Compression ratio calculator

What compression ratio is

It is a ratio of volumes, not a pressure. The smaller number is the clearance volume: everything still left above the piston when it reaches top dead centre. The larger number is that same clearance plus the swept volume, the cylinder the piston displaces between bottom and top dead centre.

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

The V8 7.0L Pushrod in the simulator has a 104.8 mm bore and a 101.6 mm stroke, so each cylinder sweeps 876 cc. At 11.0:1 the clearance has to be 87.6 cc, because 876 ÷ 10 is what the equation rearranges to. Nothing about fuel, boost or cylinder count enters it. Eight cylinders of that geometry make 7.01 L, and the ratio is the same as it would be on one.

Every term in the equation

The swept volume is bore, stroke and π. The clearance volume is four separate things stacked, and getting one of them wrong by two cubic centimetres moves the ratio by 0.23 of a point on this engine.

• The combustion chamber. The volume cast and machined into the head above the valves, with the valves and the spark plug fitted. Catalogue numbers are nominal; a burette and a plate of perspex give the real one, and a skimmed head is smaller than the catalogue says.
• The head gasket. A short cylinder of gasket bore by compressed thickness, and the word compressed is the one that matters: a gasket measures thicker in the box than it does torqued down. A 1.3 mm gasket across this 104.8 mm bore is 11.2 cc, 13 % of the whole clearance volume. Gasket bore is usually a little larger than cylinder bore, which is why the calculator takes it separately.
• Deck clearance. The gap between the piston crown and the top of the block at top dead centre. Positive when the crown stops below the deck, which adds volume. Negative when the piston stands proud of the deck, which takes volume away, and the calculator takes a negative number for that case.
• Piston dish or dome. Here a dish is positive and a dome is negative, because a dish adds volume above the piston and a dome fills it. Published calculators split on this sign, and a number carried across from one that inverts it lands a full point out. Valve reliefs count as dish, positive, on top of whatever the crown does.

Static versus dynamic

The static ratio assumes compression begins at bottom dead centre. It does not. The intake valve stays open well past bottom dead centre so the incoming column's momentum keeps filling the cylinder, and nothing is compressed until that valve shuts. Dynamic compression ratio measures from the crank angle where the valve closes, so part of the stroke is spent pushing charge back out of the port rather than squeezing it.

On this short block, 11.0:1 static with the intake valve closing 60° after bottom dead centre and a 154.1 mm rod is 9.13:1 dynamic. Close the valve ten degrees earlier and it becomes 9.71:1. Ten degrees of cam timing is worth 0.57 of a point, and buying the same swing through static ratio alone would mean cutting 4.8 cc out of the chamber. That is why a big cam and a high static ratio go together: the cam gives some of the ratio back, and the engine that reads 11:1 on paper is compressing like a 9.1:1 engine.

Rod length is in the calculation for the same reason. A long rod holds the piston near bottom dead centre for longer, so less of the stroke has passed by the time the valve shuts, and the dynamic ratio comes out higher for identical volumes and identical cam timing.

Boost and effective ratio

A compressor delivers charge that is already above atmospheric pressure, so the cylinder's ratio acts on top of the compressor's. Treating that first stage as isentropic gives the effective ratio:

CR_effective = CR_static × (P_absolute ÷ P_atmospheric)^(1 ÷ 1.4)

One bar of boost on a 10:1 engine reads 16.33:1 by that formula. The forum rule of thumb takes a square root instead and reads 14.10:1, roughly 14 % lower, which is why two people can quote different effective ratios for the same engine and both believe they are right.

Effective ratio is a knock proxy, not a work ratio. It says something about the pressure and temperature the end gas sees; it says nothing about expansion, because the piston still expands over the same geometric volume it always did. That is the reason boosted engines run low static ratios: the highest static ratio on any charged petrol engine in the roster is 10.0:1, while its naturally aspirated petrol engines reach 13.3:1.

What the roster runs

Every crank engine in the simulator, sorted by compression ratio. Bore, stroke, ratio, fuel and induction are the preset's own configuration; the peak is what that engine measured on the simulator's dyno, not a quoted figure. The spread runs from 18.7:1 on the Two-Stroke Diesel I6 7.0L down to 6.0:1 on the blown engines at the bottom, which trade static ratio for manifold pressure.

Three groups fall out of that table. Every diesel sits above every petrol engine, because a diesel compresses air alone and needs the heat to light the injection. Of the 21 naturally aspirated petrol engines, 14 fall between 10:1 and 11.5:1, with the two racing engines above them on 102 octane. The charged engines sit lower, and the wartime aero engines lowest of all, running high manifold pressure on 130 octane avgas. Why the fuel sets that ceiling is the subject of what engine knock is; this page does not print an octane verdict, because the simulator's knock model has no spark map and a single ignition timing cannot answer that question honestly.

Worked example: the 7.0-litre V8

Take the 104.8 × 101.6 mm short block, a flat-top piston, zero deck clearance and a 1.3 mm compressed gasket, and ask what chamber reaches 11.0:1.

• Swept volume per cylinder: 876.4 cc. Across 8 cylinders that is 7.01 L.
• Clearance volume for 11.0:1: 876.4 ÷ 10 = 87.64 cc.
• Gasket, deck and piston together: 11.21 cc, all of it gasket here.
• Head chamber needed: 87.64 − 11.21 = 76.4 cc.

Fit a 70 cc head to the same short block instead and the ratio goes to 11.79:1: 6.4 cc of chamber is worth 0.79 of a point. Then give it a cam that closes the intake valve 60° after bottom dead centre, and what the engine actually compresses is 9.13:1. Those three numbers are the whole argument: the same short block reads as three different engines depending on which ratio someone quotes at you.

The preset built on this geometry measures 502 hp at 5,688 rpm and 648 N·m at 4,638 rpm on the simulator's dyno. Change its ratio in the spec editor and run the dyno again to see what a point is worth on a real curve rather than in a table.

Questions people ask

Sources

• John B. Heywood, Internal Combustion Engine Fundamentals, chapter 2. The cylinder geometry, the clearance-volume definition and the crank-slider relation the dynamic calculation uses.
• Wallace Racing, dynamic compression ratio calculator The classic reference for computing from intake valve closing and rod length, and the page most forum arguments about dynamic ratio point at.
• Chevrolet Performance crate engine specifications The published 11.0:1 and 104.8 × 101.6 mm the LS7 preset is built to.
• The sign convention for piston volume is not standard across published calculators: some ask for a dome as a positive number and subtract it internally, others ask for it signed. This page takes dish positive, dome negative, and says so on the field.

Related reading

• What engine knock is The end-gas chain that sets the ceiling on the number this page computes.
• The Otto cycle explained Why raising the ratio raises thermal efficiency, and by how much.
• How a turbocharger works Where the pressure in the effective-ratio formula comes from.
• Every engine in the simulator The 38 crank engines in the table above, filterable by fuel and induction.

Other calculators

• Engine displacement calculator Bore × stroke × cylinders to cc, litres and cubic inches, with overbore.
• Bore and stroke calculator Solve bore or stroke for a target size, the ratio, and piston speed at redline.
• Horsepower calculator Torque and rpm to hp, kW and PS, or a measured estimate from displacement.
• Dyno correction calculator SAE, STD, DIN and EC corrected power from one reading.

Want this to solve for deck height or cam timing as well? Tell us.