Turbo 350 to 700R4

[twiget]

I have been contemplating swapping the TH350 in my '90 'burban out for a 700R4. Apparently I took too long to decide, and the truck decided for me.

How difficult is it to put a 700R4 into one of these things? Is the tranny physically the same size? Will I have to change the drive shaft too? Any recommendations on where to find a good tranny and conversion kit? I am leaning towards a rebuilt one, and not one from the neighborhood U-Pull-It. I know it will be more expensive, but at least this way I know what I am getting.

The truck is a 1990 Suburban 2500 with a 454, TH350, and 4.10's in the rear.

 

[sweetk30]

the year and size of the truck i will bett 20 bucks its a th400 not a th350 tranny.

if me i would have a good quality reman th400 reinstalled and if you are hard bent on overdrive go 4l80-e but thats some work as to electric and longer tranny than th400.

i would sugest pics of your tranny / tcase in your truck to make 100% sure what ya got. then we can go from there to be exact.

 

[chevyuser74]

the 700r4 is about 2 inches longer than a th350, the output shaft maybe different im not sure, and u will need something to control the lockup torque convertor, they make seperate controllers or u can have a trans shop make it controlled hydraulically.

 

[Thunder]

If you get a rebuilt 700r4 you will have to get it beefed up to handle the torque that your 454 puts out. 700r4 was never designed to be behind a BBC. You will also need a chip for the ECM that controls torque convertor lockup
Cheaper, better, easier to stick with a TH400.
You need to verify what trans you have. It should have come with a TH400.
Go here. http://www.nastyz28.com/chevy-transmission-identification.php
Check your trans pan to see what you have. You can id it by the pans shape.

 

[twiget]

Thanks for the link. It is a TH400.

I am considering the 700R4 because of the OD. This truck will be my tow rig, so there will be a lot of highway miles, and I want to bring the RPM's down to help with the gas mileage.

It is a moot point right now though. The noise I was hearing is not coming from the tranny as first thought. It appears to be coming from the rear end, though it is pretty intermittent. Part of my plan to bring the RPM's down was to change out the 4.10's with 3.73's or 3.42s first, then worry about the tranny. I should be ok with 3.73's I am concerned that if I use 3.42's, it will kill my towing capacity because the RPM's will be too low to generate decent torque.

 

[Thunder]

I would forget the 700r4 if you are going to tow. The overdrive will do you no good because you should not tow in overdrive with a 700r4.
If you want an overdrive tranny you should get a 4l80E. They are basically an electronically controlled TH400 with overdrive. But even with a 4l80E I would not tow heavy loads in OD.

 

[lectric80]

A 700r4 replacing a t400 is going to require a different input on the t-case, 27 spline versus 32? I know that the t350 and 700 both use a 27 spline output, while the 400 has a higher spline count on the output. As was said, best bet is either another 400 or a 4l80e, and make sure you have it out of OD while towing.