It's crazy to me the amount of driveline shops I've talked with that never even heard of critical speed before.
I've seen trucks come in with amazingly long 3" steel driveshafts that would have blown apart at 5k rpm. So, it is possible.
I would have a driveshaft built with at least a front CV rather than a u-joint as I would guess it would end up with a lot of working angle on the front joint? Unless the tunnel was rebuilt to bring the angles closer to zero.
You guys may be right. I don't know what shops these guys used, but I called a local shop who I thought had a pretty good reputation and the guy proceeded to tell me how "guys on the internet get way too hung up on critical speed" and said a few things leading me to believe he didn't actually know what it was. I purchased a shaft for my supercharged LS swapped impala years back from a different local shop and don't recall the shop ever asking about rear gear, tire height, trap speed or anything. If it were a 4" carbon fiber shaft in a short wheelbase car I could understand them not asking, but with the aluminum shaft my car is coming pretty close to critical speed and could have been over critical speed if the car were a bit faster, had more gear or a shorter tire.
The failures do seem like a critical speed issue to me, as me Blusumbl mentioned. They all fail just like that, and all happen down track. If it were something with pinion movement binding up the slip shaft or the slip not having enough travel, you'd things would be going south off the line and not down track.
Like Cab said though, it's definitely NOT something I want to mess with. Spraying out trans fluid and shrapnel in front of my slicks down track at a high speed. I wish I could know for sure it's a critical speed issue as that shouldn't be hard to address, and it's a short car so shouldn't be a problem building a DS that can be well in the safe range, but not knowing for sure is really making me question using the 8 speed.
I would have thought if DS's were hitting critical speed, it would have been pretty obvious to the driver with vibration but maybe not. Recently swapped from a 60e to an 80e in my Impala. Completely forgot to check pinion angle after the trans swap. . Checked it the other week and turns out the 80e crossmember sat the trans way further down than the 60e. my pinion was 1 degree down, trans 4 degrees down (this is a triangulated 4 link car). I drove the car about 2k miles on the street, long trips at 80mph, several track outings on drag radials running high 130's trap and never felt anything off, but i'm sure running a pinion angle like that couldn't have been good.
ended up shimming the trans to 3.5 degree down (as high as I could get it) and brought the pinion 2.5 degrees up. Definitely felt smoother during the test drive but may have been in my head since I never noticed any vibration before lol
