Understanding static compression ratio
Static compression ratio compares the volume of one cylinder with the piston at bottom and top dead center. A 10:1 ratio means the bottom-dead-center volume is ten times the remaining top-dead-center volume. This geometric ratio has no pressure units.
CR = (Vs + Vc) ÷ Vc; Vc = chamber + gasket + deck + dish − dome
Gasket, deck clearance, dish and dome
Gasket volume uses its bore and compressed thickness. A piston below the deck adds volume; a piston above it subtracts volume. Dishes and valve reliefs add space, while a dome removes space. Enter the net crown volume from measurement or piston documentation, and do not count a valve relief twice.
Worked example
An 86 mm bore and stroke give 499.56 cc swept volume. A 50 cc chamber, 87 mm gasket bore with 1 mm thickness, and 0.2 mm deck clearance give 5.94 cc gasket volume and 1.16 cc deck volume. With a flat piston, clearance is about 57.11 cc and compression is about 9.75:1. A 5 cc dish lowers the ratio; a 5 cc dome raises it.
Static ratio, dynamic ratio and compression testing
Dynamic compression depends on intake-valve closing and the effective compression stroke. Those inputs are absent here, so this tool reports static geometry only. A compression gauge measures cranking pressure, which also depends on valve timing, sealing, cranking speed and test conditions. There is no unique conversion from static ratio to a gauge reading.
Checking your measurements
Use chamber and deck measurements for the actual machined assembly. All volumes are per cylinder. This model omits separate top-ring crevice volume and does not check mechanical clearances. A positive clearance volume does not establish component fit, a suitable fuel octane or a safe boost level.