SkyDrill

Compression ratio calculation

Aviation Mechanic Powerplant (AMP) practice question 3 of 13 on Reciprocating Engines. ACS element AM.III.A.K3 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONA cylinder contains 140 cubic inches of space with the piston at the bottom of the stroke and 20 cubic inches with the piston at the top of the stroke. What is the compression ratio?

  1. A6 to 1
  2. B7 to 1
  3. C120 to 1
Show the answer and explanation

Correct answer: B. 7 to 1

  • A — incorrect That result does not come from dividing the bottom-of-stroke volume by the top-of-stroke volume, which yields 7.
  • B — correct Dividing 140 cubic inches by 20 cubic inches gives 7, so the ratio is 7 to 1.
  • C — incorrect Subtracting 20 from 140 gives the displacement volume, not a ratio; compression ratio is a quotient of the two volumes.

BACKGROUND

All internal combustion engines must compress the air-fuel charge to get a reasonable amount of work from each power stroke. Compression ratio compares the volume of space in the cylinder with the piston at the bottom of the stroke to the volume with the piston at the top. The comparison is expressed as a ratio, so 140 cubic inches at the bottom against 20 cubic inches at the top reduces to 7 to 1. Compression ratio is a controlling factor in the maximum horsepower an engine develops, but it is limited by available fuel grades and by the high engine speeds and manifold pressures required for takeoff.

STUDY TIP

Always divide the larger bottom-of-stroke volume by the smaller top-of-stroke volume, then reduce the fraction; subtracting the two volumes gives displacement, not a ratio.

KEY POINTS

  • Compression ratio compares cylinder volume at bottom of stroke to volume at top of stroke.
  • 140 cubic inches over 20 cubic inches reduces to 7 to 1.
  • Fuel grade, engine speed, and manifold pressure limit how high the ratio can go.

SOURCES

  • FAA-H-8083-32B ch01 "Compression Ratio"
Practice the Aviation Mechanic Powerplant (AMP) test →Timed 100-question exam, by ACS area, or review your misses. Free.
MORE ON RECIPROCATING ENGINES

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.