RSR BMWAdvanced Dynamic Compression Ratio Calculator
BMW k1200RS: Intake closes 40 deg ABDC measured at .050" lift. Bore 2.775". Stroke 2.953". Static Compression 11.5:1. Rod Length 5.118".
It's the System...Not the Parts!
To see what the effect of bore, stroke, rod length, cam timing, compression ratio, boost pressure and altitude is on your dynamic compression ratio, simply enter the seven variables and the calculator will display the results in a new pop-up window. This will give you an idea of what happens before you assemble your engine and also how you should plan your motor for your target altitude whether it is for racing or day to day operation. Of the variables, the most important is cam timing which has a dramatic effect on your "dynamic" as opposed to your static compression ratio. The more "overlap" your cams have, the lower your "actual" as opposed to your static compression ratio will be. Assuming your motor is somewhat "fixed" then this is the one area you can easily change.
Make Your Motor Work For You...Not Against Itself
If you are designing a new motor another consideration is rod length and it's effect on the engine's output. Changing the rod length will affect the piston location in the bore relative to cam timing everywhere except at TDC and BDC thus changing the dynamics of the engine. Piston design cam selection, and static compression can planned in a more thoughtful manner with the use of this calculator. The calculator will generate your "chamber volume" for you as a target for your "cc'ing" activities.
Cheat, Cheat, Never Get Beat
When you play around with this you may see why RB Racing has run "high" compression ratios on certain turbo bikes when we were running at Bonneville which is at an altitude of 4500 feet. Corrections on a particular day at Bonneville may render this to be a "much higher altitude" due to temperature and other conditions...hot thin air may actually look like 7000 feet! Building a motor that sacrifices torque for peak power by running too low a compression ratio can be a mistake in some cases, especially if you live at higher elevations like in Denver, Colorado. Secret camshafts have have big role in this...but secrets are secrets!
Keep in mind that it is airflow that we are concerned about and that RB Racing Turbochargers are designed to move large volumes of air at low boost levels. Do not confuse boost with airflow...it is CFM that makes the torque and power, not pressure alone.
A final point to consider in this scenario are the mechanical issues of bore size and combustion chamber efficiency...you have to make leeway for these. On less efficient combustion chambers you may elect to keep your static compression lower and compensate with boost. There are a myriad of considerations and experience will beat unproven theorems time after time. A bit of forethought and knowledge never hurt your chances, however.