TH350 vs. TH400

[Bob B.]

I have always wondered why in 1971 when automatic transmissions became available in 1/2 and 3/4 ton 4X4's GM used the TH350 only. The TH400 had been an option since mid-'66 in 1/2 and 3/4 4X2's but it wasn't until many years later it was used in 4X4's. Must have been a reason...

[jumpsoffrock]

Maybe it was cheaper to break a Th350 than a T-Case or a diff, so they made that the weakest link. although a good th350 being "weak" is arguable in and of itself.

Maybe the bean counters were complaining of the 4wd option adding up too much and asked them if they could please cheapen it a bit and sacrifice something that "isn't a big deal, really". So they decided to use a "cheaper" trans--again this is arguable.

Maybe something about the fitment didn't work and the simple easy solution at the time was changing the trans.

I seem to remember that adding 4wd to a truck doesn't necessarily increase it's GVWR or towing ability--Maybe they figured there was no need for a 4wd to have a th400 because they wouldn't be hauling enough anyways. Maybe they did case-studies and found that 4wd trucks were doing less hauling and more "getting to tough places", so they used a lighter duty trans.

[special-K]

^^^ Yeah, I seriously doubt that was the reason. The TH350 is by no means a weak transmission, even in a 3/4t 4wd. I am a long time owner of K/20s, have owned quick a few, and have never had one with a bad TH350 or had to have one rebuilt. One exception is one from a parts Blazer I was swapping into a 3spd Blazer. It operated fine, but I took it to a highly recommended shop to do a performance build. Whatever they did was not right for the application and it blew in a matter of weeks. Then I took it to a shop operated by a rocket scientist of transmissions. Ran my mud bogging Blazer flawlessly, then went into a '71 K/20 work truck with utility body, 4.57s, and was used to plow snow. Operated flawlessly for years.

Bob, this is a great question. One I have wondered about myself. Maybe GM only wanted to run one transmission for economics and didn't feel K/5s, K/10s, or Suburbans needed a TH400. They were right! I can't believe they used Saginaw 3spds. A recipe for failure right there

[Mike80D]

Correct me if I’m wrong, but I’m not seeing an option for the TH400 in our C series trucks as stated above....

[57taskforce]

Originally Posted by jumpsoffrock

Maybe it was cheaper to break a Th350 than a T-Case or a diff, so they made that the weakest link. although a good th350 being "weak" is arguable in and of itself.

Maybe the bean counters were complaining of the 4wd option adding up too much and asked them if they could please cheapen it a bit and sacrifice something that "isn't a big deal, really". So they decided to use a "cheaper" trans--again this is arguable.

Maybe something about the fitment didn't work and the simple easy solution at the time was changing the trans.

I seem to remember that adding 4wd to a truck doesn't necessarily increase it's GVWR or towing ability--Maybe they figured there was no need for a 4wd to have a th400 because they wouldn't be hauling enough anyways. Maybe they did case-studies and found that 4wd trucks were doing less hauling and more "getting to tough places", so they used a lighter duty trans.

I’m confident that the SM465 is by far harder on the transfer case/drive train than either a 350/400. That 205 don’t give a crap what tranny is in front of it. They are tough as nails in so many ways. It would have been nice to have the 400 in factory form for sure but I agree with Tim, the th350 is still no slouch.

[jimijam00]

I thought GM put TH400s in the higher HP engines to handle the power, and not not whether or not a truck had a transfer case. But because of the Rona, I haven't stayed at a Holiday Inn Express since March of 2020.

Originally Posted by Mike80D

Correct me if I’m wrong, but I’m not seeing an option for the TH400 in our C series trucks as stated above....

I've got the original TH400 in mine, and shows M49 on the SPID too. Original engine was a 350.

[Mike80D]

Originally Posted by jimijam00

I thought GM put TH400s in the higher HP engines to handle the power, and not not whether or not a truck had a transfer case. But because of the Rona, I haven't stayed at a Holiday Inn Express since March of 2020. I've got the original TH400 in mine, and shows M49 on the SPID too. Original engine was a 350.

It doesn’t really make sense that GM use the same M49 code for the 350 & 400 trans, but it appears that’s the case. My truck has the original TH350 in it along with 350 engine, and has M49 on the SPID. That’s what I was going by, as well as the info packet from GM heritage center.

[Mike80D]

Originally Posted by jimijam00

I thought GM put TH400s in the higher HP engines to handle the power, and not not whether or not a truck had a transfer case. But because of the Rona, I haven't stayed at a Holiday Inn Express since March of 2020. I've got the original TH400 in mine, and shows M49 on the SPID too. Original engine was a 350.

It doesn’t really make sense that GM use the same M49 code for the 350 & 400 trans, but it appears that’s the case. My truck has the original TH350 in it along with 350 engine, and has M49 on the SPID. That’s what I was going by, as well as the info packet from GM heritage center.

[MDFarms]

If anyone is looking for a rebuilt 400 I have one in parts section,

[Accelo]

GM put the 400's behind the 2 Wheel drives big blocks. There was no Big Block option in the $x4's.
If you just look at them as torque rated, not hp, you will see why it wasn't an option.

Edited. Yes, the THM 400 was mandatory behind the BBC and optional behind two wheel drive SBC.
I have never seen a THM 400 behind a 6 cylinder but I have seen C6 Fords behind 6 cylinders.

[jimijam00]

Originally Posted by Accelo

GM put the 400's behind the 2 Wheel drives big blocks.

I'm not the only 69 2wd around here that came with a 350ci and a TH400. It wasn't very common it seems, but it did happen.

[FirstOwner69]

Originally Posted by jimijam00

I'm not the only 69 2wd around here that came with a 350ci and a TH400. It wasn't very common it seems, but it did happen.

My '69 2wd 3/4 ton with 350 engine also has a TH400.

I think I read in a post on this forum a few years ago that the TH350 was used in all 2wd SBC trucks starting in 1970 or 1971. Not sure about big blocks.

[Mike C]

TurboHydromatic was an option, but TH350 or TH400 was not. GM chose which trans to put in the truck based on whatever metrics they may have had in place. 1969 was the first year for the TH350 and maybe it was unproven in the 3/4 ton arena and that’s why they used 400’s commonly that year.

I also have a ‘69 3/4 ton pickup with a 2 barrel 350 and the TH400 installed. My ‘72 350 4 bbl 3/4 ton Suburban has a 350. The ‘71 1/2 ton big block has a TH400 because big block.

I have a junky ‘69 Jeep J3000 pickup that has the GM sourced Buick 350 and a TH400 attached to the Dana 20 t-case, all factory for that application so GM could of had they wanted or felt the need.

[Bob B.]

FWIW, I have a friend that owns a '70 C20 with a 350/Turbo 350.

[special-K]

I had an all original '70 C/30 157" Dual Rear Wheel 350/TH350 low hump. Also had a '69 C/20 Stepside 250/Powerglide. on 3spd manual K-models GM used the NP205 on trucks and Dana20(?) on K/5s. Mike C is correct and that explains why either TH350 or TH400 show the same RPO code

[67carry-all]

Accelo - my '67 Suburban 2 WD has a factory 292/THM400 combo. If I recall the story correctly from my dad, he wanted a 3-spd auto trans instead of the base 2-spd powerglide. The 292 was a mandatory upgrade from the base 250 in order to get the THM400, there was no THM350 at that time. Love that this six-banger has the same tranny behind it as big-blocks and Rolls-Royces of the era!

[Caddylackn]

I have the SBC and the TH400 and a small hump floor.

Does everyone else that has the SBC 350/TH400 combo from the factory have the small hump floor?

[FirstOwner69]

Originally Posted by Caddylackn

I have the SBC and the TH400 and a small hump floor.

Does everyone else that has the SBC 350/TH400 combo from the factory have the small hump floor?

My '69 (post 12) has a low hump.

What year is your truck?

[chewychevy67]

My 67 2wd has a low hump 327 th400. Only the big block auto 2wd ( always th400) came with a high hump factory. Although I have heard from others that combo still fits under a low hump floor.

[Bob B.]

Someone once told me that a Big Block/Turbo 400 in a short bed would have the high hump floor, but a long bed would have a low hump floor. Something about the angle the engine/trans was mounted.

['68OrangeSunshine]

Originally Posted by 67carry-all

Accelo - my '67 Suburban 2 WD has a factory 292/THM400 combo. If I recall the story correctly from my dad, he wanted a 3-spd auto trans instead of the base 2-spd powerglide. The 292 was a mandatory upgrade from the base 250 in order to get the THM400, there was no THM350 at that time. Love that this six-banger has the same tranny behind it as big-blocks and Rolls-Royces of the era!

The 292 is the Big Block of the L6s. Torque monster.

I used to run a '67 K/10 Suburban. PO had upgunned it to a BBC 454/TH350/NP205.
It was originally a 283/SM420/T221 truck, I think.
Always wondered if the TH350 wasn't big enough for a heavy 4x4 Sub and Big Block. Sometimes the pan would leak all around the bottom gasket, especially after running up a mountain highway in Summer.

[factorystock]

Originally Posted by Accelo

I have never seen a THM 400 behind a 6 cylinder

Here you go http://67-72chevytrucks.com/vboard/s...ryder+c+30+292 The th 400 was first used in '66 C 20 and C 30's , was not listed on '66 C 10's. '67-'68, it was the optional 3 speed automatic for all series and most all engines. In '69, the new th350 was introduced in pickups, and now there were 2 3 speed automatics. GM started to be selective on which application got what. As others correctly stated many '69 C 20 with small blocks came with the th400. By 1970, a transition to the th 350 for small block C 20's took place. The th 400 was became reserved for big block and 1 ton applications till the late 70's.

[Bob B.]

I think the TH400 was a mid-year option on both the '66 Chevy and GMC. I have seen '66 GMC pickups with the V-6 and TH400, but the TH400 option is not in the brochure. Only the AT-218 Pow-R-Flo (Powerglide).

[bigdog55]

I have a 70 half ton GMC with a 350 and a th400 and a large hump. Also a long bed

[factorystock]

Originally Posted by Bob B.

I think the TH400 was a mid-year option on both the '66 Chevy and GMC.

I believe your right Bob. The transmission wasn't quite ready when the first '66 pickups became available for sale in Sept. '65. By January '66, the Th400 was beginning to be installed on C20's and C 30's but not C 10's. The GMC literature shows it available on 1500 series which is equivilent to C 20 series.