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Heat range choice for a high-compression engine

Aviation Mechanic Powerplant (AMP) practice question 8 of 11 on Ignition and Starting Systems. ACS element AM.III.H.K2 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONAn engine built with a high compression ratio generally uses spark plugs of which heat range?

  1. AHotter range plugs, to burn off carbon
  2. BEither range; reach sets the heat path
  3. CColder range plugs
Show the answer and explanation

Correct answer: C. Colder range plugs

  • A — incorrect Low-compression engines tend to use hot range plugs; a hot plug in a high-compression engine invites pre-ignition.
  • B — incorrect Reach is the length of the threaded portion entering the cylinder; the principal factor in heat range is the length of the insulator nose core.
  • C — correct High-compression engines tend to use colder range plugs, which have a short insulator nose that transfers heat rapidly to the cylinder head.

BACKGROUND

Heat range is a measure of a spark plug's ability to transfer the heat of combustion to the cylinder head. The plug must run hot enough to burn off the carbon deposits that cause fouling, yet cool enough to avoid pre-ignition, which happens when the electrodes glow red and light the air-fuel charge early. The length of the nose core is the principal factor in establishing heat range: hot plugs have a long insulator nose and a long heat transfer path, while cold plugs have a short insulator for rapid heat transfer. The manufacturer picks the plug after testing, based on compression ratio, degree of supercharging, and how the engine is operated.

STUDY TIP

Link the words in pairs: long nose equals hot plug equals low compression; short nose equals cold plug equals high compression. Do not confuse heat range with reach.

KEY POINTS

  • High-compression engines tend to use colder range plugs.
  • Nose core length is the principal factor in heat range.
  • Too hot a plug can cause pre-ignition; too cold a plug fouls.

SOURCES

  • sources/amp-powerplant/ch04.txt "Spark Plugs"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-32B Aviation Maintenance Technician Handbook – Powerplant, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.