Rebuild th400 or 4l80e?

Howdy all!

I have an 85 454/th400 suburban and my first travel trailer pull (6miles in) caused me to puke out new tranny fluid out the vent tubes. The fluid came out burnt and black so I’m guessing the clutches are shot. Should I rebuild this thing (much cheaper?) or swap to a 4l80e (new shaft, controller, etc?)?

The turbo 400 looked good on the inside but I wonder if being 40 years old made it take the cake when I towed with it?

Got a trans cooler on it?

idahovette said:

Got a trans cooler on it?

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Yes. External cooler

txaggie said:

Yes. External cooler

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In series with the radiator transmission cooler?

This is all a matter of opinion... it should be obvious

OD or no OD, to break it down to its lowest common denominator

I daily drive

Ricko1966 said:

In series with the radiator transmission cooler?

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not run through the radiator. Exterior only.

txaggie said:

I daily drive

not run through the radiator. Exterior only.

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Run it through the radiator then to the ext cooler. You're giving up a lot of heat exchange by not including the internal cooler in the system.

Also, Id swap in a 4L80E if it's in your budget to do so.

NickTransmissions said:

Run it through the radiator then to the ext cooler. You're giving up a lot of heat exchange by not including the internal cooler in the system.

Also, Id swap in a 4L80E if it's in your budget to do so.

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My tech told me it would be cooler if it didn’t hit the warmer radiator. More cooling time does make since.

txaggie said:

My tech told me it would be cooler if it didn’t hit the warmer radiator. More cooling time does make since.

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Your tech was wrong. A fluid to fluid cooler is more efficient than a fluid to air cooler.Burn your finger,then wave it in the air,or dunk it in water which one cools faster?And I don't actually 100 percent agree with Nick. I run through the external cooler to the radiator. The radiator regulates the transmssion fluid temperature so it is not too hot not too cold. Transmission fluid should be in the 175 to 225 range. Engineers discovered a long time ago,engines didn't wear as much if they maintained the proper oil temperature,not just as cold as they could keep them. Transmission fluid is oil.

I'd rebuild the 400 and add a Gear Vendor.

NickTransmissions said:

Run it through the radiator then to the ext cooler. You're giving up a lot of heat exchange by not including the internal cooler in the system.

Also, Id swap in a 4L80E if it's in your budget to do so.

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Nick,

Do you recommend a heavy duty tow rebuild kit and a shift kit?

This one looks good:

txaggie said:

Nick,

Do you recommend a heavy duty tow rebuild kit and a shift kit?

This one looks good:

Performance Shift Kit - GM/Chevy TH400 Transmission

www.onlinetransmissionparts.com

High Performance Super Kit for GM/Chevy TH400 Transmission

www.onlinetransmissionparts.com

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I normally dont use shift kits in TH400s and 4L80Es as they arent necessary. Dual feed the direct clutch internally, rollerize the output thrust flange, install the Sonnax TH400 boost valve/sleeve and spring kit, drill the high clutch feed out per your converter stall speed and desired shift firmness. Keep both accumulators active and reinstall all the check balls.

I would also incorporate high energy clutches into the build, though the stock clutches are fine for most mild applications. Install a Chrysler A518 forward clutch snap ring in the intermediate clutch pack in place of the factory TH400 snap ring. I usually end up using the .106 thick version and with three factory steels and clutches end up with .040-.050 clearance in that pack which is ideal, IMO.

Bennyt said:

I'd rebuild the 400 and add a Gear Vendor.

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You can swap in a 4l80 far cheaper than that, and gain the lockup converter.

The vent on the trans is probably plugged. Some mud wasps likely filled it in. It's common for something that sits. Clear that, change the fluid and go live your best life.

Matt69olds said:

You can swap in a 4l80 far cheaper than that, and gain the lockup converter.

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^^This.

Plus a 4L80E is stronger than any 400, unless you build a full on race transmission (which has it's major street drawbacks.) Buy the controller, enjoy OD, enjoy not leaking, enjoy being able to tune the transmission's shift points and firmness. Enjoy a transmission case that's at least 20 years newer, and not worn out (yes, cases wear.)

Modern GM pickups can tow much more than our squares. A big chunk of that is heavier frames/suspension and brakes. Some of it is also the transmission. I've swapped 4L80s into performance cars, into trucks, and into cruisers. Best money spent for driving daily.

We've come a long way on driveline and brake technology in the nearly sixty years since the TH400 was introduced. It's a great transmission for it's time, and still own a TH425 in my GMC MoHo, we've just progressed past that technology.

PS: You can even go steeper in your rear end. 400s/4L80s have the same relatively steep 1st gear; but with OD, you can still cruise comfortably with 3.73s, or actually get gas mileage with 3.42s. My square is a 5.3/4L60E, 3.42s, and I get 22 on the highway with the tonneau cover on... And she'll still tow 7K.