RSR Boost Compression Ratio
To see what the effect of boost pressure and altitude is on your static compression ratio, simply enter the three variables and the calculator will display the results in a new pop-up window. This will give you an idea of what happens under boost and also how you should plan your motor for your target altitude whether it is for racing or day to day operation. There are other variables, the most important of which is cam timing which has a dramatic effect on your "effective" 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. To give you an idea of this, an 11.5:1 compression motor may have cranking presures of 165 psi whereas another 10:1 motor may show 190 psi. If the barn door is open the horses are going to get out.
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.
A 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.