A 5.7 Hemi's PROPER idle range is 600-625rpms, while in gear, and at a full stop. One person above, said 800rpm's is the proper range (TOTALLY INCORRECT). If your 5.7 Hemi is idling at at 800rpm's or higher, then your having either a fuel/air delivery system issue or your Cataletic Converters are clogged up, or in the process of clogging up, making it hard for the engine to properly dispel the exhaust gases, thus causing excessive engine back pressure, in which is forcing the engines ECM (computer), to try and compensate for the problem. ALL 5.7 Hemi's are pre-set by the factory to run at 600/625rpm's (while not under load) while engaged in gear, at a full stop...........5.7 Hemi's idling at 500rpm, will generally cause the Alternator to dispel the improper voltage output (too low). There's multiple important reasons why engines are set to idle at given rpm's. Proper Alternator voltage output is one of them.