Compression Ratio Calculator
Calculate static compression ratio from bore, stroke, chamber volume, head gasket, deck clearance, and piston dome or dish volume.
Combustion chamber volume (cc'd or from specs)
Compressed thickness
Leave blank to use cylinder bore
Distance from piston top to deck surface (positive = below deck)
Positive = dome (raises CR), Negative = dish (lowers CR)
What is Compression Ratio?
Compression ratio (CR) is the ratio of the total cylinder volume when the piston is at the bottom of its stroke (BDC) to the volume remaining when the piston is at the top (TDC). It's expressed as a ratio like 10.5:1.
Why It Matters
CR directly affects an engine's thermal efficiency, power output, and fuel requirements. Higher compression extracts more energy from each combustion event, but also increases the risk of detonation (knock) if the fuel can't handle it.
- 8.5:1 – 10.5:1 — typical for pump gas street engines
- 10.5:1 – 12:1 — usually requires premium fuel (93 octane)
- 12:1+ — race fuel or very careful tuning with direct injection
- 7.5:1 – 9:1 — common in boosted builds to leave room for boost pressure
Static vs Dynamic Compression
This calculator gives static compression ratio — based purely on geometry. Dynamic (effective) compression ratio accounts for cam timing, specifically intake valve closing point. A cam with late intake closing reduces the effective compression because air escapes back out the intake before the valve closes.
The Formula
CR = (Swept Volume + Clearance Volume) / Clearance Volume
Where clearance volume = chamber volume + gasket volume + deck volume − dome volume (or + dish volume). Swept volume = π/4 × bore² × stroke.
Frequently Asked Questions
What compression ratio can I run on pump gas?
On 91 octane pump gas, most naturally aspirated engines can safely run 10.0:1 to 10.5:1 with a modern combustion chamber design. With 93 octane, you can push to 11.0:1 or slightly higher. Older wedge-chamber engines may need to stay at 9.5:1 or lower. Factors like cam timing, quench distance, and ignition timing all affect detonation threshold.
How does compression ratio affect horsepower?
Higher compression extracts more energy from each combustion cycle. As a rule of thumb, each full point of compression ratio is worth about 3-4% in power. Going from 9:1 to 10:1 might add 10-15 HP on a 350 HP engine. However, there are diminishing returns above 12:1, and the risk of detonation increases.
What is the difference between static and dynamic compression ratio?
Static compression ratio (SCR) is the mechanical ratio of total cylinder volume to combustion chamber volume. Dynamic compression ratio (DCR) accounts for the intake valve closing point — the later the valve closes, the more charge escapes before compression begins. DCR is more useful for predicting detonation. A typical street cam with 9.5:1 SCR might only have 7.5:1 DCR.
How do head gasket thickness and piston dome affect compression ratio?
A thicker head gasket increases the combustion chamber volume, lowering compression ratio. A piston dome reduces chamber volume, raising compression. A piston dish adds volume, lowering it. Changing from a 0.040" to a 0.020" head gasket on a small-block can raise CR by 0.3-0.5 points.
Related Articles
How engine displacement is calculated from bore and stroke, the difference between oversquare and undersquare designs, and how displacement affects power and torque character.
Engine BuildingCompression Ratio Explained: How to Calculate, Measure, and OptimizeEverything you need to know about compression ratio — how to calculate it, how to measure it, and how to choose the right CR for your fuel, cam, and boost level.
Engine BuildingCam Specs Explained: Duration, Lift, LSA, and Overlap DemystifiedHow to read camshaft specs — what duration, lift, lobe separation angle, and overlap mean, how they affect power, idle, and drivability, and how to choose the right cam for your build.
Related Calculators
Calculate engine displacement from bore, stroke, and number of cylinders. Results in cc, cubic inches, and liters.
Rod RatioCalculate connecting rod ratio from rod length and stroke. Shows rod angle, piston dwell, acceleration characteristics, and comparisons to common engines.
Piston SpeedCalculate mean piston speed from engine stroke and RPM. Results in ft/min, m/s, mph, and km/h with safety benchmarks for street, race, and extreme applications.
Cam CalculatorCalculate valve timing events, overlap, and cam character from duration at 0.050, lobe separation angle, and advance.