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Static/Dynamic Compression Ratio Calculator
Written by Frank Gore   
Thursday, 27 October 2005 16:21

Here's a displacement and static/dynamic compression ratio calculator. While intended mostly for Pontiac engines, it'll work just as well for any other OHV V8 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. The numbers pre-entered correspond to a stock '77/'78 Pontiac 400 W72.

Please note, this calculator requires that JavaScript be turned on in your browser.

Data Needed to Compute Static Compression and Displacement
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
Displacement Stock Bore Stock Stroke
287 3.75 3.25
316 3.94 3.25
326 3.72 3.75
347 3.94 3.56
350 ** 3.875 3.75
370 4.06 3.56
389 4.06 3.75
400 4.12 3.75
421 4.094 4.00
428 4.12 4.00
455 4.15 4.21

** the 350 actually displaces 354 cubic inches
Stroke: inches
Combustion Chamber: cc Where do I find the combustion chamber volume?
(this link doesn't work yet)
Deck Height: inches Stock deck height has always been 0.023" throughout the production of the Pontiac 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 Pontiac block is to have a 0.00" deck height.
Gasket Thickness: inches This varies depending on the gasket material, manufacturer, etc. A typical value of 0.042" has been entered based on most common experience.
Dome/Dish/Valve Relief: cc Here a positive number (6 or +6) indicates a depression in the piston like valve reliefs or a dished piston top. A negative number (-6) indicates a protrusion like a domed piston top. Valve reliefs on stock-type pistons are typically 6cc in volume.
Piston Ring Height: inches This is the distance between the top of the first compression ring and the top of the piston. The number pre-entered here was measured from my own stock pistons. 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 Accepted values for this on stock engines were 0.0025" to 0.0033". The average is pre-entered here. 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 Pontiac connecting rods were always 6.625" long. Only change this if you're using a different length.
Cam intake duration: degrees Your cam manufacturer provides these figures. Below is a chart of common stock Pontiac cams. The most common cam's numbers are pre-entered here (cam grind "N"). 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). We're still working on a way to accomodate dual-pattern cams.

Enter your own custom numbers manually, or click on the "Stamp" code in the chart below to have those numbers automatically loaded into the fields on the left
Stamp** Advertised Duration Lobe Intake Centerline
N 273 111.5
U 269 113.5
P 273 113
6 283 114
S 288 116
H 301 115.5
T 308 113.5

** The factory stamp code can be found on the nose of the camshaft
Cam intake lobe centerline: 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 intake lobe centerline fields above. Only people with dual-pattern cams should need to do this, or those that have precisely measured the intake valve closing angle themselves on their engine.
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. A value of +4 degrees is most common for built-in advance in aftermarket cams. 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 (ie. 4 becomes 8, or -3 becomes -6).



Cubic Inches Liters Static
Compression Ratio		 Dynamic
Compression Ratio
:1 :1



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