[53Chev]

Swapping rear end gears and possibly the tranny from TH350 to something w/Overdrive

Ok, I need some simple answers here. This is my dilema:

I am running a 71 Chevelle with a 350ci and a TH350 tranny. Tranny has a 2500 Fireball torque conv. and is a 3 speed automatic. Came original with the car. It also has a 10 bolt uni rear end with I think 3.55 gears.

I want to convert to a higher gear say a 3.73 posi unit. I am also aware that putting that ratio of gears will bring up my RPMS and as this is going to be my daily driver in the summer and i will be doing some hiway travelling and long trips, this will not be good for gas mileage. I was told that I could replace my tranny with one that has an overdrive to help bring down the rpms on the highway. I am also disapointed that after paying close to ten grand to get my engine completely redone with all new parts, it is putting out close to 400hp at the flywheel and I cant even get it to chirp the wheels from a standing start. The engine runs like a top and it has had my mechanic go over it with a fine tooth comb to tune it almost perfectly. The problem does not lie with the engine or its setup.

My questions are as follows:

I want to keep my automatic(my mind is set) and convert to a 3 speed with O/D. I dont plan on drag racing. (I may take it to the track once just to see its performance times).
What is the best/simplest swapover/cheapest/most reliable type tranny to replace my TH350 with? (The least painful switchover would be prefered).
Will I have any problems with kickdown brackets, other parts, shifting problems, or anything else that will have to be modified?
I know I will likely need to get my driveshaft shortened or vice versa so I am prepared for that, but will my 2500 Torq. Conv. from my TH350 fit onto my replacement tranny or is it specifically made for the TH350 only?

My budget is tight so I want to do it properly but not if its gonna be one big nightmare running around finding extra parts that I need and also spending weeks and weeks fine tuning the damned thing. My patience level is not as high as the average joe.

Any help from you veterans would be appreciated.

Thank you.

 

[Xtreme70SS396]

Re: Swapping rear end gears and possibly the tranny from TH350 to something w/Overdri

I run a GV overdrive myself - but I don't think that's your answer given what you've stated.

I would think given your power levels you could get by with a stock OD tranny, either from a buick GN (2004R I think) or a later model F body camaro/firebird (700R4 I think). I doubt you would have to modify them at all to handle your power levels.

But I know there are some changes, like the cable, electronics, etc that make those trannys work properly, and you would likely need your driveshaft shortened. I'm no expert so I'll leave that for others to comment on.

 

[Big James 4XL]

Re: Swapping rear end gears and possibly the tranny from TH350 to something w/Overdri

Do a search for 700R4 and 200R4, that will keep you reading a while and give you all the info you need to make a decision.

 

[53Chev]

Re: Swapping rear end gears and possibly the tranny from TH350 to something w/Overdri

Its not really that easy to make a descision even after reading up on the internet like you suggested.

I would like to hear from some seasoned veterans of these change overs. Its all well and true to say "go look it up in the dictionary if you dont understand", but I just thought the help would be a little more in depth.

 

[Xtreme70SS396]

Re: Swapping rear end gears and possibly the tranny from TH350 to something w/Overdri

Bump your own post like you just did - you need more people to see it to get the knowledge base you're seeking.

 

[Big James 4XL]

Re: Swapping rear end gears and possibly the tranny from TH350 to something w/Overdri

Its not really that easy to make a descision even after reading up on the internet like you suggested.

I would like to hear from some seasoned veterans of these change overs. Its all well and true to say "go look it up in the dictionary if you dont understand", but I just thought the help would be a little more in depth.

Ok, I've done the swap with a 700R4, with a stock one behind a 350 and later with a Bowtie Overdrives stage 3 700R4 behind a mildly built 454.

In short, for a 700R4 will require the driveshaft to be shortened, the 200R doesn't. The 200R MUST be strengthened to stand any horsepower, a 700R will take a little more but it is highly recomended that they be strengthened as well. 87 and later 700R have the larger input shaft and are more desirable.

The 700R has a lower 1st gear, which would help you do more than "chirp" the tires, even with the 3.55 gear. The downside is there's a big jump in ratio from 1st to 2nd. The 200R has a higher 1st and the 1-2 shift is not as big a jump.

As for electronics there are several methods for locking the converter, required for both the 200R and 700R. As simple as a toggle switch or pressure switch or B&M converter control, I prefer the B&M controller but it's more expensive than the other methods. Each of the trannies will require a TV cable, that's throttle valve position cable, not cable TV, . Proper linkage is mandatory because the TV cable adjustment is critical to get good shifts and for the life of the tranny. I made my own bracket based somewhat on the one that Bowtie Overdrives sells.

The 700R I put behind the 350 I had was a high milage unit from the boneyard, it came from a 90 model Cadillac which had a Chevy engine in it from the factory. That tranny did fine for couple of years but was starting to slip a little. When I changed to a BBC rather than have the tired 700R rebuilt by a local shop, none of which I trusted to know how to build a 700R for performance, I bought the stage 3 unit from BTO. It has performed flawlessly for routine driving and during playtime.

In short, I say leave the 3.55 gear in and go with a 700R.

 

[53Chev]

Re: Swapping rear end gears and possibly the tranny from TH350 to something w/Overdri

Thank you nig james.....this is the info i have been looking for. Will the 700R require a higher stall torque convertor like I have now (Fireball automatic 2500) or will the one i have work with the 700R? Are there any other modifications I will have to make to the shift linkage in my car? Will the stock 3 speed indicator have to be replaced with a 4 speed indicator unit? If I leave the stock 3 speed indicator will the orange pointer still line up properly?

 

[Big James 4XL]

Re: Swapping rear end gears and possibly the tranny from TH350 to something w/Overdri

Thank you nig james.....this is the info i have been looking for. Will the 700R require a higher stall torque convertor like I have now (Fireball automatic 2500) or will the one i have work with the 700R? Are there any other modifications I will have to make to the shift linkage in my car? Will the stock 3 speed indicator have to be replaced with a 4 speed indicator unit? If I leave the stock 3 speed indicator will the orange pointer still line up properly?

And it's BIG James BTW!!!

Go ahead and get the same stall converter for the 700. I don't know about the indicator for a colum shift, but it will still function OK. A kit is available for the floor shift that consist of a new lens and shifter detent that look like factory.

 

[gnicholson]

Re: Swapping rear end gears and possibly the tranny from TH350 to something w/Overdri

For simplicity and durability the trans you alredy have is tough to beat.If you have 400 hp the car should run 12's as is.The 200 or 700 properly done with a 355 or 373 would be a good chice but in my opinion the 350 with the 331 or 355 would be almost as good and not cost you anything.The car should smoke the tires as is.Take it to the strip and see what kind of mph you are running; should be at least 105 or better.

 

[53Chev]

Re: Swapping rear end gears and possibly the tranny from TH350 to something w/Overdri

Will the torque convertor i have now fit on any other transmission or is it exclusive to my TH350?

 

[Whiskey]

Re: Swapping rear end gears and possibly the tranny from TH350 to something w/Overdri

No it will not. Also a 2004r will bolt in without shortening your driveshaft. I put one in my 68. The trans mount will be located like 7 inches back farther but is the same length as the 50 turbo. My frame even had holes drilled in it that lined up! also has the same output spline. So save the money on the driveshaft work and buy a new converter for it. Bill

 

[53Chev]

Re: Swapping rear end gears and possibly the tranny from TH350 to something w/Overdri

No it will not. Also a 2004r will bolt in without shortening your driveshaft. I put one in my 68. The trans mount will be located like 7 inches back farther but is the same length as the 50 turbo. My frame even had holes drilled in it that lined up! also has the same output spline. So save the money on the driveshaft work and buy a new converter for it. Bill

Thanks.

Does the 200 tranny come with overdrive and do they make a new console shifter lens and detent for it in a kit or something? I want this to be as painless as possible. What vehicles did this tranny come in and do they still make it for modern vehicles?

Thanks

 

[Xtreme70SS396]

Re: Swapping rear end gears and possibly the tranny from TH350 to something w/Overdri

The 200 is an overdrive unit. I know there is a place that makes a retrofit console lens, but I don't have any idea who. I came across it by accident once. I'll do a little hunting and post back if I find it, it may have been at YearOne or Ground Up.

Wow, took me 30 seconds: Looks like this is the place to check out - 71-72 conversion for a 2004R is near the bottom of the page.
http://www.shiftworks.com/chevelle.htm#68-72chevelle

 

[FunkyNova66]

Re: Swapping rear end gears and possibly the tranny from TH350 to something w/Overdri

I just installed a Level 3 200-4r from Bowtie Overdrives and I love it. Awesome tranny.

 

[busterwivell]

Re: Swapping rear end gears and possibly the tranny from TH350 to something w/Overdri

I've had 3 700R4s built. I had a 56 Studebaker 2 door wagon that I put a 350/350 combo in, had 3:55 rear gears. Couldn't run 80MPH with the AC on without the engine heating up, was running something like 3400 RPM. Switched to a 700R4, idle came down, temp came down, mileage went up, noise level went down........overall, a great swap. My TH350 had a 9 inch long tail, so it was the same length as the 700R4. The TV cable is CRITICAL. I've always had mine set by the pros.

The other one was put into my 71 Chevy pickup, but it replaced a granny 4 speed. It's hooked up to a 402, and was built for towing. I pull a 5500lb trailer with it all the time, It has been in there for 5 years now, with out a problem.

The last one I have, hasn't been installed in my 47 Chevy yet, but will be soon.