Turbo 400 to 700R4 swap

[Rusty Nail]

whatcha gon do with that 400?
maybe ill buy it from you.

 

[Bextreme04]

A properly built 700r4 would be fine. See my 700r4 Information thread on here for more information. I also have a YouTube channel with a big 700r4 playlist with all manner of builds showing all the parts I chose for installation…

GM 700R4

https://www.youtube.com/playlist?list=PLMqgQ_reThtLi6qVnkI6g0OgCjwVxQp67

While I agree that a 700R4 would support a 6.2 diesel just fine... it would require a complete changeup of the entire drivetrain. If he can find a donor vehicle with Transfer case, adapter, T-case, and front/rear drivelines... then it makes sense for the right price. Swapping to a 4l80 from a TH400 would require just the 4wd 4l80 and adapter. No change to the T-case or drivelines required. You would need a way to control it and some modification to the shifter assembly, but it has been done many times and is pretty well documented how to do it.

 

[NickTransmissions]

While I agree that a 700R4 would support a 6.2 diesel just fine... it would require a complete changeup of the entire drivetrain. If he can find a donor vehicle with Transfer case, adapter, T-case, and front/rear drivelines... then it makes sense for the right price. Swapping to a 4l80 from a TH400 would require just the 4wd 4l80 and adapter. No change to the T-case or drivelines required. You would need a way to control it and some modification to the shifter assembly, but it has been done many times and is pretty well documented how to do it.

I imagine it’s a case of six on one hand, half a dozen in the other, in terms of comparative costs. 700r4 is a driveline modification, yoke swap no pcm required.

The 80 costs about the same to build, same driveline mod and may or may not require a yoke swap, depending on what sort of output shaft the TH400 came with vs what sort of output shaft the 4L80E comes with (and I’m not talking about spline count). So no matter which way he goes, driveline work will be required. Advance Adaptors also makes a 32-spline output shaft for 700r4s for NP208 t cases (no idea what t case he has, just mentioning it).

If he acquires a late 4L80E (which would be recommended for a few reasons), he will need to install the Sonnax retro short shaft to work with the 1996+ shift shaft and rooster unless he goes to a shifter cable set up in place of the rod style linkage that the 700r4 and TH400 use, which in that case he can use the long shift shaft that came stock with the 97+ 4L80E.

I think his next step should be to price everything out in an itemized table or matrix so he can see what the total cost for each option is, bumped against not doing anything, to see what option yields the best overall value.

But insofar as the two transmissions, I think the cost to build each for the purpose/application, including a new TC, would be about the same…

 

[Bextreme04]

I imagine it’s a case of six on one hand, half a dozen in the other, in terms of comparative costs. 700r4 is a driveline modification, yoke swap no pcm required.

The 80 costs about the same to build, same driveline mod and may or may not require a yoke swap, depending on what sort of output shaft the TH400 came with vs what sort of output shaft the 4L80E comes with (and I’m not talking about spline count). So no matter which way he goes, driveline work will be required. Advance Adaptors also makes a 32-spline output shaft for 700r4s for NP208 t cases (no idea what t case he has, just mentioning it).

If he acquires a late 4L80E (which would be recommended for a few reasons), he will need to install the Sonnax retro short shaft to work with the 1996+ shift shaft and rooster unless he goes to a shifter cable set up in place of the rod style linkage that the 700r4 and TH400 use, which in that case he can use the long shift shaft that came stock with the 97+ 4L80E.

I think his next step should be to price everything out in an itemized table or matrix so he can see what the total cost for each option is, bumped against not doing anything, to see what option yields the best overall value.

But insofar as the two transmissions, I think the cost to build each for the purpose/application, including a new TC, would be about the same…

Its an 85, so it will be a round pattern short output TH400 whether it is an NP208 or an NP205(which were the only two options in 1985). All T-cases that were behind a TH400 after '84 will be round pattern with a long 32 spline input. It is a direct swap for any 4wd 4L80 from a chevy truck/burb as long as you use the T-case adapter from the 4L80. There should be no driveline mod, yoke swap, shift shaft swap required. The existing 400 rod connector will swap right over and you will need to extend the linkage and linkage pivot point by 1" to the rear. That's it for the entire swap. If you can get the 4l80 from a driving vehicle, I'd be tempted to not even open it or do anything to it, just drop it in and go. A 700R4 will need to come apart and get a full work through and some upgraded parts/cooler before I would trust it to leave my driveway.

 

[NickTransmissions]

If you can get the 4l80 from a driving vehicle, I'd be tempted to not even open it or do anything to it, just drop it in and go. A 700R4 will need to come apart and get a full work through and some upgraded parts/cooler before I would trust it to leave my driveway.

You’d be surprised how many running 4L80s need something or where something is just about done internally. Worn pumps (priming crescents excessive wear causing line pressure irregularities), forward drum sealing ring groove wear, direct clutch pack on its way out, etc.

Sure, you can stab it and take your chances but a used transmission is nothing more than a core in an overwhelming majority of the cases. I say this from building countless numbers of both that were installed as “good” and last less than a year (most make it 3-6 months). This applies to pretty much all automatic transmissions where no history is known…

I really don’t care what way the OP goes but my original point was that the 700r4 can handle that engine if properly built.