Compression Ratio Calculator
This compression ratio calculator works out the static compression ratio from bore, stroke and combustion chamber volume per cylinder, or the clearance volume you need for a target ratio.
Example: What to calculate: Compression ratio · Bore (cylinder diameter) 86 mm · Stroke 86 mm · Clearance volume per cylinder 45 cc → Result: Compression ratio 12.1 : 1. Source: Wikipedia – Verdichtungsverhältnis (ε = (Vh + Vc)/Vc). Updated: .
How it is calculated
With an 86 mm bore, 86 mm stroke and 45 cc clearance volume, the compression ratio is 12.1 : 1. It says how strongly the mixture is squeezed: volume at bottom dead centre divided by volume at top dead centre.
How it is calculated
The calculator first finds the swept volume of one cylinder: Vs = π/4 · bore² · stroke. The compression ratio is then CR = (Vs + Vc) / Vc, where Vc is the clearance volume with the piston at top dead centre. If you aim for a particular ratio, rearrange to Vc = Vs / (CR − 1). That shows how large the chamber must be after milling the head or with a thicker gasket.
Notes
- A higher ratio improves efficiency but raises the tendency to knock; fuel quality sets the limit.
- Turbocharged engines usually run a lower ratio than naturally aspirated ones with the same knock resistance.
- Measure Vc yourself by liquid fill where possible; published figures vary.
Frequently asked questions
How do you calculate compression ratio?
CR = (Vs + Vc) / Vc. Vs is the swept volume of one cylinder (π/4 · bore² · stroke) and Vc the clearance volume at top dead centre.
What counts as clearance volume?
The volume in the cylinder head (combustion chamber), the head gasket, any dish or dome in the piston crown and the gap around the piston top.
How does milling the head change the ratio?
The chamber gets smaller, Vc falls and the ratio rises. Enter the new Vc to recalculate.
Is compression ratio the same as compression pressure?
No. The ratio is purely geometric. Measured pressure also depends on valve timing, sealing and cranking speed.
Sources and legal basis
- Wikipedia – Verdichtungsverhältnis (ε = (Vh + Vc)/Vc)
- Wikipedia – Compression ratio (Hubvolumen, Verdichtungsvolumen)
As of: · Editorial policy
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