i have a 5.3 sierra and they are way detuned from the factory to run the 87 octane, i put a 93 octane tune in my truck and gained almost 10* of timing. the L33 5.3 is the most desirable because of the all aluminum block and heads and the 325hp. your LS4 comes with 6* of KR and 200* IAT temps because of that big V8 stuffed under the hood with no way for the heat to escapse