I'm in the process of swapping my TBI 454 out of my 88 2 Wheel Drive Suburban and dropping it into my 85 4x4 Suburban. The swaps going fine, I'm doing the fuel system at the end of this week and I'll hopefully have it finished by sunday. My 85's currently running the THM400, and I'm going to keep it that way for a while. However, in the future I'd like to swap out the Turbo 400 for an Overdrive Transmission. What I can't figure out is what exactly to do. I'm told the 700R4 is the best, but my buddy has that trans in his Sierra and hes on his 2nd rebuild already, and just this past week 1st gear began grinding. I'm told the transmission can be beefed up and upgraded, and as long as I operate the trans the way its meant to be operated, and I use a trans cooler, as well as gear my axles for my tires, I should be able to make the transmission last for years. Which is what I'm intending to do. But will the 700R4 last in my heavy suburban with a big block pushing her? Thats what I'm most worried about. I've been looking into Gear Vendor Overdrive units for my THM400, but I'm told It wont be operational in 4x4, and I have to manually shift it into overdrive for all three gears. Which i do believe defeats the purpose. I'm also told that set up isnt good for towing, Which I do use my big block burban currently for (20 foot kona semi v-hull, 26 foot camping trailer, 22 foot car hauler trailer, etc etc) I tow with my suburban on a regular basis, maybe 4-5 times a month, so I can't afford to put all my money into a set up that ultimately will fail, or cost me more money in the long run.

If you have a recomended rebuild kit/strong upgrades (I already know about the 3-4 clutch failure, will obviously address that issue first) to do to the 700R4 to make her last with my 454 with the heavy weight of the suburban, plus towing with her without my 700R4 breaking a sweat.

I'm also curious what transfer case I should use, I'm using the NP208 in the 85 (strange, I thought up until 85 these trucks all came with NP205's.) Can this NP208 be matted to the 700R4, can I throw an NP205 on there and still retain the 4x4 shifter currently in the 85?

 

A factory 700R4 won't hold up to that and compared to a turbo 400, it's a POS (in stock form). Many shops know the weak links in this trans and can mod it to last. There are several tricks to beefing it, and I doubt it's something you'd want to attempt yourself unless you have experience with this trans. You already hit on one of the most important, and that's to have the axles geared for it. With the OD (and especially towing), you could/should probably be at 4.10's or even lower unless you're near stock tire sizes. The problem with that is the low gears aren't going to mate well with the non-OD trans right now (unless you're not in a hurry and don't care about mileage). I believe the NP205 went away in '80 or '81.

 

The 700r4 is a POS. I have burnt up 12. In less then 20,000miles behind a 300hp 350, in my 89 c1500. My problem was the t.v. cable. If you go with the 700r4 I suggest getting it built so you dont have to run the t.v. cable. Theres a way to do it, and I soon as I find it again on google I'll post it here.

 

there are proper ways to build the 700R4's with out the t.v. cable causing trouble according to quite a few sources, theres a shop by my work, and im told my many unbiassed supporters of this shop that 700R4's are his specialty, So I'm thinking I'm going to go ahead and start searching for a 700R4 core, or possibly a fresh one that just needs to be built correctly. Either way should work wouldnt you think?

 

Either would work. What I was looking at I was a little mistaken. From tci you can get a Constant Pressure Valve body. If the tv cable isn't adjusted correctly, it'll will still have constant pressure in the trans, instead of the pressure dropping to low and burning the clutch packs, basically lets you have a less precise adjustment on the cable and still be okay.