th-400 questions

[G_Body_Enthusiast]

ok i have a 9" tail shaft th-400 and i have some questions, at least one for the moment.

what length yoke do i need? they come in lengths from 4"-6"roughly. also what is the spline count?

i'm sure i need to get a book on this transmission but for now i need some questions answered that dont require a book.

also what about changing the gear ratios? i know there are parts to convert the 1st and 2nd gears to a lower ratio for better acceleration. are these ok to use on the street? i'm asking because i have a 2.41 rear gear and if i can use a lower gear set in the transmission then i wouldnt have to mess with the rear end which would still be 2.41 final drive giving me a good highway gear but acceleration would be better.

2.48 x 2.41= 5.98 (th-400 first gear)
2.97 x 2.41= 7.16 (th-400 TCI first gear http://www.summitracing.com/parts/TCI-229700/ )
2.74 x 2.41= 6.60 (200c first gear, original transmission)
2.52 x 2.41= 6.07 (th-350 first gear, current transmission)
2.75 x 2.41= 6.62 (th-400 TCI first gear, different kit http://www.summitracing.com/parts/TCI-227500/ )

if i am to avoid spending big bucks on an OD transmission then i need to work with the gear i have now so this is my alternative option to a lower rear gear and OD transmission. the gear sets for the transmission are about 840 and 685 bucks, not cheap but maybe cheaper than spending 2K on a transmission and then more money on another gear, carrier, etc.

when i went to the th-350, the car had less acceleration and i could tell a difference and i have hated it since the transmission went in and it will get worse if i use the th-400 and do nothing about the gearing. i dont want to raise the cruising RPM i just want better acceleration so i can either put in lower gears in the transmission or get an OD transmission and a rear gear. i know nothing of these gear sets other than they've been around for quite a while but i dont know anyone who has used them and that kind of input is what i'm needing.

 

[TexasT]

If you are making enough power to need a TH400, you probably need to look at upgrading the 7.5 axle. A TH350 is a pretty stout trans when built accordingly.

If you are wanting overdrive I would look at a 2004r or 700r4 and not look back. These will require a rear gear swap also as the 2.41 in there will have your rpms too low when in overdrive on the high way.

You don't say what you are powering all this with but the 700r4 has an extra low 3.06 first gear ratio to get things underway with authority.

edit: I see in your sig you are using a big Cadillac and this I believe would require an adapter to run the 700r4, though I believe the dual pattern 2004r will bolt up. Lots of good parts for the 2004r these days and should be able to withstand some abuse if built by someone who knows how.

 

[-83MONTESS-]

I dont think your gonna have to worry about changing the rear gear with a TH400. That cadillac engine is gonna be a torque monster! My uncle just put a stock rebuilt 472 in his 40s Cadillac. That thing hauls *ss!!! If anything I would get some 2.73-3.08s and be done with it. As for the yoke---I had this problem when I swapped my TH400 in. I believe mine is the 9" tailshaft and the yoke I used was pretty long, probably close to 6". I believe the stock spline count is 32 I know the one in my buddy's Blazer was a 32. Its been about 2 years since I have really messed with the trans in my car aside from maintnance so I could be wrong.

 

[G_Body_Enthusiast]

i think i might go ahead and use the lower gears in the th400 and go with a 2.73 gear. i'll get better than original acceleration without it being too low and the cruising RPMs will still be acceptable. i can always change the gears and trans later if i dont like it.

 

[martin.ca]

Why not get the TH-400 set up for a "Switch Pitch" Torque Converter?

http://www.442.com/oldsfaq/oftsp.htm

 

[G_Body_Enthusiast]

Why not get the TH-400 set up for a "Switch Pitch" Torque Converter?

http://www.442.com/oldsfaq/oftsp.htm

the low drag low gear set has 6 pinions instead of 5 increasing load capacity. it has torrington bearings instead of the usual thrust washers and they will reduce drag. so i get more load capacity, lower gears and less parasitic loss. stronger, faster and more efficient.