Compression ratio, and how to work it out

Compression ratio is one number over another: everything above the piston at the bottom of its stroke, divided by everything above it at the top.

CR = (swept volume + clearance volume) ÷ clearance volume

The swept volume is easy — it is the cylinder. The clearance volume is where every mistake happens, because it is four things added together and one of them is usually a guess.

The five volumes

Swept. A cylinder of bore diameter and stroke length: π × (bore ÷ 2)² × stroke. Times the number of cylinders, this is the displacement the engine is named after. A B18A is 81 mm × 89 mm × 4, which comes to 1,834 cc — and if your arithmetic does not reproduce the number on the badge, stop, because nothing after this will be right either.

Chamber. The pocket in the head, measured in cc by filling it with fluid against a plate. It is the only one of these you cannot calculate from a dimension, and the only one worth measuring yourself if the head has been touched.

Piston dish or dome. A dish is a hollow in the crown that adds clearance volume and lowers the ratio. A dome displaces volume and raises it. Sign conventions differ between calculators, which is a real hazard: enter a dome as a dish and you will calculate a safe engine and build a detonating one. Here a dish is positive and a dome negative.

Deck clearance. How far the crown sits below the deck at top dead centre, times the bore area. You can measure it with a dial gauge, or stack it up from the parts:

deck clearance = block deck height − (rod length + stroke ÷ 2 + piston compression height)

Stacking is where small errors land hardest — it is the difference between four large numbers, so a tenth of a millimetre wrong in any one of them lands whole in the answer.

Gasket. Compressed thickness times the area of the gasket’s own bore, which is usually a little wider than the cylinder. Using the cylinder bore instead is a common shortcut and it overstates the ratio slightly.

Effective compression

Static ratio is geometry. What decides whether an engine detonates is the pressure the charge actually starts from, and boost changes that directly:

effective CR = static CR × (ambient + boost) ÷ ambient

At sea level, ambient is 14.7 psi, so 10 psi of boost on a 9:1 engine puts it near 15:1 on the intake stroke. Altitude works the other way — ambient falls to about 12.1 psi at a mile up, which is why a naturally aspirated engine feels flat in the mountains and why a turbo car needs less timing there than the same boost gauge reading at sea level suggests.

This is an approximation, and it is worth being honest about what it ignores: intercooling, cam timing and the fact that a real cylinder never fills completely. It gives the right order of magnitude. It is not a number to tune to.

The numbers that come with it

Once you have the bore, stroke and rod length, three more fall out for free and all three matter more than they get credit for.

Mean piston speed, 2 × stroke × rpm ÷ 60, in metres per second, is the best single predictor of how long an engine lasts. Road engines generally stay under about 20 m/s; race engines run to 25 and pay for it in rebuild intervals.

Rod/stroke ratio is rod length over stroke. Below about 1.5 the rod angle gets severe and pushes the piston hard into the bore; above about 1.8 the piston dwells longer at the top. Most production engines land between 1.55 and 1.75, and the effect is far smaller than internet argument suggests.

Peak piston acceleration happens at top dead centre and is ω² × r × (1 + r ÷ l), with r the crank throw and l the rod. It scales with the square of engine speed: an 89 mm stroke at 8,000 rpm pulls about 4,200 g. That is the load trying to pull the rod bolts apart, and it is why another 500 rpm costs so much more than it looks like it should.