1. What is a Compression Ratio Calculator With Boost?

Definition: This calculator determines the effective compression ratio of a boosted engine by accounting for forced induction pressure.

Purpose: It helps engine builders and tuners understand how boost pressure affects the engine's dynamic compression characteristics.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• Effective CR — Dynamic compression ratio under boost (unitless)
• CR — Static compression ratio (unitless, e.g., 10.5:1)
• boost — Forced induction pressure above atmospheric (psi)
• atm_pressure — Atmospheric pressure at sea level (14.7 psi)

Explanation: The boost pressure ratio is added to 1 and multiplied by the static CR to determine the effective compression ratio under boost.

3. Importance of Effective CR Calculation

Details: Knowing the effective CR helps prevent engine knock, optimize timing, and select appropriate fuel for boosted applications.

4. Using the Calculator

Tips: Enter the engine's static compression ratio (e.g., 10.5), boost pressure in psi, and atmospheric pressure (default 14.7 psi). All values must be > 0.

5. Frequently Asked Questions (FAQ)

Q1: Why does boost increase effective compression ratio?
A: Boost forces more air into the cylinder, effectively increasing the pressure before compression begins.

Q2: What's a safe effective CR for pump gas?
A: Typically 12-14:1 for 93 octane, but depends on many factors including combustion chamber design.

Q3: Should I change atmospheric pressure for altitude?
A: Yes, at higher altitudes use lower values (e.g., 12.7 psi at 5,000 ft) for more accurate results.

Q4: How does this relate to dynamic compression ratio?
A: This is a simplified model - actual dynamic CR also depends on cam timing and other factors.

Q5: What's the maximum safe effective CR?
A: Depends on fuel quality and engine design, but rarely exceeds 16:1 even with race fuel.