Manual to Auto drivetrain swap

[PAPA71]

Hey guys I am building a 71 short bed 4x4 and was REALLY wanting an AUTO instead of the 4 speed I have. I believe the diff gearing are 3.73 Besides the obvious mechanical changes I would have to make, do I need to change my diff gearing also? Is there a transmission that will work (efficiently) with out the hassle?

[hugger6933]

Welcome to the site lots of help here. Regardless of the trans unless it is an overdrive the final outcome is still 1:1 so the man. vs auto is moot point in that is has no chance on how it will perform differently than it did before the change with respects to gearing. Meaning if the gears were ok with the amount of tires and motor you had before and your going back with the same it will still be ok. Jim

[HSRACER]

Originally Posted by PAPA71

Hey guys I am building a 71 short bed 4x4 and was REALLY wanting an AUTO instead of the 4 speed I have. I believe the diff gearing are 3.73 Besides the obvious mechanical changes I would have to make, do I need to change my diff gearing also? Is there a transmission that will work (efficiently) with out the hassle?

A 700r automatic trans works great with 373 gears

[Accelo]

Four speed transfer case input shaft has10 splines. The automatic is fine splines (26) and will not bolt up to an automatic. One has to change the t/c input shaft or swap the transfer case to an automatic style case.

If it was me I would likely go for a 4L60 (700R4) and the matching transfer case. Just remember the 700r4 transfer cases come both ways, a off set to the passenger side and another to the drivers side. Get the one that matches the original. By the way; the newer cased are all aluminum and much lighter.

Tire size makes a huge difference and will effect the gear ratio you want to run. Especially with the overdrive 700R4. I have driven trucks with 456 gears and large tires where the combo worked well. Use some of the gear ratio calculators available online.

[57taskforce]

If you go with a 700r I'd look into swapping the 205 behind it. I'm personally not a big fan of the aluminum np 208/241/231etc cases that came behind the 700r in it's various applications. I prefer the beef of the cast iron, gear drive monster that is the 205. The th350 205 can be mounted to the 700r with a spacer or an tranny output shaft swap depending on which way you want to go. Keep in mind a 4spd 205 can be made into a th350 205 with an input shaft/power coupler swap and the correct th350 adapter. Mostly the trans you choose will dictate what you've got to work with. If you want overdrive the 700r is likely the easiest choice due to it's lack of computer controls, not to mention rebuildable cores are still pretty plentiful in junkyards and you can buy them already beefed up from places like bow tie overdrives,monster transmission and others. It all comes down to how much you want to spend and what you have in mind for your truck in the future. Is it gonna tow? Get lifted? Just be a weekend cruiser?

I agree if you choose the 700r do you home work on gear ratios. The 700r typically doesn't play well or live long with gears that are far from factory spec. Keep in mind with and auto swap you need to be looking at things like throttle valve/kickdown cables,torque converter lock up controls, the possibility of driveshaft length changes, a transmission cooler, shifter/ shift linkage, etc. it can be a bit pricey.

[PAPA71]

Thanks guys Its just going to be a weekend cruiser but if I want go take on the road or in the Dunes here I want the torque etc. No bigger than 35s when I am done. I do not want computer controlled transmission. Old school is fine. I have a 69 Camaro that had a turbo 350 in it but previously someone hacked a hole for a 4 speed. I threw in a Muncie (you know to take advantage of said hole). Well when I took off in first it was like starting in 4th. I had 3.08 and was told it needed 3.73. had it swapped and it does fine now. Dont want the headache of putting in the auto in the truck and having the same similar issues. Thanks again.

[57taskforce]

If you were to run a 700r you'd want to run numerically higher gears than 3.73s if you run 35s. 33s might be ok with 3.73s but with 4.11s it would probably be happier. If you don't want overdrive than id slap a th400 or th350 in it and let it eat. Your 4 speed 205 can be made to work with a th400 but it requires some machine work to the case and the other parts already mentioned but specific to a th400 rather than the th350. Hope this helps some...

[Accelo]

One general transmission trend is: The newer the transmission the lower the first gear. If you go the 700R4-4L60 route you could use the 1987 to 1992 transmission. No electronics except for the lock up torque converter.

Converting to a 350 would likely be the least expensive.

[Mike C]

Please don't. The manual trans is the best thing ever in a 4x4. Besides, there are two kinds of automatics: those that have quit working and those that will quit working. My SM465 has 350k miles on it and only needed a clutch at 252k.

[Accelo]

Below are the ratios:

TRANSMISSION 1ST 2ND 3RD 4TH 5TH

Borg-Warner T-5 (GM Applications) 2.95 1.94 1.34 1.00 0.63
Borg-Warner T-5 (GM Close Ratio) 2.75 1.94 1.34 1.00 0.74
Borg-Warner T-10 2nd Design 2.64 1.61 1.23 1.00 n/a
Borg-Warner T-10 2nd Design 2.88 1.74 1.33 1.00 n/a
Borg-Warner T-10 2nd Design 3.44 2.28 1.46 1.00 n/a
GM Muncie (M-21, M-22) 2.20 1.64 1.28 1.00 n/a
GM Muncie (M-20) 2.52 1.88 1.46 1.00 n/a
GM Powerglide 1.76 1.00 n/a n/a n/a
GM Turbo Hydra-Matic 350 2.52 1.52 1.00 n/a n/a
GM Turbo Hydra-Matic 400 2.48 1.48 1.00 n/a n/a
GM Turbo Hydra-Matic 200-R4 2.74 1.57 1.00 0.67 n/a
GM Turbo Hydra-Matic 700-R4 3.06 1.62 1.00 0.70 n/a

Note: The 700-4L60 first gear is 19% lower than the 350's ratio.