It's nice that they take the time to put this stuff up there, but even better if it were correct.
Their "math" is simply multiplying the static (mechanical) compression ratio by the pressure ratio, using 14.7 as ambient atmospheric pressure. (24 boost + 14.7 ambient) ÷ 14.7 = 2.63 PR. 2.63 × 6.5:1 = 17.1:1.
Except, that's not what happens in the engine.
Their purpose is to predict the equivalent static CR with the same knock characteristics as a normally aspirated engine, and they simply made this up.
The compression ratio
never changes regardless of boost, the cylinder pressure changes, and the actual math is quite different.
Read my article:
http://victorylibrary.com/mopar/blower-engine5.htm#ECR
