Quote:
Originally Posted by Dukat
lol. As long as it's normal I'm okay with it, although a technical explanation would be interesting if anyone knows it.
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Upon cold startup (I believe Lotus explained this elsewhere but I'm too lazy to find it), the 35i engine idles at around 1200-1300 rpms and sometimes 1500 (with the understanding that one would not be experiencing the idle jumping issue). BMW claims that this high idle is to allow the engine to warm up more quickly. In addition, the exhaust valve is closed during this warm-up period to increase back-pressure (in what I assume is an attempt to keep hot air from escaping the engine). Consider the number of exhaust openings available as a function of the speed of an amount of exhaust emitted over a given period of time. With less exhaust openings, a given amount of exhaust will escape the available opening(s) more quickly than if both/all exhaust pipes were open. If we remember that the volume of the exhaust note in dB is proportional to the amount of exhaust being emitted at a given time, this higher idle generates a louder sound until the engine has heated up and back pressure is relieved.
Hope that helps! Lotus, feel free to correct me if need be; I am no expert, at least not as much as you are.