mm
mm
mm
mmSame as stock unless you fit a stroker crank.
Result
New displacement1,699 cc
Increase8.2 %
New compression ratio (same clearance volume)10.73
How it is calculated, with your numbers
- Stock swept volume per cylinder
sw1 = π / 4 × bore1² × stroke1 / 1000π / 4 × 100² × 100 / 1000 = 785.4 cc - New swept volume per cylinder
sw2 = π / 4 × bore2² × stroke2 / 1000π / 4 × 104² × 100 / 1000 = 849.5 cc - Clearance volume per cylinder
clear = sw1 / (cr1 − 1)785.398 / (10 − 1) = 87.27 cc - Stock displacement
disp1 = sw1 × cyl785.398 × 2 = 1,571 cc - New displacement
disp2 = sw2 × cyl849.487 × 2 = 1,699 cc - Increase
gain = (sw2 / sw1 − 1) × 100(849.487 / 785.398 − 1) × 100 = 8.2 % - New compression ratio (same clearance volume)
cr2 = (sw2 + clear) / clear(849.487 + 87.2665) / 87.2665 = 10.73
Working uses the metric units the formula is written in.
- bore1 Stock bore (mm)
- stroke1 Stock stroke (mm)
- cyl Cylinders
- cr1 Stock compression ratio (to 1)
- bore2 New bore (mm)
- stroke2 New stroke (mm)
- sw1 Stock swept volume per cylinder
- sw2 New swept volume per cylinder
- clear Clearance volume per cylinder
- disp1 Stock displacement
- disp2 New displacement
- gain Increase
- cr2 New compression ratio (same clearance volume)
What it means
Displacement is the swept volume of one cylinder (the bore area times the stroke) multiplied by the number of cylinders. A big bore kit enlarges the bore; a stroker crank lengthens the stroke; some kits do both.
The calculator works back from the stock compression ratio to the clearance volume (everything above the piston at top dead centre), then adds the new swept volume to it. More swept volume squeezed into the same space raises compression, which is why many kits come with their own pistons, a thicker gasket or reworked heads to bring the ratio back to what the fuel and cams can take. Use the compression ratio calculator with the kit maker's figures for the exact number.
Assumptions and limits
- Clearance volume unchanged: same heads, gasket, deck height and piston crown volume. Real kits usually change at least one of these.
- All cylinders identical.
Sources
- Heywood, J. B., Internal Combustion Engine Fundamentals, McGraw-Hill, 1988.
Related calculators
Compression ratio calculatorStatic compression ratio from bore, stroke, chamber, gasket, deck height and piston volume.Engine displacement calculatorBore, stroke and number of cylinders to cc and cubic inches.Dynamic compression ratio calculatorEffective compression once the intake valve closes, from static ratio, rod length and cam timing.