Compresion and HP. What are the relation?
 

Is there a formula for calculating HP gain by raising compression. Is it that simple or are there other factors involved. My 358 SBC have 9.1/1 compression @ 420HP. What would be the HP gain bumping to 11 or 12/1?

A good rule of thumb is each point of compression between 9:1 and 11:1 should be worth aprox. a 4% horsepower increase all else being equal. If in the process of changing the C.R. you also change the combustion chamber configuration or quench that could alter that number.
Hope this helps,
Bob

Just changing compression ratio will not do much without changing the camshaft. :)

Performance gains from increasing the mechanical compression ratio are very modest. The more important figure is the dynamic compression ratio. This is determined by the effect the intake valve closing point has on cylinder pressures during the compression cycle. DCR is also important in determining the octane requirements for you engine.
Hope this helps,
Bob