I’m totally sold on CoolBook, a Mac OS X utility that sets the voltage levels supplied to a Mac’s CPU at different clock speeds. I became tired of my 2.33Ghz MacBook Pro’s fan spinning up all the time, but I didn’t want to compromise by running it a clock speed. After using CoolBook and tweaking fan settings with Fan Control , my computer is now cool and quiet most of the time. I’ve even tweaked my settings so the fan rarely spins up when I’m on battery power (and the CPU stays relatively cool).

In order for CoolBook to work properly, you have to do a bunch of testing to get the proper settings for your particular machine — it’s not for the faint of heart and can involve a dozen or more kernel panics to get right. Be sure to read the manual and undervolting reference that comes bundled in the CoolBook archive, and follow the instructions to find your optimal settings. Here are the settings at which my MacBook Pro 2.33Ghz runs stably:

CoolBook settings for my MacBook Pro 2.33Ghz

Note that I have configured my machine to only run at 1.837Ghz when on battery power. I’m able to run at that speed stably with the minimum voltage going to the CPU: 0.95V. That means that CPU temperature should be lower even when the CPU is taxed, which means that the fan should spin up less frequently when I’m on battery power. It should also give me longer battery life.

Here are temperature levels in °F after taxing the CPU at 100% (both cores) for 5 minutes:

Ignore the hard disk temps. My SSDs don’t seem to report temperature correctly.

Even though the fan still spins up in both scenarios, the machine is running much faster with less heat generated. Instead of running at 1.002Ghz @ 0.95V (default), I’m now running at 1.837Ghz @ 0.95V. And I’m only pulling 1.0625V at 2.338Ghz instead of the standard 1.1625V.

Note that tweaking these settings are for advanced Mac users only. You can make your machine unbootable using this app; however, the manual assures users that you should always be able to boot in Safe Mode even if you screw up your settings. :)