Ever wondered why your engine sounds the best first thing after a cold start? It's angry, it's aggressive... and then it just calms down into a regular idle? This is the Cold Start Cycle, and it's your engine management doing everything it can to waste fuel so that it can warm up more quickly. Cold-start emissions are a major challenge for automakers, so they program engines to run as inefficiently as possible so that they can light off the catalytic converters more quickly. This is because cats don't operate properly until they've reached operating temperature. Wasting fuel by changing ignition and valve timing and injecting a rich mixture sends unburned fuel and extra heat down the exhaust, which can then be burned by the catalytic converter, heating it up more quickly. The result is an angry, awesome-sounding idle — that sadly lasts only a few seconds.