[speeder]

Turbo pressure and CR are generally in an inverse relationship, i.e., the more boost you have, the lower CR needs to be. That is the reason for the crazy turbo lag on 930s and most older turbo cars, until manufacturers figured out how to stagger two small turbos so that boost came on partially @ low rpms.

Higher CR engines w low-boost turbos can be really sweet, I think that is what 991.2 cars have. They also have DI and really sophisticated engine management systems that keep everything optimal all the time, protect the engine from damage, etc...

None of that is germane to modding an old air-cooled 911, except the boost/CR ratio which never really changes.