turbo Si

I have been boosting 8 psi for the longest with it and it's been sounding great. Last week I boosted on 23psi and it sounded very funny. It would give the initial sound although the high pitch toward the end, that I have grown to love, is gone . So I get an allen head and turn it toward harder, and I get the same results. I even lowered the boost to 15psi and same results. Is there some general rule or something with this thing. Like if I add boost, am I supposed to make the spring harder or softer?
Thank's!