Horse power isn't what breaks the half shafts. Torque is what will break them if your rear end starts to flex and induces the suspension into wheel hop.
The purpose of the extra rear sub frame support is to anchor the sub frame to the unibody preventing it from flexing. The flexing that occurs from the torque is from the action of the sub frame trying to lift the car off the ground by rotating it on the back wheels under a sudden acceleration. Our stock set up is very weak. Under heavy acceleration, our sub frame tries to pick the front end of the car up, but the mounting brackets to the control arms flex causing the lower control arms to start "dancing" thus inducing wheel hop.
By completely reinforcing the sub frame at the mounting brackets so they will be unable to flex will keep the control arms stable. Thus as the sub frame is reinforced, more torque can be applied to the rear end. To keep the sub frame, itself, from flexing, additional connection points need to be added between the sub frame and the unibody.
I am also changing to the GKN axels. I am in the process of trying to set up a group purchase on these axels.
I am shooting for the range of 800+ Ft*lbs of torque.
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