Compression Ratio Calculator
Enter your bore, stroke, head gasket, deck height, piston and chamber volumes to get the engine's static compression ratio. Measurements can be in inches or millimeters and volumes in cc or cubic inches.
How to calculate compression ratio
Static compression ratio compares the volume above the piston at bottom dead center with the volume left when the piston reaches top dead center. The volume at top dead center is the clearance volume, and it has four parts: the combustion chamber in the head, the head gasket, the gap between the piston and the deck, and any dish, dome or valve reliefs in the piston.
Compression ratio = (swept volume + clearance volume) ÷ clearance volume
Swept volume = 0.7854 × bore² × stroke
Clearance volume = chamber + gasket + deck + piston volume
Gasket volume uses the gasket's bore and compressed thickness, and deck volume uses the cylinder bore and how far the piston sits below the deck. Multiply cubic inches by 16.387 to get cc so every volume is in the same unit. The displacement calculator covers the swept volume math in more detail.
Getting the signs right
The piston and deck entries need the right sign. A dish or valve reliefs add clearance volume, so enter them as a positive number of cc. A dome takes up room in the chamber, so enter it as a negative number. For a flat-top piston with valve reliefs, enter just the relief volume.
Deck height works the same way. If the piston sits below the deck at top dead center, enter a positive number. If it pokes out above the deck, enter a negative one.
Example: a 350 small-block
Say you're building a 350 bored .030 over, with a 4.030-inch bore, the stock 3.480-inch stroke, 64cc chambers, a head gasket with a 4.100-inch bore and .041-inch compressed thickness, and flat-top pistons with 5cc valve reliefs sitting .025 inch below the deck.
- Swept volume is 0.7854 × 4.030² × 3.480 × 16.387 = 727.4 cc.
- Gasket volume is 0.7854 × 4.100² × .041 × 16.387 = 8.87 cc.
- Deck volume is 0.7854 × 4.030² × .025 × 16.387 = 5.23 cc.
- Clearance volume is 64 + 8.87 + 5.23 + 5 = 83.10 cc.
- Compression ratio is (727.4 + 83.10) ÷ 83.10 = 9.75:1.
What changes compression the most
Starting from the 9.75:1 example above and changing one part at a time:
| Change | Compression ratio |
|---|---|
| Example build as listed | 9.75:1 |
| .028-inch gasket instead of .041 | 10.06:1 |
| Pistons at zero deck instead of .025 below | 10.34:1 |
| 58cc chambers instead of 64cc | 10.44:1 |
| 76cc chambers instead of 64cc | 8.65:1 |
| 5cc dome instead of 5cc reliefs | 10.95:1 |
| 15cc dish instead of 5cc reliefs | 8.81:1 |
In this example the chamber and piston changes move the ratio by about a full point, zero decking moves it about half a point and the thinner gasket about a third of a point.
Compression ratios of Chevrolet Performance crate engines
| Engine | Chamber (cc) | Compression ratio | Fuel |
|---|---|---|---|
| 350/290 HP | 76 | 8:1 | 87 octane regular |
| 350 HO | 64 | 9.1:1 | Premium unleaded |
| Ramjet 350 | 62 | 9.4:1 | Premium unleaded |
| ZZ5 350 | 62 | 9.6:1 | 92 octane premium |
| LS3 | 68 | 10.7:1 | 92 octane premium |
| ZZ427/480 | 110 | 10:1 | 92 octane premium |
| ZZ454/440 | 110 | 9.6:1 | 92 octane premium |
From the Chevrolet Performance spec sheets for the 350/290 HP, 350 HO, Ramjet 350, ZZ5 350, LS3, ZZ427/480 and ZZ454/440.
Static and dynamic compression
This calculator gives static compression, which uses the full stroke. The cylinder can't build pressure until the intake valve closes, and that happens after bottom dead center, so the compression the engine actually sees depends on the cam as well. A cam that closes the intake valve later gives less effective compression from the same static ratio. The cam timing calculator shows where your intake valve closes.
Frequently Asked Questions
How do you calculate compression ratio?
Add the swept volume of one cylinder to its clearance volume, then divide by the clearance volume. Clearance volume is the chamber, head gasket, deck and piston volumes added together.
Is a piston dome entered as a positive or negative number?
Negative. A dome fills part of the chamber, so it reduces clearance volume. Dishes and valve reliefs add volume and are entered as positive numbers.
How much does a thinner head gasket raise compression?
It depends on the bore. On a 4.030-inch-bore 350 at 9.75:1, going from a .041-inch gasket to a .028-inch gasket raises it to about 10.06:1.
What compression ratio can run on pump gas?
It depends on the whole combination, including the heads, cam and timing. For reference, Chevrolet Performance rates its 350/290 HP crate engine at 8:1 on 87 octane and its LS3 at 10.7:1 on 92 octane premium.
What is the difference between static and dynamic compression?
Static compression uses the full stroke. Dynamic compression starts when the intake valve closes, so it depends on the cam and is always lower than the static ratio.
Is a compression test the same as compression ratio?
No. A compression test measures cylinder pressure in psi with a gauge to check the rings and valves. Compression ratio is a ratio of volumes worked out from the engine dimensions.
