The same reason we stress relieve any of our parts. To get them to last longer under our new found ability to make more HP.
Am I barking up the wrong tree here?
Lynn
Well, here's some thoughts and questions:
1) I can see where a winston cup car that only goes 501 miles before it blows up could use the stress relief. I would think a huge part of that is that you want the block stress relieved so the machining can be done very very accurately.
2) But those blocks are new, right? So they would likely have casting stresses in them and the lack of heat cycling would give them basically no time whatsoever to relax. I remember reading old books where the people making mills and lathes and such would stack up new castings for a year and let them sit. Obviously this was waaaay before things like meta-lax. So it's obvious to me that new castings move as you take metal out and machine them. This would lead to distortions of the cylinders and main bearing bores, etc. And that would surely make the thing last longer no matter what the application.
3) So what does this do for old blocks that have been heat cycled a zillion times? I dunno. I can see where you might think the stresses are gone because the thing has been shaken and twisted and heated for years and years....so what pent-up stresses could be left in a daily driver motor? My guess is, not many. Now if you're a big wheel and have a 1500 hp+ pulling truck and every week you twist the ever living hell out of it, it has no cooling water, you heat it and shut it down real quick....now that block is probably stressed. But a daily driver? I doubt it would have that much.
4) I don't see where the stress relief could possibly hurt it....so if it's cheap, why not? Do it and be sleep better at night...until you blow it up from something else.
I look at it kinda like cryo. There are some situations where it definitely works without a doubt. There are some situations where it isn't doing anything but draining someone's wallet. Can it hurt? Probably not. Does it do anything? Hard to tell in some cases.
So, there's my half baked opinion.
Personally I would think stress relieving other parts might be of more benefit...like for example a crank someone has welded the crap out of. But a block? How many block failures do you see? Seems to me it's kinda rare in Cummins engines.
