How to use the engine displacement and compression ratio calculator
- Choose inches or millimeters, then enter the bore (cylinder diameter) and the stroke, and the number of cylinders.
- Read the displacement per cylinder and in total in cubic centimeters, liters and cubic inches.
- For a compression ratio, also enter the head chamber volume in cc, the piston dish (or negative dome) volume, the gasket thickness and the piston deck clearance.
- Press Copy result or print it. The steps show every calculation so you can check them.
Formula
Displacement per cylinder = pi / 4 x bore squared x stroke
Clearance volume = chamber + dish (or minus dome) + pi / 4 x bore squared x (gasket thickness + deck clearance)
Static compression ratio = (displacement per cylinder + clearance volume) / clearance volume
One inch is exactly 25.4 mm and one cubic inch is exactly 16.387064 cubic centimeters. One liter is 1,000 cc.
A 4.000 inch bore, 3.48 inch stroke V8
Bore 4 inches is 10.16 cm and stroke 3.48 inches is 8.8392 cm. One cylinder holds pi / 4 x 10.16 squared x 8.8392 = 716.62 cc, and eight cylinders hold 5,733.0 cc, which is 5.73 liters or 349.8 cubic inches.
With a 64 cc chamber, a flat piston, a 0.040 inch gasket (8.24 cc) and a 0.005 inch deck clearance (1.03 cc), the clearance volume is 73.27 cc. The compression ratio is (716.62 + 73.27) / 73.27 = 10.78 : 1.
What displacement is
Engine displacement is the volume swept by the pistons as they move from the bottom to the top of their stroke, added up over every cylinder. Each cylinder is a short circular tube, so its swept volume is the area of the circle times the stroke. Total displacement is the volume of one cylinder times the number of cylinders. It is usually quoted in liters or cubic centimeters in most of the world and in cubic inches in older American listings.
The stated size on a car often differs slightly from the exact geometric value, because makers round, and the bore and stroke can vary by small amounts. The bore to stroke ratio is also shown: more than 1 is called oversquare, less than 1 undersquare. Bore and stroke are the only numbers used for displacement, so this works for any engine you have the measurements for.
Compression ratio
Static compression ratio compares the total volume in the cylinder when the piston is at the bottom to the small volume left when it reaches the top. That small volume, the clearance volume, includes the combustion chamber in the cylinder head, any dish in the piston, the thickness of the head gasket and the space between the piston top and the block deck. A dome on a piston takes away volume, so you enter it as a negative dish.
Enter the chamber volume in cubic centimeters, measured or from the head maker, and the gasket thickness and deck clearance in the same unit as the bore. If your piston top has a valve relief that adds volume, include it in the dish value. The tool refuses to compute when the clearance volume works out to zero or less. See the unit converter to move between cc, liters and cubic inches, and the circle calculator for areas of circles.
What it does not tell you
Static compression ratio is geometry only. Cam timing closes the intake valve late, which lowers the pressure the engine actually reaches, and boost, fuel and engine design set what ratio is safe. Use the result for planning and comparison, not as a guarantee. To see how engine size relates to fuel costs, the MPG calculator and tire size calculator can help with other vehicle numbers.
Assumptions and limits
- Cylinders are perfect circles with the bore diameter, and the stroke is the full piston travel.
- Gasket volume is a disc of the bore diameter and the compressed thickness. Real gasket openings can differ from the bore.
- Static compression ratio ignores cam timing, valve overlap and ring land volume above the top ring.
- Conversions use exactly 25.4 mm per inch and 16.387064 cc per cubic inch.
Frequently asked questions
How do I calculate engine displacement?
Multiply pi divided by 4, the bore squared and the stroke to get the volume of one cylinder, then multiply by the number of cylinders. Use the same unit for bore and stroke.
How many cubic inches is a 5.7 liter engine?
One liter is 61.02 cubic inches, so 5.7 liters is about 348 cubic inches. A bore and stroke of 4.000 by 3.480 inches in eight cylinders gives 349.8 cubic inches.
What is the difference between bore and stroke?
The bore is the diameter of the cylinder. The stroke is how far the piston travels from the top to the bottom of the cylinder.
Why does my compression ratio come out different from the manual?
Small differences in chamber volume, gasket thickness and piston position change the result a lot, and some makers quote a dynamic ratio. Check each measurement and the unit you used.