Thoughts on going to a 4spd overdrive?

[RLC2020]

I'm debating swapping out the trans in my newly acquired '83 burb. It is a C20 with the original (I believe) 3spd (TH400?). I was told it has a shift kit and it does shift rather hard so I'm assuming that's true. It runs great and the only issue I have with it is that I would like to be able to run 65-70 on the highway and I really don't like going faster than about 55mph or so with it winding out as much as it does with this setup. I figure a 4spd overdrive would give me much more range with the truck as I really only drive it around town as it is right now. I'm only going to tow max about 5k pounds behind the truck and that's only a few times a month so i don't need crazy pulling power.

This isn't something I'm going to do immediately, it's just a thought and depends on if it's a relatively easy swap and/or worth it in the opinion of the experts on here. Truck has the original 6.2 oil burner in it and is in excellent condition with 99k miles, zero rust and the all original interior so I plan on keeping it for a long time, perhaps forever if it remains trouble free for me. Is it worth going to a 4spd, or is this set up great and just run the thing as is?

Thoughts? TYIA

 

[Vbb199]

With the torque output of the 6.2 + towing, i would reccomend maybe a 700r4 thats built, not a generic junkyard trans and add a good external cooler.
Make sure its a 87 or newer. The 82-86 700r4s have problems.

Theres something specific to a 6.2 700r4 vs a gasoline 700r4, but i think its just governor springs.
I recommend a tru-cool 40k and a temp gauge so you dont burn it up when you get on it, as in, with a gauge you can watch it and ensure temps arent getting too high and viscosity of the fluid doesnt get so thin it burns and you starve the clutches plates.

Generally 5000 or less is safe with a stock 700r4, but opinions will vary here.

I stand behind 700r4. Some say its junk.

That's my input here. Id only run a 3 speed auto if it was for high performance or big tire apps where MPG and rpms on the highway really doesnt matter.

 

[Vbb199]

https://www.gmsquarebody.com/threads/how-to-identify-a-700r4.1865/


Heres a good guideline if you do decide ti scour the yards for a donor.

If you buy one reman or built for towing and performance, itll already be a "K" case transmission usually.

 

[SquareRoot]

700r4 is junk. If you're going thru all the hassle there's better alternatives.

Look into adding a gear vendors overdrive. Not cheap ($3K) but super easy installation and bullet proof. The other cheaper option given its a 2wd and has diesel torque is simply swap the rear gears to something more highway friendly.

If you want ultimate badass setup, do like me and convert to a NV4500 manual trans, but its a lot of money, time and effort.

 

[RLC2020]

700r4 is junk. If you're going thru all the hassle there's better alternatives.

Look into adding a gear vendors overdrive. Not cheap ($3K) but super easy installation and bullet proof. The other cheaper option given its a 2wd and has diesel torque is simply swap the rear gears to something more highway friendly.

If you want ultimate badass setup, do like me and convert to a NV4500 manual trans, but its a lot of money, time and effort.

I would absolutely love a manual trans, but I figure that's out of the budget. I believe it's a 4.10 rear, the fender tag just says "standard" for rear axle, would going to a 3.55/3.73 make a big difference? any idea what rpm I'm running at at different speeds? This truck was straight piped by the previous owner and it sounds like it's screaming to me, but I'm sure it isn't.

 

[Catbox]

Why not source a 4L80 and have the best of everything?
They can be found in junkyards for a decent core price, then have it rebuilt.
You will have to purchase the trans controller for an additional $500, but meh.
If you are going to keep it for a good long time, do it one time and be happy.

You will have all the power handling capability you want and an overdrive.
No rear end work needed.

I found a 2wd 4L80 and yanked it and its matching big block out of a junkyard truck.
Both cost me just over $400 and will make excellent building blocks.

 

[Bextreme04]

^^^^This

A 2wd 4L80 will bolt right in place of a TH400. It is 1" longer than a TH400 but has the exact same output shaft and mount so you wont need to change anything else. You can get a core for $200 and have it rebuilt at a local shop for less than $800 usually. Its only about $150-400 for a complete rebuild depending on how much you want to overbuild it. You will need a controller of some kind for it, but a standalone 4L80 controller and harness can be had specifically for a 6.2 for pretty cheap. They came stock in later year 1 ton squarebody diesels. There is a specific torque converter for the BB and diesel.

 

[RLC2020]

4l80 sounds like the best choice. If I decide to do it $ won't really be the issue, it's having a reliable set up that will last. I'm sure the 400 in there is probably bulletproof so I'd feel silly putting something in there that isn't as reliable. I'll keep my eyes out for one. I'm in the middle of closing on a house that I need to do a lot of work to so this will have to be on hold for a few months most likely.

 

[Wumbo]

Why not source a 4L80 and have the best of everything?
They can be found in junkyards for a decent core price, then have it rebuilt.
You will have to purchase the trans controller for an additional $500, but meh.
If you are going to keep it for a good long time, do it one time and be happy.

You will have all the power handling capability you want and an overdrive.
No rear end work needed.

I found a 2wd 4L80 and yanked it and its matching big block out of a junkyard truck.
Both cost me just over $400 and will make excellent building blocks.

What about speedometer?

 

[Bextreme04]

What about speedometer?

They make mechanical speedometer tail housings for ~$350 or you can convert to an electronic speedometer. I'm converting to a 1991 electric speedometer cluster on my 454/4L80E swap. I paid $60 for the whole cluster and another few bucks for a roached older tach cluster and made a single complete electric speedo w/ odometer and tach cluster

 

[Craig 85]

The other option is a Gear Vendors overdrive. Cost is probably more than the 4L80. Sometimes you can find these on old motorhomes with the P30 chassis.

 

[WFO]

How bout this bad boy.

https://speedrevolutions.com/equipp...ntroduces-6-speed-tranzilla-4x4-transmission/

 

[Bextreme04]

How bout this bad boy.

https://speedrevolutions.com/equipp...ntroduces-6-speed-tranzilla-4x4-transmission/

That seems.......a bit excessive. I like it!

 

[Matt69olds]

The 6 speed transmission is a 4l80 with valve body modifications to allow the overdrive clutch to be engaged in gears other than overdrive. In other words, 1st gear, 1st gear + overdrive, 2nd gear, 2nd year + overdrive, etc.

A 700 properly built with some common cheap upgrades will easily handle anything a 6.2 diesel will put out. Sonnax Drive shell, 3-4 Z-PAC, good input sprag, wide band, etc, and careful assembly.

700 transmissions really are t as bad as people make them out to be. The early years did have some problems, but they figured them out.

 

[Daveo91Burb]

If you go with a 4L80e there's a "magic year" range for us square owners: '94 and I think '95. '91-'93 had this weird cleaning cycle for the force motor that the PCM controls. I'm not sure the aftermarket controllers can deal with this cycle - you'd have to do some research. And if you don't account for the cleaning cycle that tranny will be short lived for sure. In '94 they redesigned the force motor so no cleaning cycle necessary. So that year and I think '95 are good years for us, but in '96 and later the shift linkage changed from the rod-style we're used to, to a cable. It's still possible to use the later tranny, but you have to do some mods. I'll be figuring out those mods in short order since I have a '97 4L80e in my shop waiting to get rebuilt and go into my '91. I also have a new PCM from a '94 or '95 to go with the later transmission. (desire for the newer PCM is actually what's driving the transmission upgrade)