Displacement is the easy half
Swept volume for one cylinder is the area of the bore multiplied by the stroke: pi times the square of half the bore, times the stroke. Multiply by the cylinder count and you have the engine. Everything after that is unit conversion — a cubic inch is 16.387 cc, and a litre is 61.024 cubic inches — which is why the same engine gets called a 350, a 5.7 and a 5,735 cc depending on who is writing.
The part people get wrong is the bore they type in. Nominal bore is what the engine left the factory as. Finished bore is what is in the block now, after however many rebuilds. A 4.000 inch bore taken 0.030 over becomes 4.030, and across eight cylinders that is five extra cubic inches. Measure the bore rather than reading it off a casting number, especially on anything with unknown history.
The clearance stack, item by item
Compression ratio is swept volume plus clearance volume, divided by clearance volume. The swept volume comes straight out of bore and stroke. Clearance volume is the total space left above the piston at top dead centre, and it is the sum of four separate things that are measured four different ways.
| Volume | Where the number comes from | Effect on ratio |
|---|---|---|
| Combustion chamber | Cast figure, or cc the head yourself with a burette and a plate | Larger chamber lowers the ratio |
| Head gasket | Gasket bore and compressed thickness, calculated as a short cylinder | Thicker gasket lowers the ratio |
| Deck clearance | Piston crown to deck surface at TDC, measured with a dial indicator | More clearance lowers the ratio |
| Piston dish or dome | Manufacturer figure, or cc the crown yourself | A dish lowers it, a dome raises it |
Note that three of the four lower the ratio and only one raises it. That is why the sign on the piston volume is the error that keeps happening.
The domed piston sign error
A dished piston has a bowl in the crown. That bowl is extra space above the piston, so it adds to clearance volume and pulls the ratio down. A domed piston sticks up into the chamber and fills space that would otherwise be there, so it subtracts from clearance volume and pushes the ratio up. Enter a dome as a positive number and the calculation adds volume that physically is not there, and you end up building an engine you believe is 9.5 to 1 that is actually somewhere near 11. On pump fuel that difference is the difference between an engine that runs and an engine that detonates itself apart under load on a hot day. In this calculator a dish is positive and a dome is negative, and if you are unsure which one you have, put the piston on the bench and look at it rather than trusting a part number.
Static compression is not dynamic compression
Everything above is static compression: pure geometry, valves ignored. What the engine actually squeezes depends on when the intake valve closes, and on a long-duration camshaft that happens well after bottom dead centre, so part of the stroke pushes mixture back out of the cylinder. An engine with 11 to 1 static and a big cam can behave, on the octane it needs, like something considerably lower. That is why the same static ratio is fine in one combination and a disaster in another, and why nobody can tell you a safe ratio without knowing the cam, the chamber, the fuel, the cooling system and the altitude. Use this page to know what you built. Use a person who has built the same combination to decide whether it is the right one.
If you are working through the rest of a build or a purchase, the gear ratio and speed calculator covers what the engine sees at cruise, and the used car checklist covers what to look at before you inherit somebody else's compression problem.
Questions people ask
Why does my calculated displacement not match what the engine is called?
Because engine names are marketing and rounding, not measurements. A 5.7 litre engine is rarely exactly 5,700 cc, and the same casting sold in cubic inches in one market and litres in another gets rounded differently in each. On top of that, any engine that has been rebored is physically larger than its badge. If your arithmetic says 355 cubic inches and the fender says 350, the arithmetic is describing the engine and the fender is describing the family.
How do I get the chamber volume if I do not have a spec sheet?
You cc the head. The chamber is sealed with a flat plate over the gasket surface, held with grease, with a small hole to fill through, and a graduated burette meters a liquid in until the chamber is full and the reading tells you the volume. It is fussy, it needs the valves in and sealed, and it is the only way to know what a head that has been milled or repaired actually holds. Cast figures are a starting point and are frequently wrong on a head with history. If the ratio matters to the build, measure it.
Does the head gasket really change the ratio enough to care about?
Yes, and it is the cheapest lever you have. A typical gasket contributes somewhere near eight to ten cc per cylinder, and swapping between a thin and a thick gasket can move a whole cc or more. On a small chamber that is a couple of tenths of a point of compression. Builders use gasket thickness deliberately to trim a ratio into range and to set quench clearance at the same time, which is why the compressed thickness matters and the boxed thickness does not.
What compression ratio should I build?
That is not a question a calculator answers, and anyone who gives you a single number without asking about the camshaft, the fuel, the chamber design, the cooling system and where you live is guessing. Static ratio interacts with valve timing, quench, ignition timing, intake air temperature and octane, and the failure mode when you get it wrong is detonation, which destroys pistons and head gaskets quickly and expensively. Work out what you have with this page, then take the whole combination — cam card, head casting, piston part number, fuel you can actually buy — to someone who has built that combination before.
Can I use this for a diesel or a boosted engine?
The geometry is identical, so the displacement and the static ratio come out correctly. What the numbers mean is different. Diesels run static ratios far above anything a petrol engine tolerates because they compress air alone, and boosted engines run lower static ratios because the turbo or supercharger adds cylinder pressure on top of what the geometry produces. The clearance stack arithmetic does not change; the judgement about what ratio is appropriate changes completely, and it is not made from a page.