| Static and Dynamic Compression
When deciding what Static compression ratio to use on your engine build, you must calculate your Dynamic compression ratio first.
Static C/R is the most commonly used term when describing compression ratios.
Static compression Takes: Bore, Stroke, Deck Height, Head Gasket Thickness, Piston Top Volume, and Combustion Chamber Volume and calculates the difference (ratio) between when the piston is completely at "Bottom Dead Center" compared to the piston being completely at "Top Dead Center".
This Image is located on the RB Racing website.
They not only have informative articles but, they also have several engine calculators.
www.rbracing-rsr.com Dynamic Compression Ratio uses the Static Compression and adds valve timing into the equation.
Here is a link to a free download of a very good Dynamic Compression Calculator.
Static compression is calculated as if the valves are closed. Dynamic compression is calculated using cam timing as if the valves are open and closed. This basically means that you can have as much "static" compression as you want but, if the valves are not closed, you will not have any compression (or pressure).
I guess you could say that "Static" compression is theoretical and "Dynamic" compression is realistic or actual.
Most people that are familiar with dynamic compression, thinks that the ideal dynamic compression ratio is around 8.5:1. A dynamic C/R that is too high can cause detonation.
When you use a dynamic compression calculator you first want to determine what what DCR (dynamic compression ratio) that you want to target. We usually target higher that the recommended 8.5:1 ratio usually around 9:1 - 9.5:1. After putting the figures into the calculator, you can change the static C/R accordingly to get the correct DCR that you are targeting.
Here is an example: You have a 103 ci, you are targeting 9:1 DCR, you have purchased pistons that will give you a 10.25:1 SCR and a cam with an intake valve closing of 40 degrees (such as a TW-6). You are at an altitude of 1,000 ft altitude. I will be using the DCR calculator from RB Racing.
This engine will have a DCR of 9.27:1 with a cranking pressure around 192 lbs.
You will have to lower the 10.25:1 static compression to 10:1 to achieve your DCR target of 9:1.
This will also lower the cranking pressure to around 184.
You can also see how that cam timing affects DCR. Using the same engine listed above change the cam to a SE 264 with an intake valve closing of 60 degrees.
The DCR would be 8.3:1 with a cranking pressure of around 166 lbs.
You will need to raise the 10.25:1 SCR to 11.1:1 to achieve your DCR target of 9:1
This will give you a cranking pressure of around 185 lbs.
Click here for a link to a Kool Interactive Compression Animation
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