Engine Compression Ratio Calculator

What Engine Compression Ratio is for

Engine Compression Ratio Calculator is the page to open when you need engine compression ratio from measurements you already have. It belongs with the other automotive calculators because that is the work this arithmetic supports. Power at the crank for that engine is the engine horsepower. Type the entries first. The result stays hidden until you press Calculate, so a half-finished form never flashes a number you might mistake for the answer.

How to fill in this calculator

1. Bore Size. Enter the value this part of the job needs. The unit list on this field includes in, mm, cm.
2. Stroke Length. Enter the value this part of the job needs. The unit list on this field includes in, mm, cm.
3. Chamber Volume. Enter the value this part of the job needs. The unit list on this field includes cu in, cu cm.
4. Piston Style. Enter the value this part of the job needs. The choices include flat top, dish top, dome top.
5. Dome Volume. Enter the value this part of the job needs. The unit list on this field includes cu in, cu cm.
6. Dish Volume. Enter the value this part of the job needs. The unit list on this field includes cu in, cu cm.
7. Deck Clearance. Enter the value this part of the job needs. The unit list on this field includes in, mm, cm.
8. Gasket Thickness. Enter the value this part of the job needs. The unit list on this field includes in, mm, cm.
9. Gasket Bore. Enter the value this part of the job needs. The unit list on this field includes in, mm, cm.
10. Cylinder Count. Enter the value this part of the job needs.

The formula in plain language

Compression ratio is the cylinder's full volume divided by the volume left when the piston is at the top. Full volume is the swept volume plus the clearance. Clearance is the chamber, the head gasket, the deck height, and a dished piston, minus a dome.

A worked example

A cylinder that sweeps 43.7 cubic inches and has 5.0 cubic inches of clearance has a ratio of (43.7 + 5) / 5 = 9.74 to 1.

Displacement, compression, airflow, and the torque curve describe one engine. This engine compression ratio result should be read with engine torque and engine displacement.

Mistakes that move the answer

• A dome reduces clearance. Entering it as a dish makes the ratio look too low.
• Gasket bore is often larger than the cylinder bore. Using the cylinder bore understates the gasket volume.

How to check the result

After Calculate shows the engine compression ratio result, redo the last operation with a pencil using the rounded figures on the screen. Matching that check means the inputs were read correctly. If you edit a label or a sentence in this HTML file, reload the page. The form and this article both live in the file, so the edit is what visitors will read.