I had Monster highflow camshaft and valve spring sets installed on my ZC31.
The skilled mechanic had the E-Manage U re-tuned to match the camshaft at the same time.
After installation the car was put on wheels-dyno and pulled out a maximum 128.5 ps at 7,300 rpm before the rev limiter cut-off kicked in, from the dyno chart it shows more power is likely to come if I could remove the rev-limiter.
Dyno chart showed 8.5 ps (deleted "on") gain on wheels, the torque and ps increased from 3,500 all the way to 7,300 rpm.
The increase of power became more significant over 5000 rpm and beyond. The engine feels of easy breathing became apparent especially at full throttle input.
Driving impression revealed that engine revving is much smoother and the sense of power increasing along with engine speed is much improved.
I can not really feel loss of power at low rpm, or if any, maybe insignificant, the engine has become more responsive to throttle input, which makes it more fun and enjoyable to drive.
I have installed these valve springs on my car with the addition of a set of mild cams with a maximum lift of 9.0 mm. Valve lifter to camshaft clearance was professionally and accurately set. After ECU re-programming, I was able to take the car to 8000 rpm with no issues on the dyno. Occasional pulls on the road to 8000 have also been safe.