MacBook Pro Early 2015 – Understanding and Managing Temperature

The short answer: We can't tell you exactly.

The algorithm that controls this is sure to be Apple-proprietary. I'm sure they've spent lots of work perfecting it, so they wouldn't share it for other hardware manufacturers to 'take'.

Basically, Apple's algorithm tries to strike a balance between not turning on the fan when unnecessary, like for a short spike in load, (so the computer is quieter and the battery lasts longer) and not letting things get too hot. Just how hot things are allowed to get probably depends on the components in your computer, the various temperatures of those components, how well the computer is ventilated at the moment, the ambient temperature, and maybe even things like load average.

Bottom line: If the computer's doing a lot of work, the fans go on to cool it off. There's no way to say exactly what temperature triggers the fans because it varies based on a lot of things.

Sorry I can't provide anything more concrete.

While Intel may produce versions of that processor that use an interchangeable socket, all Mac laptops (and almost all other laptops) have their CPUs soldered directly onto the logic board. This means it's effectively impossible to replace them, unless perhaps you have some high-grade industrial soldering knowledge and equipment. You can see this in iFixit's logic board replacement guide for your MacBook Pro.

In addition to that, I'm not aware of anywhere that you can buy individual Intel mobile CPUs. Because they're only used in laptops, they tend to only be sold to OEMs.