Static/Dynamic Compression Ratio Calculator

📐 How to use this calculator: Enter your engine specifications in the sections below. The calculator will automatically update results as you type. Click section headers to expand/collapse. Use quick presets to load common configurations.

📊 Calculation Results

Cubic Inches
Liters
Static CR
:1
Dynamic CR
:1
DCR @ Altitude
:1
Quench
"
IVCA
°
🔧 Engine Basics (Displacement)
Cylinders:
Cylinders
Change only if using this for a different engineThis calculator is designed for 6-cylinder engines. Only change this value if you're calculating compression for a different engine configuration (4-cylinder, 8-cylinder, etc.).
Bore:
inches
Cylinder diameterThe bore is the diameter of the cylinder. Stock 4.0L has a 3.875" bore. Overboring increases displacement and can accommodate larger pistons.
Stroke:
inches
Crankshaft throwThe stroke is the distance the piston travels from top to bottom. Stock 4.0L has a 3.413" stroke. Using a 4.2L crankshaft (3.895" stroke) creates a "stroker" build with increased displacement.
🔩 Head & Gasket Specifications
Combustion Chamber:
cc
Most common volume pre-enteredThe combustion chamber volume is the space above the piston at top dead center. The most common volume (58cc) has been entered. Different cylinder heads will have different chamber volumes.
Deck Clearance:
inches
Stock 4.0L: 0.0215" (Common mod: 0.00")Stock deck clearance has always been 0.0215" throughout production of the 4.0L engine. This only needs to be changed if your engine differs from factory spec. One of the more common modifications is to deck the block for 0.00" deck clearance, which increases compression.
Gasket Thickness:
inches
Typical value: 0.051"Gasket thickness varies depending on material and manufacturer (composite, MLS, copper, etc.). A typical value of 0.051" has been entered based on common experience. Thinner gaskets increase compression ratio.
Gasket Bore:
inches
Most I6 head gaskets: 4.0"If left blank, the calculator will use the cylinder bore for gasket volume calculations. However, most I6 head gaskets have a 4.0" bore, which differs from the cylinder bore. A value of 4.0" has been pre-entered for accuracy.
⚙️ Piston Details (Advanced)
Dome/Dish/Valve Relief:
cc
Positive = dish/relief, Negative = domeA positive number (13 or +13) indicates a depression in the piston top, such as valve reliefs or a dished piston, which decreases compression. A negative number (-13) indicates a protrusion like a domed piston top, which increases compression.
Piston Ring Height:
inches
Top ring to piston top (minimal effect)This is the distance between the top of the first compression ring and the top of the piston. If you don't know this value, leave it blank. It has very little effect on the final compression calculation.
Piston to Bore Clearance:
inches
Check piston manufacturer specsClearance between the piston and cylinder wall varies depending on piston type (cast, hypereutectic, forged). Check with your piston manufacturer for the correct value. This has minimal effect on the final compression calculation.
🏔️ Camshaft & Altitude (Dynamic Compression)
Connecting Rod Length:
inches
Stock 4.0L: 6.123" | Common: 5.875" (4.2L)Stock 4.0L connecting rods are 6.123" center-to-center. Only change this if you're using a different length. A common practice is to use 5.875" rods from the 4.2L engine (click the link to auto-fill).
Cam Intake Duration:
degrees
Advertised duration (not @ 0.050")Use the ADVERTISED duration from your cam manufacturer, not the duration at 0.050" lift. Compression doesn't begin until the valve is fully seated. These numbers assume a single-pattern cam (most common). You can select from preset cams below or enter your own values.
Cam Lobe Separation Angle:
degrees
From cam manufacturer specsLobe Separation Angle (LSA) is the angle between the intake and exhaust lobe centerlines. This specification is provided by your cam manufacturer and affects the power characteristics and dynamic compression ratio.
Advance or Retard:
degrees
Crankshaft degrees (negative for retard)Enter cam timing in crankshaft degrees (not camshaft degrees). Use negative numbers for retarded cams (-4) and positive for advanced cams (+4). This applies even if the advance/retard is "built in" or "ground in". If you have camshaft degrees, double them for crankshaft degrees (e.g., 4 cam = 8 crank).
Intake Valve Closing Angle:
degrees
Check box to enter IVCA directly (Stock RENIX: 75°)If you know the exact intake valve closing angle, check the box to enable this field and enter it directly. This will override the cam duration, LSA, and advance/retard fields above. Stock RENIX IVC angle is 75 degrees.
Altitude:
feet
Leave blank for sea levelAltitude has a significant impact on dynamic compression ratio! Enter your elevation in feet above sea level. Higher altitudes result in lower atmospheric pressure, which reduces effective compression. Leave blank if you're at or near sea level.