My 4L85E is holding up just fine at ~1,700 @ 55 mph and ~2,500 @ 80 behind a 383 making 450 ft/lbs at that rpm pulling around a 7,000 lbs van. The gearing is not the problem, the 4L60E being a POS is! My POS 60E and 65E went out in my OE 3.42 geared van twice in 78K miles. The OE 60E lasted 38K miles behind the stock 350. I threw it in the scrap pile, put a 110K mile junkyard 4L85E and have not looked back. The 4L80E in my Tahoe held up to the 8.1L with 3.42s and 32" tall tires as well. Cruising RPM was like 1,850 @ 75 mph.
Too-tall a final drive gearing will stress the transmission more than otherwise, shortening it's lifespan...Stock 4L60Es are very weak in a lot of areas but once those areas (3-4 pack, sun shell, valve body/shift calibration, etc) are addressed they can take some power though anything north of 550hp, I strongly recommend a 4L80E just for a cost/benefit/risk perspective...I've done both built 60s and mild 80s in that same 400-500 hp power regime and application with no issues. Both will work, just depends on what the right choice is for the individual user and application...I've also rebuilt countless transmissions where folks have put big tires on their vehicles with no change to ratios and burnt the trans up then it comes to me...Unless the final drives have been updated with correct gearing for their tire size, I turn the job away, explaining why.
The 4L80 is clearly the right trans for you however you're countering the norm in two respects when it comes to both transmissions: Most 4L60Es don't die at only 38k behind stock 350s while most junkyard transmissions go south anywhere from immediately to 6 months or so. They are a crapshoot at best and usually fail w/in 6 months of installation from what Ive seen from customers who bring me their junk yards units to rebuild but so far your luck is holding and is making up for your bad 4L60/65E luck so nice when things equal out.
I appreciate your input guys. I guess I'd have to see what's all involved either way.
I would love to learn to tear into it myself, but there are time and money constraints. If I planned on doing more transmissions, I could better justify the cost of the specialized tooling. It's sucks, but I'm kinda strapped in that way.
If I did to the 80 route, it would be nice if someone chimed in on measurements fro their drive shafts on a similar application. Also curious what that costs nowadays. To shorten/lengthen and balance, I mean.
@Astro - You'll need to weight the pros and cons of each and determine what's best for you, as Im sure you know. Whichever route you go, you'll have options insofar as build set up, etc. Don't worry about the exact measurements as you can grab all of that once you have the 4L80 installed (assuming you go that route) but the 4L80E OAL is 26.4″ whereas the 4L60E is 23.5″ so that should give you a rough idea of the difference in the meantime.
Time is usually the biggest barrier for folks when it comes to learning transmissions as most of the special tooling pays for itself with one-two builds.
