How this calculator works
Static compression ratio compares the volume above the piston at bottom dead centre with the volume left at top dead centre. Swept volume is the bore area times the stroke. Clearance volume is built from the compressed head gasket, the deck clearance, the combustion chamber and any piston dish or dome. In dynamic mode the intake valve closes after bottom dead centre, so the engine traps less than a full swept volume. The piston position at valve closing is found from the crank radius and connecting rod length, and the trapped volume is used instead. Dynamic ratio is always lower than static ratio when the intake valve closes late.
Formula
Swept volume = pi x bore^2 / 4 x stroke. Clearance volume = gasket + deck + chamber + piston dish. Static CR = (swept + clearance) / clearance. Dynamic CR = (bore area x x + clearance) / clearance, where x = r(1 - cos theta) + l - sqrt(l^2 - r^2 sin^2 theta), r is half the stroke, l is the rod length and theta is measured from top dead centre.
Worked example
A 100 mm bore and 90 mm stroke give 706.86 mL per cylinder. A 101 mm gasket of 1 mm, 0.5 mm of deck clearance, a 60 mL chamber and a flat piston give 71.94 mL of clearance volume. Static compression ratio is (706.86 + 71.94) / 71.94 = 10.83:1. With a 150 mm rod and the intake valve closing 50 degrees after BDC, the effective stroke is 77.93 mm and the dynamic ratio is about 9.50:1.
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Frequently asked questions
Why is dynamic compression ratio lower?
The intake valve stays open after bottom dead centre, so some of the fresh charge is pushed back into the port before compression starts. The cylinder only traps the volume above the piston at the moment the valve closes, which is smaller than the full swept volume.
How do I treat a domed piston?
Enter the dome volume as a negative number, because the dome fills space that would otherwise be part of the combustion chamber. A dished piston adds volume and is entered as a positive number.
Does this replace a burette measurement?
No. Calculated compression ratio is only as accurate as the chamber, gasket and piston volumes you enter. A physical measurement with a burette and a flat plate remains the reference for engine building.
Can I use it for a two-stroke engine?
The geometry is different because the ports control the trapped volume rather than a poppet valve. This page models a four-stroke engine with an intake valve closing point.