Compression Ratio Calculator
Static compression from your engine's real volume budget, dynamic compression from your cam timing - and where the error actually comes from.
Enter Engine Measurements
Compression Ratio Results
Volume budget per cylinder
| Volume | cc |
|---|
Hit a target ratio?
Dynamic compression (optional)
Enter rod length and where the intake valve closes, and the effective stroke that remains is computed from exact crank geometry. Use the advertised (seat) closing point - if you only have the 0.050 in figure, add about 15 degrees, per KB Pistons' convention.
How the ratio is computed
Static compression ratio is the cylinder's full volume at bottom dead center divided by what remains at top dead center: (swept + clearance) / clearance. The clearance volume is a budget with four line items - combustion chamber, compressed head gasket, deck clearance, and the piston top, where a dish adds volume and a dome subtracts it. The default build here is the worked example from JE Pistons' compression ratio guide, a 496 big-block Chevy that lands on 10.25:1, so you can check this calculator against their published arithmetic line by line. The displacement figures use the same bore and stroke, so one set of measurements answers both questions.
Static vs dynamic compression
The static ratio assumes compression starts at bottom dead center, but it cannot start until the intake valve closes - and on a performance cam that is well after BDC, with the piston already partway up the bore. The dynamic ratio uses the effective stroke that remains at that point, computed from exact rod-and-crank geometry, which is why it needs the rod length. It is a comparison number, not a measurement: conventions differ on which closing point to use (seat timing, or the 0.050 in figure plus about 15 degrees, per KB Pistons), so use one convention consistently when comparing builds. As a widely used rule of thumb from the same source, pump-gas builds aim for roughly 8.5:1 dynamic or below - and boost changes the picture entirely, which is why forced-induction engines run lower static ratios (the boost to horsepower calculator covers that side of the trade).
Where builds actually go wrong
Almost never in the formula - in the inputs. Chamber volumes are advertised nominally and change the moment a head is milled; serious builders cc the chamber with a burette. Deck clearance is the quiet one: pistons rarely sit exactly at the published deck height, and hot-rodded blocks have usually been decked at least once. Measure it, and remember it can be negative when the piston pops above the deck. Gasket catalogs list both compressed and uncompressed thickness - only the compressed number belongs in this math. Get those three right and the second decimal of the result starts meaning something. Those same two inputs also set the quench distance shown under the table: deck plus compressed gasket is the squish gap between the piston's flat rim and the head, and the commonly cited street target of roughly 0.035 to 0.045 in is as important to detonation resistance as the ratio itself.