It's 8.9 on the Fiero/Celebrity/Citation/U-Van iron head HO engines. The standard engines were 8.5.
The boost is a balance of pressure and spark advance. You can crank up the boost but you have to back off the timing. Rule of thumb is you have to trade 1 degree of advance for every 2 psi of boost. When you are starting with only 10-12 degrees of advance you don't have a lot of room. Intercooling will allow more boost and/or advance. You obviously get more power from boost. As the compression ratio goes up the boost/advance has practical limits. Unless you have a knock sensor you must be conservation with both. On the flip side if you lower the compression in an effort to increase the boost/advance you run the risk of the engine having little power when there is no boost (like before the turbo spools.) 8.5 is a nice balance for boost. 8.9 is starting to get up there on these engines if you want to run some serious boost. Only the laster aluminum head engines with intercooling and better pistons could start to run boost much over 15 psi and definitely with a knock sensor.
I am still amazing that my HHR SS runs 20-23 PSI of boost and gets 290HP out of 2L. Of course the advance is only about 11 degrees at WOT.
[This message has been edited by TK (edited 11-08-2014).]