WS6 Auto Trans!!!!!
Our stock 10 bolt rear end is not exactly setting any records for strength. With an auto they tend to last longer, but they have broken even at stock power levels. FWIW I've seen people run it on the strip in the 700 hp range though, but never with an m6.
A 4L80E conversion and a 9" would set you up to be pretty much bullet proof for what you're application will likely ever produce.
The ten bolt could be good for low 1.4 shorts times or let go with a 2.1 on street tires.
Auto could support 600Rwhp or start messing up tomorrow.
You should be fine with a ten bolt cutting 1.6s for a long time. You should be fine making 400Rwhp for a long time.
Either could go at any second.
As for the 10-bolt, I would say that they are weaker than the 4l60E transmission! I blew one with my bone-stock V6 firebird! I've already had to have another one put in this car.
Bottom line: 4l60E is OK if not pushed/abused. 10-bolt=crap.





