TH350 to 4L80E Tranny Swap in 77 SBC K25 Suburban

[mistaake]

The Suburban originally came with a TH400. At some point in the past before I owned it a TH350 was installed. Why, I have no idea, but it is what it is!

I'd like to do a 4L80E conversion when the TH350 dies. I can't afford to do it right now, especially because there isn't anything wrong with the current transmission, but when (not if) the TH350 dies there's no way it would end up with anything but a 4L80E. I drive the old 77 pretty harshly (when it's running). The rest of the drivetrain can handle me... no worries there... I just have no confidence in that TH350.

I want to get together a complete parts list and have all the considerations taken care of in advance.

I understand I'll need a standalone controller/computer for the 4L80E. Is this my best option? http://www.tciauto.com/tc/ez-tcutm-...it-gm-4l60-4l65-4l70-4l80-and-4l85e-to-09html and http://www.tciauto.com/tc/4l60e-4l80e-remote-tps-w-mounthtml/

What about things like mating the tranny up to the engine, starter, transmission linkage, transfer case location, transfer case linkage, and will i need custom driveshafts too since the 4L80E and TH350 are different physical sizes?

The main reason I want a 4L80E is to improve highway driving engine sound and fuel economy.

Thanks for any information you can provide.

 

[foamypirate]

It's pretty straight forward for the most part. You'll need an aftermarket controller, new driveshaft (or shortened factory, anyway), possibly modified cooler lines, depending on which year 4L80E you get, and misc odds and ends. You should be able to reuse the factory crossmember, just need to slide it back some.


On a side note, why the hate for the TH350? It's not a weak transmission by any means. Mine survived 4 years of torture without a single hiccup. It's a lot stronger than a 700R4. If it's in good shape, I think you'll be waiting a lot longer for that "when, not if", than you think.

 

[MikeB]

I agree. The only issue with the TH350 is it doesn't have OD. My guess is it's good up to 400-450 lb/ft torque, and even more with aftermarket internal parts.

My oldest TH350 is the original trans in a 69 C10, and it's still going strong after being rebuilt in 1989. For 4-5 years it lived behind a 383 (that I did not baby). Of course I changed fluid and filter several times and replaced the shift shaft seal. And it does have a TransGo shift kit that firms up the shifts a bit, making for less clutch wear.

Oh, yeah, the truck runs a B&M 70265 stacked plate trans fluid cooler like this:

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[mistaake]

It's new to me that the th350 is a strong trans. I've heard mostly bad stuff about them. I know the 700r4 is garbage but I was under the impression the th350 was not much better.

If it never dies, I will never do the conversion. I simply can't throw out a functioning transmission. On the other hand, I drive the old burb very aggressively and I do not plan on ever servicing the tranny.

How the truck drives at 80mph would be much better with the 4L80E.

I do plan on installing a tranny cooler and tranny temp gauge when the 4L80E swap happens.

Would I need new driveshafts for the front and back? And will it mate up to the old sbc350 just fine? And will it mate up to the existing transfer case?

 

[Georgeb]

"This one time at band camp" I was manualy shifting my TH350 while field beating my truck. WOT shifts wound out all the way till the valves floated. Anyway I put it into reverse. It killed the motor. Started back up and drove amazingly well. It eneded up lasing another six months through the winter. I used two 55 gallon drums full of water for traction weight that year so about 800 pounds. I was not easy on that trans. It started to slip a bit when it shifted. Once I took the weight out of the truck it still drove ok. Put a junk yard trans in and that one lasted 15 years until I sold the truck. They are tough!

 

[MikeB]

One of the best features of a TH350 is that most experienced trans shop guys can just about rebuild them blindfolded. Parts are plentiful and very cheap. And around here you'd be hard pressed to spend more than $400-$500 parts and labor, and then you'd be good to go for another 150K miles. To me that beats the heck out of scrapping one because the clutches are slipping, and then taking a chance on a used one.

Regarding the 700R4 being garbage, the earlier versions had issues behind bigger engines, but the 87-up models are fine. Turns out it's still THE go-to OD trans for hot rodders. And shops like Bowtie Overdrives, Monster, and Phoenix can't build them fast enough.