Compression Ratio Calculator
The Compression Ratio Calculator determines an engine's compression ratio from swept volume (displacement) and clearance volume (combustion chamber). Optimize your engine build for power, efficiency, and fuel octane requirements.
🕐 Recent Calculations
What is Compression Ratio?
Compression ratio is the ratio of the total cylinder volume (when the piston is at bottom dead center) to the clearance volume (when the piston is at top dead center). A 10:1 compression ratio means the air-fuel mixture is compressed to 1/10th of its original volume before ignition.
Higher compression ratios increase thermal efficiency and power output but require higher octane fuel to prevent detonation (knock). Stock gasoline engines typically run 9:1 to 12:1. Diesel engines use 14:1 to 25:1 because diesel fuel ignites through compression alone.
Formler & Ekvationer som används
Denna Compression Ratio Calculator använder 5 kärnekvationer:
1 Compression Ratio ▼
A cylinder with 500cc swept volume and 55cc clearance: CR = (500 + 55) / 55 = 10.09:1.
2 Swept Volume (Single Cylinder) ▼
Bore = 86mm, Stroke = 86mm: V = 0.7854 × 86² × 86 = 499.6 cc.
3 Clearance Volume from CR ▼
To achieve 9.5:1 with 500cc displacement: CV = 500 / (9.5 - 1) = 58.82 cc.
Hur man använder denna räknare
För att använda denna Förhållande Räknare, följ 3 steg:
Ange Värden
Skriv in de kända förhållandevärdena i inmatningsfälten. Lämna ett fält tomt — det är det okända värdet som Förhållande Räknaren löser.
Välj Läge
Välj förhållandeläge — Lös, Förenkla eller Skala. Varje läge tillämpar olika ekvationer på dina inmatningsvärden.
Få Resultat
Klicka på Beräkna. Resultatskärmen visar svaret med ett visuell förhållandestapel, cirkeldiagram och steg-för-steg-lösningsuppdelning.
Exempelproblem & Steg-för-steg-lösningar
Här är 3 exempelproblem med steg-för-steg-lösningar som använder denna Förhållande Räknare:
Inmatning 1 Engine with 600cc displacement and 60cc chamber
Inmatning 2 Calculate displacement for 88mm bore, 82mm stroke
Inmatning 3 Find clearance volume for 12:1 CR, 450cc displacement
Vanliga Frågor
What compression ratio is best for pump gas (87 octane)? ▼
For 87 octane regular gas, stay at or below 9.5:1 static compression ratio. Modern engines with direct injection and variable valve timing can safely run 10:1 to 11:1 on regular fuel.
How does compression ratio affect horsepower? ▼
Each 1-point increase in compression ratio typically yields 3-4% more power, up to a limit. Going from 9:1 to 10:1 might add 10-15 HP on a 300 HP engine. Beyond about 13:1, gains diminish and knock risk increases.
What is the compression ratio of a diesel engine? ▼
Diesel engines run 14:1 to 25:1 compression ratios. The high compression heats air enough to ignite diesel fuel without spark plugs. Modern common-rail diesels typically use 16:1 to 18:1.
How do I measure clearance volume? ▼
Fill the combustion chamber with fluid (isopropyl alcohol or light oil) through a burette with the valves closed and piston at TDC. The volume of fluid needed equals the clearance volume. Typical values range from 40-70cc.
Does milling the head increase compression ratio? ▼
Yes. Milling (shaving) the cylinder head reduces clearance volume, increasing compression ratio. Removing 0.020 inches from the head surface typically raises CR by 0.3-0.5 points depending on chamber size.