I was wondering how much power a built 8" rear end could take before breaking. I know a 9" would be ultimate but I have the 8" and really dont feel like swaping it out.

 

It depends on a few things;
auto or manual tranny
sticky tires or 'all season'
ratio
etc.
If you don't push it too hard, it should take what a mildly built small block can put out. And it probably won't break right away unless you really shock it.

 

i have heard of them holding up to about 350 to 400 horse weather it is true or not i dont know. i have a few friends that have dirt track cars they run nine inch rears and they are pushing about 600 to 700 horse and they hold up just fine, i cant say so much for drive shafts and u joints but that is beside the point .
now if you are looking for a kick butt rear end you can get a quick change rear end, they have a cover that comes off with 10 or 12 bolts then bam you jut take the two small gears out then put the gears back in then you have a diffrent ratio takes about 10 to 20 min to do, but it is not cheep that is the kind of rear ends they run in sprint cars and some late mod cars.thats if you dont care about the cost of things