Spark plugs: gap and heat range (and why it matters on boost)

TL;DR

• Tuning is changing targets and limits so the ECU can control torque safely under your real conditions.
• Modern ECUs are torque-based: they decide airflow, fueling, and spark to hit a torque request.
• Validation beats screenshots: consistent conditions and repeat runs are the only way to trust changes.

A mental model (text diagram)

Driver intent → Torque request → ECU torque model → Airflow (boost/throttle) + Fuel + Spark
                 ↑                                  ↓
               Sensors (air, temps, knock, fuel) ← Outcome (torque delivered)

Gap vs boost (conceptual)

Higher boost increases cylinder pressure, which makes it harder for the spark to jump the gap. Too large a gap can lead to spark blowout (misfire under load).

Heat range vs detonation margin

Heat range affects how quickly a plug sheds heat. A plug that runs too hot can increase risk; too cold can foul depending on use.

Misfire vs knock confusion

Misfire can feel like breakup. Knock correction can feel like power falling off. Treat ignition stability as a prerequisite before chasing more load or timing.

Validation (repeatability checklist)

Use this checklist any time you change hardware or calibration. The goal is not a single “hero pull.” The goal is repeatable behavior you can trust.

1) Control the variables

• Use the same gear and the same RPM range for comparisons.
• Use the same road and direction (grade and wind matter).
• Keep tires and pressure consistent when testing performance.
• Watch temperatures: compare runs at similar IAT/coolant/oil conditions.

2) Change one thing at a time

• If you change hardware and the tune at the same time, you will not know which change caused the outcome.
• Make one change, log it, validate it, then move on.

3) Interpret results as trends

• One run is noise. Multiple runs under similar conditions create a trend.
• If the first pull is great but later pulls fall off, you likely have a heat/margin problem—not a “peak power” problem.

If you see this, stop (safety signals)

This list is intentionally conservative. Reduce load, verify maintenance and fuel quality, and diagnose before continuing.

• Persistent knock correction under the same conditions.
• Misfire under boost (often feels like breakup or “stuttering”).
• Rapidly rising temperatures run-to-run (heat soak) with worsening behavior.
• Sudden torque reductions or throttle closures that weren’t present before a change.

If you are not sure what a log means, default to less load and more margin. This page is educational and not legal advice.

Quick checklist (before you change anything)

Use these questions to keep the process disciplined. You can answer all of them in a few lines of notes, but skipping them is how builds become confusing.

• Goal: What are you optimizing for (daily drivability, track consistency, drag times, or “balanced”)?
• Baseline: What is the car doing right now, and under what conditions?
• Constraint: Is the limiter traction, heat, fueling capacity, ignition stability, or torque limits?
• Variable control: Can you repeat the test with the same gear, road, and temperature range?
• Single change: What is the one thing you are changing today?
• Expected outcome: What should improve if the change works (and what tradeoff might get worse)?
• Stop condition: What would make you back off immediately (knock, misfire, temperature, or intervention)?

Common mistakes

• Changing plugs and immediately “testing” with hard pulls before verifying.
• Assuming misfire is always fuel or tune.

Diagnostics / what logs tell you (high level)

FAQ

Do I need colder plugs for a mild tune?

It depends. Use symptoms and goals. Avoid giving specific plug part numbers as universal advice.

Related guides

• Hub: Tuning hub
• More in this pillar: - /academy/tuning/knock-correction-explained/
  • /academy/tuning/what-to-log-on-a-tuned-car/
• Related (other pillars): - /academy/mods/spark-plugs-gap-heat-range/
• Reference: Glossary
• Brands: Brand pages