Estimate cranking pressure in psi from dynamic compression ratio and altitude, or calculate engine displacement, bore, stroke or cylinders from the others.

An idealized estimate from DCR, altitude and a 1.30 exponent; not a substitute for the engine’s service specification.

Effective trapped-volume ratio after the intake valve closes; must exceed 1.

Blank means sea level. Standard-atmosphere model: โˆ’2,000 to 36,089 ft.

Optional settings
1.30 is a typical assumed value for cranking; change it only if you have a better figure for your engine.

Engine Displacement (Swept Volume) Formula

The following formula is used to calculate total engine displacement (total swept volume) from bore, stroke, and cylinder count. (This formula calculates volume, not cranking pressure.)

DISP = (ฯ€/4) BD2 SL CN

Variables:

  • DISP is the total engine displacement (swept volume) (inยณ if BD and SL are in inches)
  • BD is the bore diameter (inches)
  • SL is the stroke length (inches)
  • CN is the number of cylinders

To calculate displacement, square the bore diameter, multiply by the stroke length and the number of cylinders, and then multiply by ฯ€/4 (โ‰ˆ 0.7854).

What is Cranking Pressure?

Cranking pressure is the pressure generated inside an engine’s cylinders when the engine is being cranked by the starter motor before it starts running. It is an important parameter for diagnosing engine health and performance. Compression readings must be interpreted against the engine service specification and test conditions; higher pressure alone does not establish engine health. Cranking pressure is typically measured with a compression gauge; it cannot be determined from bore, stroke, and cylinder count alone. Use the default โ€œCranking pressureโ€ mode to explore a polytropic model based on DCR and standard-atmosphere altitude. The original default exponent of 1.30 is a modeling assumption, not a universal calibration. Actual compression-gauge readings depend on engine condition and test procedure; compare measurements with the engine service specification.

How to Calculate Engine Displacement?

The following steps outline how to calculate the engine displacement (swept volume).

  1. First, measure the bore diameter (BD) in inches.
  2. Next, measure the stroke length (SL) in inches.
  3. Determine the number of cylinders (CN) in the engine.
  4. Use the formula DISP = (ฯ€/4) ร— BDยฒ ร— SL ร— CN to calculate total displacement.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem :

Use the following variables as an example problem to test your knowledge.

bore diameter (BD) = 4 inches

stroke length (SL) = 3.5 inches

number of cylinders (CN) = 6

Using the formula, the total engine displacement (DISP) is calculated as:

DISP = (ฯ€/4 ร— 42 ร— 3.5 ร— 6) โ‰ˆ 263.89 inยณ