Compression ratio and deck height calculator

Enter your bore, stroke, head volume, and deck height.  Then press the "Calculate" button to run the program.  Try various deck heights until the desired compression ratio is indicated.

The octane rating needed to prevent detonation is also estimated. There are many factors that affect the octane requirement, including: air density, humidity, air-fuel ratio, ignition advance, head temperature, carburetion, and the camshaft.  .

Sample size is B.S.A 500 cc
Cylinder bore millimeters (mm)
Crankshaft stroke millimeters (mm)
Cylinder head volume cubic centimeters (cc's)
Valve relief pocket, gasket or other volume. Use minus number for dome or pop-up. cubic centimeters (cc's)
Deck height deck is in: inches mm
Compression ratio
to :1 ratio
Estimated octane requirement based on 11.5 times Compression ratio. (R+M)/2 pump gas rating.


The change in horsepower due to the change in compression ratio is relative but not directly proportional. That is to say that a change from 8:1 to 9:1 will give you a larger increase than would the change from 13:1 to 14:1.
I seem to recall for every one point change around 7:1, that is the change from 7:1 to 8:1, would be slightly more than a 3% power increase. Once you get up to around 13:1, that same one point change is only good for about a 1.5% power increase.
The rule of thumb for the compression ratios run in most street engines is: for every point change in the compression ratio your power output will change by 2%. Using this rule of thumb on an engine that produces 100hp, every 1 point change in compression ratio will result in approx. a 2hp change in output.

Displacement Calculator

Enter the cylinder bore and the crankshaft stroke in millimeters. 
The program will calculate your engine size both in cubic centimeters and cubic inches. 
Click on the "Calculate" button to see the results.
bore millimeters (70 to 110)
stroke millimeters (50 to 100)
cylinders (enter 1 to find single cylinder size)
displacement cubic centimers
displacement cubic inches