Compression Ratio Calculator
Calculate engine compression ratio using cylinder specifications and chamber volumes
Engine Specifications
Total number of cylinders in the engine
Cylinder bore diameter in millimeters
Piston stroke length in millimeters
Head gasket thickness (typically 0.9mm)
Gasket bore diameter in millimeters
Distance between piston and deck at TDC
Volume of the combustion chamber
Volume displaced by piston dome/dish (0 for flat top)
🔧 What is Compression Ratio?
The compression ratio is the ratio of the volume of the cylinder when the piston is at the bottom of its stroke (BDC) to when it's at the top (TDC).
Formula:
CR = (Vd + Vc) / Vc
Where Vd = displacement volume, Vc = compressed volume
📊 Typical Ratios
🚗 Regular Gasoline
8:1 to 10:1 compression ratio
⛽ Premium Gasoline
10:1 to 12:1 compression ratio
🏁 Race Fuel
12:1 to 15:1 compression ratio
🚛 Diesel Engines
14:1 to 22:1 compression ratio
🔍 Volume Components
📐 Displacement Volume
Volume swept by piston from BDC to TDC
🏠 Chamber Volume
Volume of combustion chamber at cylinder head
🔧 Gasket Volume
Volume occupied by head gasket
📏 Deck Clearance
Volume between piston and deck at TDC
Understanding Compression Ratio
💡 Why It Matters
The compression ratio is a crucial engine specification that affects performance, efficiency, and fuel requirements. Higher compression ratios generally produce more power but require higher octane fuel.
Understanding your engine's compression ratio helps you:
- Choose the correct fuel octane rating
- Predict engine performance characteristics
- Plan engine modifications safely
- Diagnose potential engine issues
⚡ Performance Impact
Higher compression ratios offer several benefits:
- Increased Power: More compression creates higher cylinder pressures
- Better Efficiency: More complete fuel combustion
- Improved Throttle Response: Faster pressure buildup
However, too high compression can cause:
- Engine knock or detonation
- Required premium fuel
- Increased stress on engine components
Static vs Dynamic Compression Ratio
📏 Static Compression Ratio
The static compression ratio is calculated using the full stroke volume and compressed volume. This is what our calculator computes.
Formula: CR = (Vd + Vc) / Vc
This ratio assumes the intake valve closes exactly at bottom dead center (BDC), which doesn't happen in real engines.
🔄 Dynamic Compression Ratio
The dynamic compression ratio accounts for the actual intake valve closing point, which occurs after BDC in most engines.
This provides a more realistic measure of actual cylinder pressure and engine behavior.
Dynamic ratios are typically 1.5-2.5 points lower than static ratios, depending on cam timing.