Not to derail the thread, but a local shop was warning me against the 6L80. They said the 6L80 is longer, heavier, and prone to torque converter issues than the 4L transmissions.
Could you explain, other than the awesome write-ups above, why the 6L80 is better than the 4L line?
The 6l80 is shorter than the 4l60,65,70's by about 1 1/2". It is taller which is no problem in a JK, it can be an issue in early muscle cars like Camaro's or Vettes with low ground clearance.
The 6l80 has been available in the SUV's since 2007(maybe earlier) with the 6.2 engine. Early on there were some issues with some shift flares but the architecture of the transmission is similar to the HD Allison and is very strong.
We have run 6l80's in 400, 500, 600 and 750 HP LS engines that have destroyed D60's and Atlas TC's without a single failure of a 6l80.
The 6l80 torque converter is very different than the typical TH350 converter and is less tolerant to modifications. The tolerances are very tight, we work with Edge converters to mod the converters for LS engines with big cams. If you run a truck converter designed to idle at 500 rpm with a cammed LS7 idling at 900 rpm you will have a problem.
Remember the 6l80 is an adaptive can driven transmission with it's own controller built in. No more wires running from a TCM or PCM for each solenoid, range switch, VSS, etc. The can bus has reduced the wire count to a few wires eliminating a problem with early GM 4 speed transmissions- wire chaffing, grounding out, etc.
The 6l80 pretty much became standard in the truck line in 2009 right across the board and millions and millions have been made. The failure rate is low compared to Chrysler transmissions or other similar adaptive transmissions. My new Mercedes with a 7 speed auto has been to the dealer 3 times for transmission problems and they are not exactly sure what to do.
The 6l80 has been a workhorse of a transmission and can handle over 700 ft lbs of torque in my experience, compare that to the import transmissions Chrysler uses.
Another great thing with the 6l80 is software, with readily available software the same 6l80 can take a 10 second Vette down the 1/4 mile or push your grandmas Cadillac around, it is that versatile. No more shift kits or governors or spinny things with weights that fly out.
The 6l80 is not tolerant to improper set up. Overfilling a 6l80 will cause it to run hot and burn up. I see this often and the transmission is blamed when the fluid level is 1 quart over full.
If you are going to make mechanical mods or software changes to a 6l80 make sure you know what you are doing, they respond well to tuning but with all the networking a change here will affect something over there. A common example is guys raise the rev limiter in the engine tune to 6,500 rpm for more top end, then they don't change the WOT settings in the T43 TCM, so the engine is trying to rev to 6,500 rpm and the transmission is programmed to shift at 5,700 so it short shifts and runs funny and the transmission is to blame.
Guys that grew up with muscle cars(me) who built TH350's and 400's need to rethink the way modern transmissions work, they do not respond to traditional mods so you will have problems. Work within the operational range of the transmission and the benefits are big. Better mpg, high torque rating, axle protection, great launch performance, low friction cool running, smaller trans cooler, etc.
Up till 09' GM did have several issues with shift flares, redesigned oil pump and some software updates; however, if you consider how many millions of 6l80's are out there I think you will find the failure rate very low.
We have not had one failure of a 6l80 in a swap we have done in the last 5 years so we will keep using them, in fact we stopped supporting the 4 speed autos at this point.
One concern we had with the 480 HP engine was the high idle, looser converter and the fact there is no production tune for this combination. We were conservative and wrote our own tune for the combination based on the GM performance E67 480 HP tune married to the Camaro SS 6l80 tune. It has worked well so far, we have a few 480 HP 6l80's out there with no failures.
