Alignment Diagnostics and Fleets
Engine Compression Test Variation Calculator
Calculate compression-test spread and percentage variation. Cranking speed, throttle position, test strokes, temperature, gauge, altitude, and engine design affect readings.
Inputs for maximum cylinder variation
Do not combine values from different vehicle configurations merely because their units match.
The relationship under review
Calculate compression-test spread and percentage variation — reference values, sensor readings, and physical measurements should remain separately labeled.
Cranking speed, throttle position, test strokes, temperature, gauge, altitude, and engine design affect readings — that condition defines when maximum cylinder variation is comparable with another result.
Document Highest cylinder pressure as maximum cranking compression reading — this means you should record gauge type, temperature, and whether the value is absolute or relative pressure.
Lowest cylinder pressure. Minimum cranking compression reading — for this measurement, record gauge type, temperature, and whether the value is absolute or relative pressure.
Average cylinder pressure: Mean reading across all cylinders — a compatible entry should record gauge type, temperature, and whether the value is absolute or relative pressure.
The Fleet Vehicle Utilization is the appropriate follow-up when the vehicle review also needs to calculate use of available fleet capacity.
Arithmetic behind the estimate
In “variation = (highest reading − lowest reading) ÷ highest reading,” the entered measurements must use the reference points described above.
No term beyond highest cylinder pressure, lowest cylinder pressure, and average cylinder pressure is introduced in “variation = (highest reading − lowest reading) ÷ highest reading.”
What to retain from the result
Maximum cylinder variation answers “Calculate compression-test spread and percentage variation.” The additional display, Pressure spread, is a different view of the same entered measurements.
Follow manufacturer test procedures and specifications — when that condition changes, compare separate calculator runs instead of blending the inputs.
Because cranking speed, throttle position, test strokes, temperature, gauge, altitude, and engine design affect readings, a disagreement between maximum cylinder variation and an outside reference should trigger a review of highest cylinder pressure and average cylinder pressure.
Practical limitations
A sensor or comparison value can narrow an investigation, but it cannot identify the failed part without the specified physical tests — for highest cylinder pressure, the page specifically expects maximum cranking compression reading.
Tracing the displayed example
The example data set consists of Highest cylinder pressure = 195 psi, Lowest cylinder pressure = 166 psi, and Average cylinder pressure = 183 psi.
Those entries produce Maximum cylinder variation = 14.9% and Pressure spread = 29.0 psi.
Because a different input set is required to add vehicle accessory loads and convert total power to current, use the Vehicle Electrical Accessory Load for that calculation.
Practical questions for this calculator
What measurement source fits Highest cylinder pressure when it represents maximum cranking compression reading?
Because highest cylinder pressure represents maximum cranking compression reading, use a source tied to the exact vehicle, component, and operating period described by the other fields.
How does the warning “Cranking speed, throttle position, test strokes, temperature, gauge, altitude, and engine design affect readings” affect Maximum cylinder variation?
The condition “Cranking speed, throttle position, test strokes, temperature, gauge, altitude, and engine design affect readings” is not corrected automatically by the numeric inputs, so create a separate engine compression test variation case when it changes.
What assumption is expressed by “variation = (highest reading − lowest reading) ÷ highest reading”?
In “variation = (highest reading − lowest reading) ÷ highest reading,” highest cylinder pressure and lowest cylinder pressure are treated as parts of one vehicle case.
How narrowly is Lowest cylinder pressure defined by “Minimum cranking compression reading”?
The definition “Minimum cranking compression reading” excludes a similarly named rating or a measurement taken at another reference point.