|A typical compression ratio calculator as found in Motion Software's Desktop Dyno program. It relies on measured values to calculate your static compression ratio.|
Your engine actually has two compression ratios, static compression ratio and dynamic compression ratio. Static compression ratio is what you measure when you cc' the combustion chambers, calculate the gasket and deck height volumes, figure in dome or dish volume, valve reliefs and so on. Its a fixed number that doesn't change based on those particular values.
Oddly, a simple misunderstanding of the basic compression stroke leads many of us to conclude that static compression ratio is the real deal and that a 10:1 engine is really a 10:1 CR engine. Well yes, but only when the engine is not running. Static compression ratio (CR) simply represents the ratio of the swept volume of a given cylinder (displacement) to the volume above the piston at top dead center (TDC) as defined by the actual cylinder volume and the combined volume of the spaces above the piston at TDC. Because we all learned the basic four stroke theory of engine operation (intake, compression, combustion, exhaust) we tend to think the compression stroke is the same length as the crankshaft stroke. Its not.
If the intake valve is open at all there is no compression ratio. Compression does not begin until the intake valve closes; not at bottom dead center which is the physical beginning of the commonly stated compression stroke, but at some point up the bore where the intake valve closes and the piston can actually begin compressing the trapped mass in the cylinder. That point is defined by the intake valve closing point or IVC.
|Piston position within the bore at the time of intake valve closing IVO dictates your overall dynamic compression ratio. Note the center piston is rising in the bore, but the intake valve is still open so the compression cycle has not yet begun.|
Consider a 565ci BBC with a 4.250-inch stroke and 6.535-inch rods. Say your cam card indicates that the intake valve closes at 69° ABDC.That means the piston has now passed through Bottom Dead Center and is rising in the cylinder on the compression stroke. How far it travels up the bore before it reaches 69° ABDC depends on the geometry of the stroke and rod length. This can be calculated and some engine modeling programs indicate it. The geometry in this case puts the piston .1.055-inch up the bore at 69° of crank rotation past BDC. The intake valve closes and compression begins. If the engine has a 4.250-inch stroke, the length of the compression stroke is now 4.250 inches minus 1.055 -inch or 3.195inches. This represents the Dynamic Stroke length or the distance remaining to TDC after the intake valve closes. It is the specific dimension needed to calculate the Dynamic Compression Ratio. After calculating the Dynamic Stroke length, that dimension becomes the new stroke length for calculating the Dynamic Compression Ratio. Enter the new dimension as the stroke on any CR calculator and it will calculate the DCR.
Your DCR is generated within this space. Note that the cylinder volume is less than that of the full physical stroke of 4.250-inches. By starting late, (69° ABDC) you are compressing a smaller volume, plus you light it off before it reaches full compression at TDC. The ratio of the cylinder volume at IVC over the volume above the piston at TDC dictates the dynamic compression ratio.The reduced volume that gets compressed represents your dynamic compression ratio and it is always less than the static compression ratio. In most cases it is up to 2 full points lower. Because it is based on fixed values static compression ratio never changes and neither does the dynamic compression ratio unless of course you change the cam itself or alter cam position by advancing or retarding the closing point.
Calculating your DCR when planning an engine is a valuable step that can keep you out of trouble. As a rule a DCR of 8.25:1 is about ideal. Beyond 8.4:1 you run the real risk of breaking a motor even with the best fuels. Here is a handy link to a free online DCR Calculator written by enthusiast Pat Kelley. Thanks to Pat it will calculate your dynamic stroke length, dynamic compression ratio and static compression ratio. While you're there be sure to say thanks.