How much power can a rear end from a 1981 rx7 handle? I have a "monster" im thinking about building, and thought about using a spare 7 rear axle I have laying around untill I can get a 12bolt shortened. Incase you are wondering this "monster" isnt a V8 powered 7....its a 57 triumph that im wanting to stuff a V8 in. I plan to change the front suspension and rear axle since parts for a triumph would be hard to find and high priced.

I have heard they can hold around 250hp.

 

mine is still holding at around 320 rwhp

 

Man,this sure is getting asked a lot lately.....

It doesnt really matter how much HP you are making at the flywheel.Torque output and how you deliver it,is what kills a rearend or trans.
V-8s make lots of lowend torque.When you launch in 1st,your applying the maximum amount of torque to the rearend that can be made.At lower launch RPMs,the V-8 is pushing its max torque and 1st gear is giving maximum torque multiplication.This is when you break a R&P or twist an axle.Add sticky tires or drag slicks and it gets worse.
A high power rotary will tend to be more kind to a rearend.Radical N/A porting reduces lowend torque drastically.Turbo rotaries,while torquey, tend to have a bit of lag which also cuts lowend torque(except REW rotaries)You can be making LOTS of peak HP from a rotary and still have no problems with a stock rearend because they just dont have much displacement or grunt.Ive been doing it for almost 5 years with my 300HP 13BT.Vehicle weight also makes a difference.
A big cube V-8(compared to a rotary) can make short work of a stock RX-7 rearend,especially if you beat it.Thats not to say it might not hold up for a while,but itll be very stressed and its likely to already have 100K or more miles on it,being 20 years old.