The Sonic and Cruze comes with very light valve springs from the factory.

Whenever you bump the rev limiter and/or boost the valve springs will start to float. This is bad for a few different reasons, it is hard on the valves/valve seats themselves, and it reduces power because the valves are not opening and closing properly like they should. When you install our ZZP valve springs, you will be able to rev the car out further and make power power up there. The stock valve springs are measured at 42# and these are 72#. We highly recommend bringing the car to us for the install, we designed a tool that will avoid having to remove the head to install them.