Easy way to disable processor cores on the fly Hey i'm looking for a fast way to disable processor cores of my macbook to save power when i'm stranded away from a charger. I'm away that instruments can do this from it's preferences, but that's a bit inconvenient. There also used to be a "Processor" system pref, but doesn't work on lion since it's 32-bit.

Is there any other way? perhaps a menubar app or terminal command that can be scripted?
Disabling cores does not save power.  In fact, it is likely to use more power.

OS X supports SpeedStep, which automatically scales frequency and voltage to support the operation being performed.  So if a core is not being used, it will not use (much) power.

But there is more:

Suppose you have a task which can be run in parallel, such as encoding video.  If you have two cores, you can run it at least twice as fast with one.  This means that the computer can stop using energy for other tasks needed for the process - processor overhead, RAM allocation, hard drive I/O, etc much sooner.

Also, while that long process is running, it has to keep switching all the other tasks the computer normally does.  This wastefull switching itself is going to use power and resources.

See this StackOverflow thread for a longer explanation.

(Here is the Apple page on the Processor preference pane and how to disable cores from the command line.)