The 911 returns, and it's quite a special car. The RSR Turbo 2.1 packs wild bodywork and an even wilder engine. While 2.1-liters might not seem like a lot, this mill packs a gigantic turbo to push lots of additional air into the cylinders.
The sound that this engine makes is unbelievable. The turbo layout lets the exhaust gases spin the compressor and then immediately spits the gases out of the back out of a pipe that's only a few inches long. This setup results in the only entry on this list the requires hearing protection for the people experiencing the sound live. The mill belts out an impressive 138.1 decibels.