thinking about swapping in a turbo 400 tranny from summit when mine goes out. i currently have a stock th350 and 10 bolt with 3.23 gears. anyone know the ratios for the th400? i would like a good towing/dd gear for the rear. the 3.23 is a bit high for me. i was thinking 3.55 or 3.73's. any suggestions? the motor is already built .060 over, flattop pistons, midrange cam, edlebrock manifold, carb, 14" cleaner, headers, no cats, roller rockers, stock heads ported, double roller timing chain, hardened push rods. i am going to install a set of trick flow heads and a little bigger cam, msd ignition,and elec. fan. after the tranny goes out(which it will, from towing and hp) the turbo 400 goes in and the 12 bolt. i jsut need the th400 gear ratios so i can match my rear end 12 bolt gears with it better. thanks.
greg

First gear in a TH400 is only a few hunderedth different from a TH350. For the most part they have the same ratios.

3.73 will hurt your highway speed. Very high RPM at freeway speeds. I have 3.73's and usually only go around 55 MPH or the MPG goes way down to GPM. I would find a 12 bolt with a factory 3.42 gear or if I was regearing I would probably buy a 3.55. If 65 to 70 MPH travel isn't important I would get a 3.73 alxe out of a straight 6 truck or 4.10 for real heavy towing. It also depend on how tall your tires are. My 2WD truck has 29" tires, bigger tires will need bigger gears.

The newer 10 bolts are damn near as strong as 12s. There are lots of them laying around with 3:42 gears. And they are plenty strong for what you want to do. You also might could find a semi-float 14 bolt. Wont have to mess with gear swaps(course that is part of the fun eh)
Unless your set on a 12 bolt this is a good option.

Huh?

TH350: 2.52, 1.52, 1.00

TH400: 2.48, 1.48, 1.00

700R4: 3.06, 1.62, 1.00, 0.67

I see no need to change axle gears for a tranny swap, what size tires are you running? No 10-bolt rear will hold up to much abuse, the 12-bolt is marginally better. I assume this is a street truck/DD/tow rig, so if you're looking for highway mileage, I'd go with a well prepped 700R4. The TH400 is stronger than the TH350, but a TH350 can be built to handle what you are doing, the TH400 robs a lot more horsepower to run, and is a wast of time in my opinion. A 700R4 can be built to be very strong, and the lower first and second will help, plus with the OD you can gear lower to help towing without sacrificing highway mileage. The 700R4 is only a little longer than the TH350 and shorter than the TH400, and if this is a 4wd application the 700 shares the same bolt pattern as the TH350, the TH400 does not.

I would find a good trans shop to rebuild a trans before buying one from Summit, Jegs, TCI, Jasper, etc. I've seen the insides of quite a few of those "hi-performance" transmissions, nothing but plain-jane stock parts, and rusty ones at that, not worth the money.

thanks for the help. the reason for the gear swap is not for the tranny swap. i just bought the truck and towed my nissan to arkansas and it does wonderful at 65-70 mph, no problem. it's just difficult to get it moving, especially at a hill. the reason for the 12 bolt swap is because when i get my new heads, ignition, etc...i want/need to extra strength. this truck is my new dd and tow rig, but it will be built fast as well. i just want it strong. the guy i bought it from used to have a set of heads and little bigger cam, but he was blowing out 700r4's and stock th350's left and right and 3 10 bolt rear ends. so i am going to swap in a th400 tranny and 12 bolt and while i'm at the axle swap, might as well re-gear. i was thinking a 3.55 would be good all around. this truck already hauls ass. has all kinds of internal work. only thing left is heads then a few other bolt ons. and eventually a small super charger. it already has close to, if not 300 hp, just lacking on the low rpm torque some. with the new heads, ignition, and future mod holley 144 super charger, i should be in the 450 hp or so range farely easily. i'd rather not have a 14 bolt if a 12 bolt will hold up to it. and i know the th400 is a lot stronger the the th350 and i can get one from summit rated to 450 hp for $700. thanks for the help.
greg

also the tires i am running now are 265/70/15...future tires are 285/65/17. farely close, a little larger.
greg

That guy sounds like my bro in law. Dont ever let him drive your truck.
Here is one more option. A Dana 60. Came in Fords, Dodges, and pre 73 chevys. A step up from the 12 bolt but not quite a 14. Might be worth checking into.

is this 2wd or 4wd.. if it's 2 go w/ the th400.. if it's 4 go with a built th350 or 700r4.. your tcase will still bolt up and the drivelines should need no mods.. and forget the 12 bolt.. 35 spline dana 60, dana 70 or FF 14 bolt.. u want something tough right?? :rolleyes: 8 lug! :flipoff2: :D

it's 2wd. i was thinking about the 14 bolts....but they are so doggone big...how much hp/torue is a 12 bolt GENERALY good for?
greg

12 Bolt in a 2 wheel drive should be stong enough. running one in my truck with 35's and a stroker and have not broken it "yet"....
that being the key word. sounds like for your application it would do just well. plus you won't have to worry about 2 different lug patterns if you put in a 14 bolt.
Just my .02