how can i calculate the safe boost for any engine ?

 

There's no way to calculate it how much, it's trial and error. It's much easier if someones done it before you and saves you the trouble.

 

keep turning it up until something breaks, then back it off a bit.

 

its the perfect answer i hear at all lol

 

i know it depend on the C R. but is there any equation to calculate it ?

 

It does not depend on compression ratio, it depends entirely on tuning. Otherwise there would only be one compression ratio used for every application. Generally speaking, almost all engines can handle some positive boost pressure over ambient (atmospheric). A majority of decently designed stock engines can at least handle a half an atmosphere of positive boost pressure which would be 6-8 psi since atmopheric pressure is 14.7 psi as is.