Ok, i got an 04 Silverado that i installed a lq9 with just a ls6 cam. It has great power on low rpm but it feels like it falls on its nose on higher rpms. So now i'm wondering if i need to change to some stiffer valve springs(i left the stock lg9 ones on there). I'm guessing that the valves are not able to close fast enough and that's why it would lose power but i'm not sure, any idea or comment is welcome. Please i need to get this problem fixed.

 

The stock springs and pushrods are barely strong enough to support the factory cam, an ls6 cam has more lift than the stock lq9 and springs as well as hardened pushrods are always recommended for a swap. That being said, a decrease in power could be lots of things - how many miles on the vehicle and how recent is the problem? how many rpm's when the power drops out?