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Old 06-30-2022, 01:54 PM   #1
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Cummins to TH400?

I have not exhausted this site, but in all the 545 transmission swaps to put behind a 6BT, I have never run across a Turbo 400 as an option.

Here is an adapter

https://www.speedwaymotors.com/G-Force-GF-C-S-1989-02-Cummins-to-Chevy-Trans-Adapter-Flexplate,455883.html

The later Turbo Hydro 400ís had lock up converters and are still the go to trans for high torque and HP in racing.

Has anyone looked into this? Maybe they have and there is a reason that it is a bad idea, but I canít see one from where I am.

The adapter is 1K, and a switch to lock up the converter is not going to be much. It is non computer, parts are easy,

New driveshaft👎🏻

But I canít think of much else.

Maybe have to make a shift lever to fit the trans that is the same length, or the proper length. Might take a few tryís.

What am I missing?

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Old 06-30-2022, 02:23 PM   #2
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Admittedly I had to read the wikipedia page to learn about this transmission. At a high level, I see two problems:
1. It only has 3 forward speeds so you're going to be turning pretty high RPMs to get up to highway speeds given the typical rear end gearing of a school bus
2. You could also have issues with acceleration (or lack thereof) depending on how your rear end matches up with the gear ratios on that transmission.

Basically I think you'll want to do some serious math to determine your top speed with that engine, transmission, and rear end, then work backwards to make sure you can actually get going in 1st going uphill.
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Old 06-30-2022, 02:24 PM   #3
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The 4L80 is not as strong, but a lot more common. A stand alone lock up mod is very inexpensive for it <$200.

And the internet says a stock or rebuild will handle 450hp/torque. Now the hp is not a problem for the Cummins, but the torque might be one! Y’all will know better than I.
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Old 06-30-2022, 02:44 PM   #4
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Quote:
Originally Posted by dbsoundman View Post
Admittedly I had to read the wikipedia page to learn about this transmission. At a high level, I see two problems:
1. It only has 3 forward speeds so you're going to be turning pretty high RPMs to get up to highway speeds given the typical rear end gearing of a school bus
2. You could also have issues with acceleration (or lack thereof) depending on how your rear end matches up with the gear ratios on that transmission.

Basically I think you'll want to do some serious math to determine your top speed with that engine, transmission, and rear end, then work backwards to make sure you can actually get going in 1st going uphill.
Does the 545 have more than three forward gears?
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Old 06-30-2022, 02:50 PM   #5
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Originally Posted by ChurchBuz View Post
Does the 545 have more than three forward gears?
The 545 has 4 forward gears. https://www.dal-trans.com.au/files/AT-545.pdf

Range Ratios*:
First . 3.45:1
Second 2.25:1
Third. . 1.41:1
Fourth . 1.00:1
Reverse 5.02:1
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Old 06-30-2022, 04:46 PM   #6
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the 400 is a great trans and you can build it to take the power... the 4L80 (E) was used behind the duramaxx diesels in the van chassis that couldnt fit the allison 1000s... you can build a 4L80E up to be pretty strong...



the issue comes about that the clutches arent as beefy... in fact in many TH400 builds, they use AT545 clutch steels / frictions because they were heavier duty... some 4L80E builds get the allison clutches as well...



if the bus is a Van chassis then id say sure put a Built 4L80E behind your cummins and go for it...



if its a full chassis bus, you'll wreck the 3-4 clutch in a matter of weeks... no matter how much you drill the plate for the 3-4 accumulator.


a TH400 will wilt... 3 forward gears will trash the converter from heat....
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Old 06-30-2022, 05:00 PM   #7
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Originally Posted by cadillackid View Post
the 400 is a great trans and you can build it to take the power... the 4L80 (E) was used behind the duramaxx diesels in the van chassis that couldnt fit the allison 1000s... you can build a 4L80E up to be pretty strong...



the issue comes about that the clutches arent as beefy... in fact in many TH400 builds, they use AT545 clutch steels / frictions because they were heavier duty... some 4L80E builds get the allison clutches as well...



if the bus is a Van chassis then id say sure put a Built 4L80E behind your cummins and go for it...



if its a full chassis bus, you'll wreck the 3-4 clutch in a matter of weeks... no matter how much you drill the plate for the 3-4 accumulator.


a TH400 will wilt... 3 forward gears will trash the converter from heat....
Thank you!!!

Now I know.
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Old 07-01-2022, 06:44 AM   #8
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I should add that the weight is the issue... much like the old dual-stage-start AC electric motors.. these motors are often on food disposers, blowers, pumps, etc.. they incorporate a really high power start-winding.. once the motor spins up a switch throws and the motor runs on its normal torque...



the start windings make tomns of power and are strong.. but only for short periods of time.. if that start switch ever sticks 9which they do) then the motor burns up in a matter of minutes (or kicks out on thermal overload)...


with a Built auto trans.. a smaller unit can be built to take lots of power.. but short period of time.. thus why towing is so hard on drivetrains in pickup trucks.. an empty truck with a 400 HP engine and a built transmission.. if you punch it, pretty quickly that truck is up to speed where that transmission's input RPMs and ratios are not nearly as offset from the engine's RPMs.. so the load reduces.. at some point the truck is flying and the driver lets off the pedal...



put that same transmission in a 35 foot school bus loaded with a conversion.. now you have to maintain the engine's power input into that transmission for a LOT longer time to get the bus up to speed.. and since the bus is heavier you may need to maintain a higher continuoous load.. heat builds up.. even with coolers.. gears under load squeeze out more of their oil.. drums under thrust can cause thrust bearings to spin continuous that are really designed to be momentary or short-duration..



Billet shafts and hardened shells are great for taking power.. firming up shifts to reduce heat buildup from slipping clutches during engagement all help.. but if youve ever driven a hot rod wit ha "built" transmission.. notice during that firm shift your car Pops / lurches forward during the shift.. this is an energy dissipation.. the transmission didnt have to eat the whole inertia energy of changing that gear ratio (which slows down an engine thats making power.. and also has to either stop or spin-up a drum (deoending on the shift).. put the setup in a bus.. now the bus doesnt lurch forward.. the engine has to slow down to match that ratio fully (rather than speed the car up).. so that tranny just had to eat a good bit more energy internally than in a hot rod car..



modern transmissions handle this with 'SEM', shiftsense, etc other brand names.. essentially they reduce the power output of the engine during shifts.. once the transmission reports the shift is completed the engine powers back up... similar to how a good stick-shift driver can make a clutch last multiples of 100k miles.
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Old 07-01-2022, 10:33 AM   #9
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Like others said, a 400 and 545 are pretty similar capability wise, but the 400 only has 3 speeds. The heavier your are, the more value the 4th speed becomes.

I'm not aware of any 400's from the factory with converter lock up. 700r4 offered that, but that's not a th400. The aftermarket offers kits to give the 400 lockup that drag racers used. Those kits changed the pump, input shaft, converter, etc. to do so. A lot of people also built the trans in the process, because the lockup engagement can be harsh on components.

If you were a machinist/fabricator, you could probably frankenstein one of those kits to work with a 545, as a lot of the components are similar/same. Torque converters are, so long as they're both 6 pad units.

Some of the gmc p30 chassis used 4bt/th400 powerplants, and you could probably use the adapter parts from that with a 6bt-th400 marriage. But once again, why would you want to?
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Old 07-01-2022, 10:54 AM   #10
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the only 3 speed units factory from GM with lockup were the TH-200C and the TH-350C,


like booyah says Ive never seen a lockup 400 althiough there were kits to add lockup to them..



GM ran 3 lines of transmissions for awhile and each on lived (lives) on its own way..


the 200's were the lighter duty often found on the V6 series mid size cars.... they ran these into the early 80s.. in the form of the lockup TH-200C but they fizzled out.. later to be reincarnated as a pretty nice unit , the 200-4R in 86 (firsrt year I remember them).. they had a few weak links .. however these would run foot to the floor in overdrive..



the 350s were a fully mechanical unit with a vacuum modulator and a TV cable... these were pretty stout units asnd of course could be built.. there was a 1 or 2 year version with lockup called the 350C.. the neat thing about a 350 was it had the ability to shift based on engine load (vacuum) and throttle positiion (TV cable).. you could control easily when and how hard these things shifted!. starting in 1982 the 350 got Overdrive and became the 700R4 (and lost the modulator)... in88-89 it became the 4L60 and i think around 92 became the 4L60E... they used it till the 6 speeds came out..(azn interesting tidbit is the early 700's would NEVER go into Overdrive at anything more than 3/4 throttle.. EXCEPT on the 9C1 (police) cars...



the TH400 was the heaviest of all of them.. it used a vacuum modulator and an electric kickdown... the 400 was a beast!!.. Later to become the 4L80 (E) and the 6L80E (6 speed).... like booyah mentioned, the allison 545 was a 4 gear (still 1:1: top) cousin of the TH400..
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Old 07-01-2022, 11:59 AM   #11
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That is all really great info!!!!

Thank you both!!!’

My parts counter guy says that they did release a late limited number of lock up TH400’s. He is absolutely sure of it. Has been a parts counter guy since the 90’s.
But that is just info.

Y’all have educated me out of considering the idea.

Thank you again
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