It will all depend on your setup. I have seen stock axles works up to 600 whp, I have also seen 700 whp axles break at 300 whp.
These are some of the things that you need to look for, I wrote this back long time ago.
A lot of drivetrain issues or so called "luck" all rely on your setup.
Many things to watch for on drivetrain and lonegtivity on your axles are:
Alignment - A perfect alignment will ensure that you have an equal amount of stress distribute evenly on both sides.
Ball Joint - This takes impact on the upward and downward movement, lack of this or blown/leaky ball joint will cause vibration that will put more stress on the axles.
Traction Bars - Having a traction bars will reduce the forward and backward movement of your LCA, again, less stress on the axles.
Clutch - Not so much of a type but how you drive the
car, dumping the clutch from no enagement to full engagement will cause a lot stress from momentum that the clutch created. This is why we always suggest of pre-loading the clutch at launch or burn out. Your axles are assembled with specific tolerance and clearance, this is what enable it to be flexible with left and right turn.
Some race axles have tighter clearances that requires wider radius of turn and this is why most of full race axles are not meant to be driven on the street Not saying that you can't drive it on the street, but you'll experience some clicking that is normal for thsi type of application, depending on the material that the internal are made of and clearance. Back to the pre-loading the clutch. Because there are clearance that needs to be met, at no engagement, you'll have that clearance space empty, as you dump the clutch, it'll create a momentum before the internal is locked and engage. This is another reason axle breaks as well.
LSD - Limited Slip Differential, without it, one of your axles will spins first causing all the transfer of power to distribute UN-Evenly on one side, versus EQUALLY on both sides. Again more stress on one side, causing that side to break first before the others.
Ride Height - Your car has a specific ride height from the factory to keep the axles as straight as possible. Lowering your car will change the ride height causing the axles to bind (bend) at an angle. Again, angled axles will not be able to take more stress than a staright axles. This is why we enforced of having your ride height as close as you can to factory height. This goes to
tire height as well.
Motor Mount - Having a solid mounts or inserts will help tremendously in keeping your engine from rocking back and forth during acceleration, tremendous engine movement will cause more stress on the drive train.
There are more reasons and ways you can do to keep your axles in the best shape as possible. These are just a few that I can think of.
Keep in mind, Axles does not gives you any power, does not make your car lighter, does not make your car go any faster or run more MPH, doesn't give you better 60 or 330.
All axles does is takes abuse and stress and ensuring that you'll get to the finish line
So the more things that you can do to keep your car as solid and as straight as possible will ensure the longetivity of your axles.
I have seen stock axles handle 400-500 whp and runs 10's all day long, Joe Simpson is a prime example, I also have seen 500 whp axles break at 300 whp. Again it all depends on your setup, nothing more than that. Hope this helps at all...
