Calculate engine effective compression ratio, required boost, or static compression ratio with boost inputs in psi, bar, kPa, or atm.
Related Calculators
- Bore To Stroke Ratio Calculator
- Drivetrain Loss Calculator
- WHP to HP (BHP/Crank HP) Calculator
- Engine KW to Wheel KW Calculator
- All Automotive Calculators
Boost / Compression Ratio Formula
The calculator uses atmospheric pressure of 14.7 psi as the reference pressure. If you enter boost in bar, kPa, or atm, the boost value is first converted to psi for the compression ratio formula.
ECR = SCR * (1 + B_psi/14.7)
B_psi = 14.7 * (ECR/SCR - 1)
SCR = ECR / (1 + B_psi/14.7)
- ECR = effective compression ratio
- SCR = static compression ratio
- B_psi = boost pressure in psi
- 14.7 = standard atmospheric pressure in psi
Effective Compression Ratio mode: enter boost and static compression ratio to estimate the effective compression ratio under boost.
Required Boost mode: enter static compression ratio and a target effective compression ratio to find the boost pressure needed to reach that target.
Static Compression Ratio mode: enter boost and a target effective compression ratio to estimate the static compression ratio that would produce that target.
Boost Unit Conversion Factors
These are the conversion factors used when boost is entered or returned in units other than psi.
| Unit | To psi | From psi |
|---|---|---|
| psi | 1 | 1 |
| bar | 14.5038 | 0.0689476 |
| kPa | 0.145038 | 6.89476 |
| atm | 14.6959 | 0.0680459 |
Typical Effective Compression Ratio Ranges
| Effective Compression Ratio | General Meaning |
|---|---|
| Under 12:1 | Moderate combined compression and boost level |
| 12:1 to 16:1 | Common range for many boosted gasoline setups, depending on fuel and tuning |
| 16:1 to 20:1 | Higher pressure range that usually needs careful fuel, timing, charge cooling, and engine setup |
| Over 20:1 | Very high combined pressure estimate, commonly requiring specialized parts and tuning |
Example Problems
Example 1: Find effective compression ratio
You have 10 psi of boost and a 9.5:1 static compression ratio.
ECR = 9.5 * (1 + 10/14.7)
ECR = 15.96
The effective compression ratio is about 15.96:1.
Example 2: Find required boost
You have an 8.5:1 static compression ratio and want a 15:1 effective compression ratio.
B_psi = 14.7 * (15/8.5 - 1)
B_psi = 11.24
The required boost is about 11.24 psi.
FAQ
Is effective compression ratio the same as actual cylinder pressure?
No. Effective compression ratio is a simplified estimate that combines static compression ratio and boost pressure. Actual cylinder pressure also depends on cam timing, volumetric efficiency, intake temperature, intercooling, exhaust backpressure, altitude, fuel, ignition timing, and engine speed.
Why does the formula use 14.7 psi?
The formula compares boost pressure to standard atmospheric pressure. At sea level, atmospheric pressure is commonly approximated as 14.7 psi. Higher altitude has lower atmospheric pressure, so this simple formula may overstate the pressure ratio if you are not near sea level.
Can a higher static compression ratio use less boost for the same effective compression ratio?
Yes. For the same target effective compression ratio, a higher static compression ratio needs less boost. A lower static compression ratio needs more boost to reach the same target. This does not mean both setups behave the same, since combustion efficiency, knock resistance, turbo response, and tuning can differ.