4L80E to TR400 Trans swap 1995 Chevy P30 chassis motorhome

[Wulfdan]

Hello GMT400 Members, nice to be here. I am hoping that one or more of you can share some wisdom.

I have an old 1995 Class A Motorhome built on a chevy P30 chassis. I wasn't able to bring the 454 around to where it would run without falling on its face so I bought a 454/TR400 used out of another motorhome. This will be the new engine/trans combo because the original is over 270K miles, and the replacements are around 70k. Aside from adapting the older intake for TBI, the big problem I'm facing is the driveshaft changes required to go to the shorter TR400. I tried the googles, but nobody was doing a straight swap, there was other custom involved.

My question is what do I need to swap or replace regarding the yoke/driveshaft to swap from the longer 4L80E to the shorter TR400. My brother bought a kit to take care of the speedo, so we're down to figuring out what needs to happen with the driveshaft. Don't worry about exact length, this monster has a 3 piece segmented driveshaft. I need to know how to properly measure the length, do I need to change the yoke? etc.

Thank you for all your help. Sorry I don't have a super cool truck to show you, but she's got some junk in her trunk ;-)

 

[Erik the Awful]

Typically a driveshaft shop will want the measurement from the end of the transmission output shaft to the center of the u-joint on the rear pumpkin. With a three-piece they'll probably want the measurements to each carrier bearing as well. It's a good idea to call and ask them first and find out what they want. If you also let them know you're going from a 4L80 to a TH400 they might know offhand how much to add.

Personally, I would swap in another 4L80. They're a solid step up from the three-speed TH400.

 

[Wulfdan]

I didn't want to go to a TR400 either but it's the path of least cost. This RV will be semi-permanent housing for me, and will only drive to somewhere with RV service and back. I won't be touring America in this one. I got the 454 and TR400 pretty cheap for the low miles they have. Thank you for the advice on how to measure the driveshaft.

 

[NickTransmissions]

I didn't want to go to a TR400 either but it's the path of least cost. This RV will be semi-permanent housing for me, and will only drive to somewhere with RV service and back. I won't be touring America in this one. I got the 454 and TR400 pretty cheap for the low miles they have. Thank you for the advice on how to measure the driveshaft.

What's a "TR400"? You're referring to a TH400, correct?

I agree with Erik, go with the 4L80E as it's already set up to take one and if your existing '80 is in good working order, just put it behind the replacement 454 and go. No sense in spending a whole bunch of money downgrading (not to mention the additional funds required if the Turbo-Hydramatic requires a rebuild.

In short, dont fix what isnt broken.

 

[Wulfdan]

Nick, I have a unique set of circumstances and I'm trying to work with what I've got. If you have a free 4L80E to send me I'll be happy to stay with the 4L80E. Do you really think I should stay with a 250K transmission over one with 70k? I don't even know if it works right or not.

 

[NickTransmissions]

Nick, I have a unique set of circumstances and I'm trying to work with what I've got. If you have a free 4L80E to send me I'll be happy to stay with the 4L80E. Do you really think I should stay with a 250K transmission over one with 70k? I don't even know if it works right or not.

No, and you don't need me to do so as you already have one...Since you bought the th400 used, do you have proof of the mileage on it and/or it's source vehicle?

250k vs 70k is nearly the equivalent of 6 in one hand, half a dozen in the other since the newer 4L80E will last longer all things equal to the older TH400 but either can go south at any time. It would be different if the Th400 was either freshly rebuilt or had less then 5K on it...

I'd price out the cost of refreshing your 4L80E (new filter, paper/rubber, bushings, clutches, harness/solenoids) against the cost/risk/benefit of integrating the TH400 as-is into your powertrain and make a decison on a best-value basis.

If you determine the th400 is your best route the go for it. Likewise for the 4L80E...

 

[92Raiderburban]

Nick, I have a unique set of circumstances and I'm trying to work with what I've got. If you have a free 4L80E to send me I'll be happy to stay with the 4L80E. Do you really think I should stay with a 250K transmission over one with 70k? I don't even know if it works right or not.

in both circumstances, "not knowing" what is good and assuming the other is because of heresay, is not the mindstate you should be in when deciding. If i were you test that 80 out first, and go from there. i've had mine go 400k in one of my work trucks while not need anything except fluid and filter servicing.

 

[Supercharged111]

Why not just swap the engine and keep the existing 4L80? You haven't even said anything is wrong with it? Mileage be damned, if it works it works.

 

[Schurkey]

1. Until you tell us the length of the TH400 tailshaft, there's no way we can advise you on driveshaft length changes. There's four tailshafts that I know of for the TH400 in a 2WD application:
1. The "short shaft" as used in intermediate and compact cars,
2. What folks call the "long shaft" but in fact it's a medium-length, as used in B and C body full size cars,
3. The actual "long shaft" as used by Cadillac on the D-body, and
4. The goofy "TH375" shaft with smaller-diameter/fewer splines, and "375" cast into the underside of the tailhousing. Used on various vehicles--often station wagons--especially when the plant building TH350s was overloaded (due to a fire, I think.)

There's 4WD output shafts, and there may be others I don't know about.

Driving a motorhome is going to have serious consequences at the gas pump. Scrapping a possibly-good overdrive transmission with converter clutch, and instead dumping money into a lengthened driveshaft seems like a recipe for going broke if you actually drive the thing.

How are you going to control the kickdown solenoid on the 400?

 

[Nad_Yvalhosert]

So let's see. High mileage, heavy duty, desirable overdrive transmission, stock to the vehicle: Means tranny x-member is properly located, wiring harness is plug & play, cooler lines are properly sized, routed and designed specifically. Drive shafts are directly usable too.

Or, lower mileage, unknown/untested, non OD tranny, possible non lockup torque converter: Means relocate x-member, rebuild/redesign driveshafts, cooler lines, vacuum shift modulator vacuum line & port on intake, burn a specific chip for an OBD1 ECM.

How about sell the 400 and OE big block, then send the OE 4L80 to a quality tranny rebuilder?