How do I get engine audio to scale with edited HP?

I figure a 15% boost in displacement will get me around 300hp in my custom SF260, and I've edited the prop diameter to keep my RPMs where they should be, but the problem I'm having is that the engine produces full throttle audio at a MP of 23, where the default settngs produce full throttle audio around 27. The more I boost the power the sooner the full throttle audio file kicks in... Is there a way to adjust this so it scales correctly?