Hey guys, finishing up a 6.2/8hp90 swap into my 70 challenger. Getting to the final stages wiring up the trans controller and finishing up some of the coolant plumbing when I started reading about people specifically in 8hp70/90 swapped cars having issues with the trans case exploding at the tail section down track.

All the failures seemed to happen at higher speed down track, they are all catestrophic failures and everyone reported no vibration prior and claimed their driveline angles were correct (but were they really?).

Almost everyone was using the same setup, a Sonnax yoke adapter along with a driveshaft built using a slip section.

Most of the failures i've seen were in longer wheelbase cars or trucks so some have speculated DS critical speed was the issue, but is it really likely so many DS shops screwed up in making sure the critical speed of the shaft they built was adequate for the application?
Some guys think it was pre-existing damage from the donor car since most of these are pullouts from wrecked vehicles. Possible but seems unlikely to me, especially with no abnormal vibration/noise prior.

Kind of at a loss right now. Not sure if it's an issue with slip sections binding up with pinion movement, the DS somehow leveraging the output shaft because of the sonnax yoke adapter moving the U joint back or some other reason. These transmissions seem to hold up great in 800+WHP 4400lb hellcats so I really figured it would be fine with my setup (about 800whp in a 3400lb car) but this has me worried.


What do you guys think? Thought it would be really fun on the street to have the 8 Speed, also great for some of the roll race runway events they do here but unless I can figure out how to address this I may have to go back to an 80e

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.

I’ve worked on a few oem applications where the vehicle speed limiter was specifically set for driveshaft critical speed- not tires or anything else. When that speed was exceeded the parts that came back broken were astounding. Transfer cases, transmissions, blocks… etc. all end up getting destroyed in spectacular fashion.

Have a new one built for your car from Strange Engr or Mark Williams or and Dr. Diff.
I've broken a couple of U joints and yoke while racing, that is NOT fun or SAFE

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

From my drive shaft debacle, local shop, Strange, & now a Mark Williams , the MW was the ONLY place I found that could spin the shaft at a real world HIGH rpm, not the 500 max rpm that all the other use ,the MW solved my problem on my r46rh/A518 trans in my 68 Charger.

The local driveshaft guy that all the local racers used has retired. He understood critical speed and asked all the "critical questions" before he would build you a shaft. I had my last driveshaft built by Strange but I still have the driveshaft that Dave at C&U made for my 5-speed set-up. Hope you come up with the solution that you can have a stress free build. I'm sure there are racers out there with similar builds that live and run fast.

Gus

Well... maybe don't use that

This crap?

https://www.sonnax.com/parts/4430-8-bolt-adapter-flange-yoke

It's [censored] aluminum.... I wouldn't touch that with a 10 foot pole

With all these failures - what failed ? The driveshaft, the adapter, what?

I haven't seen one fail, ever, out of any of these trans grenading incidents. I have one sitting here now, it's pretty heavy duty billet aluminum. I'm no engineer but i'd trust it handling a bit of power. As far as whether or not it creates an issue with the output shaft in moving the U joint back, that i don't know. It's either that or use a factory style Hellcat yoke which supposedly isn't as strong as a 'trusted' 1350 yoke so I don't know if that's an improvement. Keeping the U joint close to the output shaft though does seem beneficial.












Almost every one I've seen, the entire case around the output shaft grenaded. The output shaft itself was intact, on some even the driveshaft was intact and undamaged. I don't know what exactly inside of the housing exploded to cause the housing to crack though, most guys didn't get too far into figuring that out other than "the trans exploded". I could say for certain though I haven't seen an adapter fail, and more than a few times I've seen the driveshafts were unscathed. Doesn't mean they didn't hit critical speed though, but they definitely didn't break in a few of the failures i've seen.

Everyone who this has happened to seems to have a different opinion on why it happened. Some say critical speed, some say they believe the trans took a hit from the doner car it was in and something inside or the case was damaged already, some say the factory yoke may have loosened up and created a wobble (apparently there is an issue with the bolts not being torqued or backing out from the factory) but the weird thing is more than a few guys CLAIM they felt no vibration while driving or prior to the failure. Some have speculated it's something to do with the the 8hp90 output shaft getting leveraged somehow by the adapter yoke or something to do with the slip shaft binding.

Again, everyone CLAIMS their driveline angles were correct but that gets complicated and sometimes people don't seem to know what correct is. The fact that none of them felt vibration driving/cruising/racing the car, even prior to failures though is what scares me since it really does seem like that rules out bad geometry.

All I know is that catestrophic failures like this really never happen in factory hellcats other than by very obvious causes (people doing burnouts in the top of 8th gear clearly running into critical speed issues with the DS or cars pushing like 1k+ wheel through the stock trans)

Well extending the pivot point off the end of the output shaft isn't good, nor is it being an aluminum piece... I'd use what the factory heck cats use.....

He's definitely on the top of my list of people to contact about building this DS. Also wanted to reach out to the driveshaft shop and PST since they do a lot of Hellcat stuff.

Which brings me to another weird thing. PST is one of the more widely used shafts for the Hellcats and they even offer a billet flange adapter (looks like the sonnax one or one of the same design) for use with a spicer U joint as an upgrade. If the yoke adapter was the problem by itself it doesn't seem to be an issue with factory hellcat cars if it's being offered as an upgrade by a pretty reputable driveshaft shop.


I'll let you guys know what MW, PST and the driveshaft shop says when I hear back.

Mark Williams 4" Aluminum with 1350's in both my cars. Best out there, and smoothest running.

I put a 5" diameter Aluminum shaft with 1480 or some huge u-joints in my truck with 36's on it from a shop in Texas. It's pretty long too. He told me to keep it under 135mph to avoid exceeding Critical Speed. I don't trust the tires much over 100mph, but every now and then I got to let the 3L Whipple 427LS loose to pass idiots that won't get over in the passing lanes.

http://driveshaftspecialist.com/Truckhtml/Chevy-GMC.html