler’s full heat-range ladder lives in the aluminum category.
All-out race on iron heads (ported round-port, aftermarket iron): colder plugs in the NGK racing series exist, but matching them to your specific reach and seat is a call-Butler item — don’t assume a number off an aluminum-head chart will fit an iron casting.

Most iron-head Pontiacs are carbureted, but the same logic applies. Carbureted, points: gap .035″. Carbureted, MSD/electronic: gap .040″–.045″; resistor plug or wires recommended. EFI on iron heads (uncommon but valid): use resistor plugs or wires to keep RFI out of the ECU/sensors, and gap per your ignition. Heat range window for pump-gas iron-head street: HR 6–7, with the R5670-6 as Butler’s stocked answer.

Pull a plug after a few heat cycles or a pass:
Light/white, blistered insulator, melted electrode → too hot / too lean / too much timing → go a step colder, richen, or back off timing.
Black, sooty, wet/oily → too cold / too rich / fouling → go a step hotter or fix tuning.
Light tan/brown insulator, ground strap colored ~halfway up → correct.
When unsure, start one step colder than you think you need — a cold plug only risks fouling, a hot plug risks the engine — and read from there.

Heat-range and gap guidelines per Butler’s published spark plug information; part numbers and fitment reflect Butler’s current catalog. Verify against your specific head, compression, and ignition combination. When a setup is borderline, call Butler — every combination is different.