How high can a 289 engine idle be?

There is no definitive answer to this question as it depends on a number of factors, such as the specific engine configuration, camshaft profile, and throttle body size. However, a 289 engine can typically idle at around 650-800 RPM without any issues. If the engine is idling higher than this, it is important to have it checked out by a qualified mechanic to rule out any potential problems.