Here's 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
|
| Stroke: |
inches |
| Combustion Chamber: |
cc |
Where do I find the combustion chamber volume?
(this link doesn't work yet) |
| 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. |
| 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. |
| 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 the "Model Number" in the chart below to have those numbers automatically loaded into the fields on the left.
| Manufacturer |
Model Number |
Intake Duration |
Lobe Separation Angle |
Advance / Retard |
| OEM |
RENIX |
270 |
112 |
-8 |
| COMP |
68-231-4 |
250 |
111 |
.5 |
| Crane |
753905 |
260 |
111 |
-2 |
|
| Cam lobe separation angle: |
degrees |
| 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 and LSA fields above. The stock RENIX IVC angle
is 75. |
| Advance or retard: |
degrees |
If your cam is advanced or retarded, include the amount of advance
or retard here in crankshaft degrees. Use a negative number (-4)
for retarded cams, and a positive number (4 or +4) for advanced cams.
Some manufacturers build advance or retard into the cam, and some
installers purposely advance or retard the cam. If you don't know what
this is, don't worry about it and leave it as-is.
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). |
