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Category Engineering Converters

Presets:

Compression Ratio (r : 1) Range: 2.0 – 50.0

Adiabatic Index (γ) Air: 1.4 | Fuel-air mix: 1.25–1.35

Polytropic Index (n) Typical engines: 1.25 – 1.35

Intake Pressure P₁ (PSI) Sea level: 14.696 PSI. Add boost for turbo.

Altitude (feet) Adjusts P₁ if using atmospheric default.

Output Unit

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About

Miscalculating cylinder pressure from a given compression ratio leads to detonation, ring failure, or head gasket blowout. The relationship between geometric compression ratio r and peak cylinder pressure P₂ is governed by the isentropic process equation P₂ = P₁ × r^γ, where γ is the heat capacity ratio of the working gas. For dry air γ ≈ 1.4. Real engines lose heat through cylinder walls, so a polytropic exponent n between 1.25 and 1.35 produces more accurate estimates. This calculator applies both models and corrects for altitude-dependent atmospheric pressure so the intake charge pressure P₁ reflects actual operating conditions.

Limitations: the tool assumes a sealed cylinder with no blowby and does not model forced induction boost independently. Turbo or supercharged setups must add manifold absolute pressure to P₁ manually. The dynamic compression ratio (DCR) option approximates effective ratio based on intake valve closing angle but does not account for cam profile harmonics or variable valve timing. Pro tip: if your DCR exceeds 8.0:1 on pump gasoline (91 octane), knock risk rises sharply above 160 PSI cranking pressure.

Formulas

The isentropic (adiabatic, no heat loss) compression model relates final pressure to the compression ratio raised to the power of the heat capacity ratio:

P₂ = P₁ × r^γ

For real engines with heat transfer through cylinder walls, the polytropic model replaces γ with the polytropic index n:

P₂ = P₁ × rⁿ

Altitude correction for atmospheric intake pressure uses the barometric approximation:

P₁ = 14.696 × (1 − 6.8756 × 10⁻⁶ × h1)^5.2559

Where: P₂ = final cylinder pressure PSI. P₁ = intake manifold absolute pressure PSI (atmospheric at sea level = 14.696 PSI). r = compression ratio (dimensionless). γ = adiabatic index (heat capacity ratio, 1.4 for air). n = polytropic index (1.25 - 1.35 typical). h = altitude in feet.

Reference Data

Engine Type	Typical CR	Effective γ	Approx. Cranking PSI	Fuel Requirement
Economy Gasoline (Port Injection)	9.5:1	1.30	150 - 170 PSI	87 Octane
Performance Gasoline (GDI)	11.0:1	1.30	180 - 200 PSI	91 Octane
High-Performance NA	12.5:1	1.28	200 - 220 PSI	93 - 100 Octane
Turbocharged Gasoline	8.5:1	1.30	130 - 155 PSI	91+ Octane
Supercharged Gasoline	9.0:1	1.30	140 - 165 PSI	91+ Octane
Diesel (IDI)	21.0:1	1.35	400 - 450 PSI	Diesel #2
Diesel (CDI / Common Rail)	16.5:1	1.35	350 - 400 PSI	Diesel #2
Two-Stroke Gasoline	7.0:1	1.28	110 - 130 PSI	87 - 91 Octane
Rotary (Wankel)	10.0:1	1.28	100 - 120 PSI	91 Octane
CNG / Natural Gas	12.5:1	1.32	190 - 215 PSI	CNG (130 Octane equiv.)
LPG Converted	10.5:1	1.30	165 - 185 PSI	LPG (105 Octane equiv.)
Ethanol E85	13.0:1	1.28	205 - 230 PSI	E85 (105 Octane equiv.)
Methanol Racing	14.5:1	1.26	220 - 250 PSI	Methanol
Aviation (Lycoming / Continental)	8.5:1	1.32	135 - 155 PSI	100LL Avgas
Model Engine (Glow Plug)	15.0:1	1.30	280 - 320 PSI	Nitromethane Blend
HCCI Experimental	18.0:1	1.33	350 - 390 PSI	Gasoline / Diesel

Frequently Asked Questions

The adiabatic model assumes zero heat exchange with the cylinder walls, which never occurs in practice. Real engines lose heat to coolant and oil, lowering the effective exponent from γ ≈ 1.4 to a polytropic n of 1.25-1.35. The polytropic result is always lower and more representative of actual cranking pressure measured with a compression gauge.

Atmospheric pressure drops approximately 1 PSI for every 2,000 feet of elevation gain. At 5,000 feet, P₁ falls from 14.696 PSI to roughly 12.23 PSI, reducing peak compression pressure proportionally. A 10:1 engine that makes 180 PSI at sea level will only produce about 150 PSI in Denver. This directly impacts knock threshold and power output.

Static (geometric) compression ratio is the volume ratio from BDC to TDC based on physical dimensions. Dynamic compression ratio (DCR) accounts for the fact that the intake valve closes after BDC, so the cylinder does not trap the full swept volume. DCR is always lower than static CR. An engine with 11:1 static CR and late intake valve closing at 70° ABDC might have a DCR of only 8.2:1.

Most naturally aspirated gasoline engines should read 125-180 PSI on a cranking compression test. More critical than the absolute number is cylinder-to-cylinder variation: all cylinders should be within 10% of each other. A single cylinder reading 20%+ below the others indicates a valve, ring, or head gasket problem regardless of the absolute value.

Yes, but you must adjust the intake pressure P₁. A turbo running 15 PSI of boost adds that to atmospheric: P₁ becomes 14.696 + 15 = 29.696 PSI at sea level. Enter this value as your custom atmospheric/manifold pressure. The compression ratio remains the geometric ratio of the engine. The result represents approximate peak cylinder pressure before ignition.

The value γ = Cp/Cv depends on the molecular composition of the gas. Pure dry air (mostly diatomic N₂ and O₂) gives γ ≈ 1.4. When fuel vapor is mixed in, the larger polyatomic molecules have more degrees of freedom, lowering γ to 1.28-1.35. Diesel engines compressing pure air before injection use γ closer to 1.35-1.40. EGR and residual exhaust gases further modify the effective ratio.