Thinking about doing some mods to the GC8, anybody have an idea of what the stock boost is on a 99 STI? Also the maximum boost? Thanks!

 

Think its around 1.2 bar and the max it runs is what the ecu controls, hence any modifications need mapping, dont just up the boost and think it will take it, it wont.

 

max boost is irrelevant, mapper will set it for a safe limit

 

Just having a look at the standard map on my v3sti and max boost target is 1.08bar with boost limit at 1.35bar so I would think the figures for your car will be very similar.
As has been said tho,you will need to remap the fuel and ignition tables when increasing boost,do not just stick a boost controller on there and hope for the best when you increase boost.