Compression Ratio Calculator

Adjust the calculator values below

Cylinder bore Piston stroke Combustion chamber volume Piston crown volume Enter a dish as positive volume, a dome as negative volume, or 0 for a flat-top piston.

Head gasket bore Compressed gasket thickness Piston-to-deck clearance Positive when the piston is below the deck at TDC; negative when it projects above the deck.

Connecting rod length Intake valve closing (degrees after BDC) Use the cam card's advertised (seat-to-seat) closing angle for an estimate.

Static compression ratio 10.12:1

Dynamic compression ratio 8.38:1

Swept volume per cylinder 499.56 cc

Clearance volume 54.75 cc

Effective dynamic stroke 69.58 mm

Head gasket volume 4.75 cc

Deck clearance volume 0 cc

Cylinder volume at BDC 554.31 cc

10.12:1

Static compression ratio Cylinder volume at bottom dead center divided by clearance volume at top dead center

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Compression Ratio Calculator

Use this compression ratio with cylinder bore, piston stroke, and combustion chamber volume to get fast results, understand the formula behind the...

Static compression ratio formula

Static compression ratio equals swept volume plus clearance volume, divided by clearance volume. Swept volume comes from bore and stroke. Clearance volume combines the combustion chamber, compressed head gasket, piston-to-deck space, and piston crown volume. A dish adds volume; a dome subtracts it.

Dynamic compression estimate

Dynamic compression starts when the intake valve closes, so a later closing angle shortens the effective compression stroke. The estimate uses crank radius, connecting rod length, and the advertised intake closing angle after bottom dead center. Cam timing conventions differ, so compare results only when the same timing convention is used.

Measurement and tuning limits

Use measured chamber, piston crown, gasket, bore, stroke, and deck-clearance values for engine planning. Catalog values and nominal gasket thickness can differ from assembled measurements. Compression ratio alone does not determine fuel octane, cranking pressure, detonation margin, or safe ignition timing.

Frequently asked questions

Common questions about compression ratio, useful assumptions, result interpretation, and mistakes to avoid.

How do I simplify compression ratio?

Simplify by finding a common factor and dividing both parts by it. For ratios and fractions, the relationship stays the same as long as both sides are changed consistently.

Can compression ratio be written as a decimal or percent?

Yes. A fraction or ratio can often be converted into a decimal or percentage, but the best format depends on whether you are comparing parts, rates, shares, or totals.

Why does the order matter in compression ratio?

Order matters when the calculation compares one value to another. Reversing the numerator and denominator can completely change the meaning.

What is the most common mistake with compression ratio?

The most common mistake is mixing part-to-part and part-to-whole comparisons. Make sure the denominator is the total only when the formula calls for the total.

How do I check a compression ratio answer?

Convert it into another equivalent form or multiply back through the relationship. If the converted value does not match the original comparison, recheck the setup.