I started running BOINC and SETI@Home again recently, on a new PC. I found that my CPU temperature was reaching about 70-75C when running BOINC tasks, including SETI@Home. More recently, I've noticed some SETI@Home tasks appearing with "(alt)" in the name (such as "SETI@home v8 8.08 (alt)", vs. "SETI@Home v8 8.00" and "SETI@Home v8 8.05"), and with the (alt) tasks, my CPU temperature has been around 80-83C. I'm curious if these (alt) versions are doing something a bit more that the other SETI@Home versions don't do?

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They use the full capabilities of a modern 64-bit CPU more efficiently (both power draw and heat output will increase because they're working harder), but they should produce the same results as all other versions.

Just quicker, that's all.

They crunch faster, so they crunch harder, so the higher temps.

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Seti@Home classic workunits:20,676 CPU time:74,226 hours

A proud member of the OFA (Old Farts Association)

If they are using the 64 bit architecture, is that the same as the application being 64 bit? My understanding from before was that most Seti applications were 32 bit because there was not much to gain with running at 64 bit. I am talking stock applications, not the special sauce.

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Seti@home classic: 1,456 results, 1.613 years CPU time

No the cpu architecture is 64 bit. Doesn't mean you can only run 64 bit applications. The Seti apps are ALL 32 bit apps, even the special app.

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Seti@Home classic workunits:20,676 CPU time:74,226 hours

A proud member of the OFA (Old Farts Association)

Look at the applications page. The two app versions 8.05 and 8.08 (alt) - the ones mentioned by the OP - are the ONLY ones listed as "Windows/x86 running on an AMD x86_64 or Intel EM64T CPU" - in other words, they will only run on 64-bit versions of Windows, which in turn will only run on 64-bit CPUs.

Look at the applications page. The two app versions 8.05 and 8.08 (alt) - the ones mentioned by the OP - are the ONLY ones listed as "Windows/x86 running on an AMD x86_64 or Intel EM64T CPU" - in other words, they will only run on 64-bit versions of Windows, which in turn will only run on 64-bit CPUs.

Correct.

Why do we have a 64 bit and 32 bit Lunatics installer.
Also some of the Linux apps are 64 bit only.

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With each crime and every kindness we birth our future.

OK, I stand corrected . The gpu apps I thought were all based on 32 bit since the developers Petri and Raistmer, I thought, have stated there was no speed improvement for compiling them as 64 bit. 64bit just gives you more memory addresses to have to wrangle in the code. I may be wrong and would like confirmation of which is true. Are the gpu apps 32bit or 64bit?

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Seti@Home classic workunits:20,676 CPU time:74,226 hours

A proud member of the OFA (Old Farts Association)

32-bit - and you're correct, for GPU apps the memory transfer efficiency between host and GPU is the most important thing.

It's only the CPU apps where the extra benefit of 64-bit registers and so on comes into play.