will th-400 hold up?

[BAnsley]

geting ready to swap a 454 out for a 12v 1993 engine in a 1987 v30 chevy and was wanting to get some feed back on how well the th400 will handle the torque. I know about having to switch out the vac. modulator with a mechanical. Ive truck pulled and have really put some 400's through the ringer and have had great sucess, but not behind a cummins. tring to keep this swap simple and i have the adapter already but do need a flexplate.



Thanks Brian

 

[BAnsley]

I also have acesss to a thm475 unit from a 4bt truck and thought about swaping a 4wd shaft in to it and rebuilding it. you can install a 34 element sprag clutch ''the weak link'' and drum and will increase the torque capasity 110% over stock 16 element. My question is what breaks when behind a 12v?



Thanks Brian

 

[zstroken]

input shafts. I would be a little concerned about only having a 1:1 ratio for top gear in the th400 though.

 

[BAnsley]

thanks for the reply, the no overdrive isn't that big of a deal no long interstate trips or anything, mainly short trips and truck pulling and tug a truck. talked to the transmission shop to day and a 300m hardend inputshaft with a 34 sprag clutch and a good torque and i wont hurt a 400. He said with a full manual valve body it will be more than enough and you set your shift points when you need them.

 

[BJMarshall]

The only bad thing about no modulator is you will have harsh shifts all the time. 400Raptor.com the Ultimate 400 and 350 Transmission Website, TH400 and TH350 Transmissions PATC has the cable operated modulator. I would go this route rather than full manual. If you've got the 475 with the cast pan it really strengthens up the case, but the adapter cocks the engine to the side a little.

 

[tgordon]

Just a question, is that why the 400's behind the 4bt wouldn't hold up in the bread trucks? I see a lot of them wuctioned off and about every single one with the cummins the trans is bad.

 

[benhall]

How would a 700R4 hold up behind a 4BT?

Ben

 

[SRath]

A TH400 is a pretty tough transmission when built right. If you can find one from a motorhome with straight-cut gears, they are even stronger. I'm a stickshift guy, but the two vehicles I own with TH400's (my K30 crewcab 4x4 with a 454 and my class C motorhome) have never had a transmission issue. We pull a fairly heavy motocross trailer full of bikes and tools and fuel, powerwasher, etc behind the motorhome. Two men cannot lift the tongue. It has about 100k on it and has never been touched. I did add auxillary transmission coolers to both vehicles.



If I were to use an automatic behind a 12-valve, my first choice would be an Allison, hands down. But my second choice would be a TH400.



Much depends on what type of vehicle and use it will see. As for no overdrive; if it is a light vehicle, just gear higher. The automatic multiplies torque and the Cummins has plenty to begin with.

 

[Skydiver]

While I am not a transmission tech I have owned 2 GM vehicles with the TH 400. 1st was at age 19 in 1979 and I could not kill it. I found out by mistake if you hold a TH400 in 1st gear and run the rpm's up to around 3500 or just keep it floored then place it in 2nd for less than 1/2 second and pull it back to first gear it will go into 2nd gear and make the tires bark. I did that hundreds of times and the transmission just kept begging for more. I abused my transmission as a teenager and it never failed. My engine did not have the power that a Cummins will produce ether. The TH400 does not have bands I've been told.

 

[JacobsD]

Remember, the turbo 400 is the same trans they put behind 2000 horse alcohol dragsters. So I'm sure if it doesn't hold up the first time, build it right the second and it will.

 

[BAnsley]

the thm 400 has 2 bands, on on the low/reverse drum and the other is for engine braking, the low clutch one is a heavy monster, and the braking one is pretty small but either seldom ever fail.

 

[SRath]

"How would a 700R4 hold up behind a 4BT?

Ben"



Again, it depends on how it is used (vehicle weight, engine power, towing, etc. ) and who builds it and how they do it.



In stock form, the early ones were easy for a mild small block to tear out. That was also easy to overcome for a competent rebuilder. My transmission guy assured me once that he could build one as strong as one of his very strong TH350's, but that still is nowhere near a TH400.



The TH400 was probably the strongest automatic ever made for light-duty vehicles by any of the Big 3. Maybe I should say 3-speed auto, since I don't know anything about modern OD automatics except that they fail alot and cost thousands to rebuild vs. a few hundred for the TH350 and TH400.

 

[Skydiver]

A good question. Would the TH 400 suffer low pressure problems with the slow turning high torque diesel?

 

[BAnsley]

just about any good upgrade kit for a 400 will have a boost valve spring to raise up line press for more aggresive shift quality,and i suspose if you needed more press you could shim that spring to a certain extent.

 

[tgordon]

Thanks, I had always thought the issue was the low RPM torque and pressure available there. Now I want to buy one of those, rebuild the trans and try it out.

 

[Gary - K7GLD]

A good question. Would the TH 400 suffer low pressure problems with the slow turning high torque diesel?

BINGO - EXACTLY!



The strength of the transmission may not be nearly as big an issue as is obtaining enough fluid PSI at low engine RPM - a high power V8 gasser shifting at high RPM is lots different than the Cummins down at about half that RPM.



Biggest reason for all the early DC transmission failures starting in mid-'91 (in my opinion), was that they added a overdrive in mid '91 to the stout 727 automatic, but expected the same pump to then supply the added system load - it couldn't, and the rest of the DC automatic premature transmission failures is well known history...

 

[MChildress]

While I am not a transmission tech I have owned 2 GM vehicles with the TH 400. 1st was at age 19 in 1979 and I could not kill it. I found out by mistake if you hold a TH400 in 1st gear and run the rpm's up to around 3500 or just keep it floored then place it in 2nd for less than 1/2 second and pull it back to first gear it will go into 2nd gear and make the tires bark. I did that hundreds of times and the transmission just kept begging for more. I abused my transmission as a teenager and it never failed. My engine did not have the power that a Cummins will produce ether. The TH400 does not have bands I've been told.

My 75 GMC 400 behind a 454 would shift to 2nd by itself in 1st, when you were in it but you would have to shift to drive to get 3rd. I know the 350 trans would hold 1st and you would have to manually shift.