Static/Dynamic Compression Ratio Calculator

Here is a displacement and static/dynamic compression ratio calculator. While intended mostly for Jeep engines, it'll work just as well for any other engine too as long as you can provide all the necessary specifications. If you can't find some of the numbers, just leave them as is.

Data Needed to Compute Static Compression and Displacement
Cylinders: Cylinders Change this only if your using this calculator for a different engine.
Bore: inches Enter your own custom numbers manually, or click on any of the numbers in this chart to automatically load them into the fields on the left.
The "Base Stroker" below gets its increased stroke by using a 4.2L (258ci) crankshaft.
Displacement Bore Stroke
Stock 4.0L (242ci) 3.875 3.413
Base Stroker 4.5L 3.875 3.895
+.030 4.6L 3.905 3.895
+.060 4.7L 3.935 3.895
+.080 4.8L 3.955 3.895

Stroke: inches
Combustion Chamber: cc The most common volume has been entered for you.
Deck Clearance: inches Stock deck clearance has always been 0.0215" throughout the production of the 4.0L engine. This only needs to be changed if your engine differs from factory spec in this respect. One of the more common changes to a 4.0L block is to have 0.00" deck clearance.
Gasket Thickness: inches This varies depending on the gasket material, manufacturer, etc. A typical value of 0.051" has been entered based on most common experience.
Gasket Bore: inches If a value is not entered here, the the gasket volume (needed to complete the equation) will use the main cylinder bore for its calculations.
A Value of 4 has been entered for you since most I6 Head gaskets have a 4" bore which is not the same as the cylinder bore.
Dome/Dish/Valve Relief: cc Here a positive number (13 or +13) indicates a depression in the piston like valve reliefs or a dished piston top. A negative number (-13) indicates a protrusion like a domed piston top.
Piston Ring Height: inches 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, just leave it as is. It has very little effect on the result anyways.
Piston to Bore Clearance: inches This number can vary depending on the type of aftermarket piston you use. If in doubt, check with your piston manufacturer. An accurate value here has very little effect on the final result.
Altitude: feet Altitude has a large impact on Compression! Enter your altitude or leave blank for sea level.
Data Needed to Compute Dynamic Compression
Connecting Rod Length: inches Stock 4.0L connecting rods were always 6.123" long. Only change this if you're using a different length. One common practice is to use a 5.875" 4.2L rod.
Cam Intake Duration: degrees Your cam manufacturer provides these figures. The ADVERTISED duration is the number that should be used, not the duration at 0.050" since compression doesn't begin until the valve is fully seated. These numbers assume a single-pattern cam (most common).
Enter your own custom numbers manually, or click on a manufacturer from the chart below to view a predefined list of cams.

Cam Lobe Separation Angle: degrees
Advance or Retard: degrees If your cam is advanced or retarded, include the amount of advance or retard here in crankshaft degrees. If the advance or retard of the cam is "built in", or "ground in" it is still important to enter it here. Use a negative number (-4)for retarded cams, and a positive number (4 or +4) for advanced cams.
Remember to use crankshaft degrees, and not camshaft degrees. If you have a value in camshaft degrees, double it to get the right crankshaft degrees (eg. 4 becomes 8, or -3 becomes -6).
Intake valve closing angle: degrees If you'd rather enter the intake valve closing angle directly yourself, you can do so with the field on the left. You'll need to place a checkmark in the box beside the field to make sure the calculator ignores the cam duration, LSA and Advance / Retard fields above. The stock RENIX IVC angle is 75.




Cubic Inches Liters Static Compression Ratio Dynamic Compression Ratio Dynamic Compression Ratio (@ Altitude) Quench Intake Valve Closing Angle
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