Compression ratio – Working it out
Working out the compression ratio (CR) of your engine is a simple matter, providing you have the relevant formula, a calculator, and the necessary pieces of data to apply. The data needed is as follows -
Piston dish capacity, if any
Piston to block deck height capacity, if any *
Combustion chamber capacity
*To calculate the piston to block deck height capacity you need to first calculate the capacity of one cylinder, then divide the result by the crankshaft stroke, then multiply this by the measurement.
Note - you should not include the static/cold ring land capacity in the calculation. This gap all but disappears when the engine is at full running temperature. It is why when the engine is over-heated the first section of teh piston you see damage on is the section above the top ring.
Generally the crank stroke and bore diameter dimensions need to be in metric since capacity is widely stated in 'cc's., a metric measurement. And generally, work to 3 decimal places as this makes it as accurate as possible without going mad on decimal places included.
The formula for the calculation is -
Swept volume + Unswept volume
Swept volume is that created by the crankshaft stroke and bore diameter for one of the cylinders in the engine. The formula for that calculation is -
Pi r squared x h
Pi = 3.142
r squared = half the bore diameter multiplied by its self
h = crankshaft stroke in mm.
Unswept volume is the total of all the capacities detailed above.
For a 1275 engine with a standard stroke that has been bored out to +0.060”.
Standard stroke is 3.20”. Converted to metric (multiply inches by 25.4, millimeters in an inch) = 81.28mm
Standard bore diameter is 2.780”. +0.060” makes that 2.840”. Converted to metric (multiply inches by 25.4, millimeters in an inch) = 72.136mm
Stroke is then 81.28mm
Bore diameter is then 72.136mm.
Typical engine build components will give capacities of -
Piston dish capacity - 8.0cc
Piston to block deck height capacity - 1.02cc
Piston ring land capacity - 0.75cc
Gasket type/capacity - 4.0cc
Combustion chamber capacity - 25.0cc
Swept volume =
Bore diameter 72.136 divided by 2 = 36.068
36.068 x 36.068 x 3.142 (Pi) = 4087.429, x 81.28 (h – stroke) = 332.226cc (engine capacity is this x 4 = 1328.905 – 1330cc).
So the swept volume is 332.226cc.
Unswept volume =
8.0 + 1.02 + 0.75 + 4.0 + 25.0 = 38.77cc
So the CR of this engine build is
332.226 + 38.77 = 370.996 = 9.568:1
To find out what capacity you need in a specific component to achieve the CR you want, or what the capacity change of a specific component will make to the CR you can use this formula -
Capacity of a single cylinder
Required CR – 1.0
Basing it on the data from above to work out what combustion chamber capacity would be needed when sorting the cylinder head out -
Single cylinder capacity is 332.226
CR required 9.568:1
332.226 = 332.226 = 38.77cc combustion chamber capacity
(9.568 – 1.0) 8.568
Combining these formulas you can go back and forth to assess the various capacities needed to achieve the CR required, or the CR you will end up with given the component capacities you have.