Mac Pro Tower - Which Fans Cool All of What?

I think you are "barking up the wrong tree".

The fans in the Mac Pro are controlled by Software in the System Management Controller. They make a feedback loop based on measured temperatures. If you are getting racing fans, that is probably based on Bad Temperature inputs. Fixing the temperature inputs is where you should place your energy. The fans are slaves in this process.

One possible exception: Most of the fans do have an RPM output indicator, which tells whether they are spinning properly. If the diagnostic indicates a MOT error, that is likely to be a Fan Motor that is not showing high RPMs at High speed settings, and may be defective. The power supply fans were designed by a different team, and may not have the same controls.

What SMC Fan Control and similar software allows you to do is put a new Floor under the fan speeds. They will spin faster at idle, and it is possible they may move up to even higher speeds more slowly. What is does NOT allow you to do is any actual reduction in minimum fan speeds, regardless of whether it allows you to set lower numbers.

When the numbers get extremely large, the magnitude bits overflow into what is sometimes considered the sign bit. That has no meaning in this context -- all numbers should be treated as positive numbers. That probably indicates they took a shortcut and used general-purpose routines for signed numbers that were lying around instead of writing more correct versions for positive numbers from scratch.

Infran wrote:

...I've been trying to find a diagram showing which fans cool which parts of the system, but I mostly get results about fan noise, or minimalist diagrams that don't tell me all I want to know...

If you download the service manual for your Mac Pro from this link, you should be able to tell from the diagrams and illustrations where all the fans are and the air flow they create; you can also determine what's in the way of that air flow to establish what gets cooled.

I had this question, too. What I ended up doing was running SMC Fan Control and Hardware Monitor simultaneously, tweaking the base fan speeds, and watching how doing so affected the temperatures of various components. In general, the intake and exhaust fans seemed to me to be best for overall lowering of temperatures without any distracting fan noise. My Mac Pros temps were mostly within acceptable range to start with, but the Northbridge gets hot. I found with just a slight pump to those two fans, I could lower the temp on it ~20C at idle. Will doing so make any difference in the life of the components? I have no idea. But as long as the fan speed is not bothering me, then why not keep the insides as cool as possible?

Gina