Saturn S series Twin Cam cylinder head had now featured a new roller-rocker setup with hydraulic tappets and roller cams replacing the conventional bucket lifter setup of the previous engine in the S series. Also introduced for the 1999 model year S-series vehicles that were sold in California was a different type of an exhaust manifold that housed an upstream catalytic converter (this converter had heated up much faster than the conventional unit, and it had also decreased exhaust emissions faster as well), and it also had provisions for an air-injection reaction system (This had injected fresh air into the exhaust to reduce
Exhaust system can be made to be quieter than 70dBa at 6000 rpm without sacrificing any power gains. The latest generations of 4-stroke street motorcycle engines that can come close to a 2T in redline RPM but still having the extra weight and inertia of cams and valves make the differences in being "cleaner burning" negligible, with the decreased engine service life to be expected, while also having the sound of a 4T exhaust carrying for miles in many cases. Additionally, "the nature of the beast" is that a 4T exhaust note above 6000 rpm is unduly harsh to the human ear.