Please sanity check my tow rig transmission choiceg

thecarman

Well-Known Member
Joined
Mar 17, 2005
Location
Apex, NC
Any downside to going from Th700 to th400 for towing, other than losing overdrive?

I'll be towing a bumper-pull car trailer up to 7k gross, and 27ft Airstream that is 6-7k loaded.

I've got a 91 suburban that I'm fixing up to tow with. It has a tbi 350 that runs great. I've swapped in 8-lug axles with 3.73 gears. It has a 700r4 that was rebuilt at some point and works great, and a 241 t-case. Up to this point, the truck has only ever been used to tow 2-3k max

I need to replace the 241 tcase because one of the support rod bolt holes is cracked out. I think it got dropped when the trans was rebuilt. Plus it's leaking fluid between the trans and tcase, and at the front output, which would need to be fixed anyway. And I don't think this case has ever been rebuilt - it's got over 200k on it

I have a known good th400 and np241 from a 90 burb (thanks Qwk87Coupe). I was going to swap the input bearing from my 27-spline 241 into the replacement 32-spline t-case from the th400. That would let me keep the th700

But then I thought - why not replace the th700 with the th400? I won't use overdrive when towing anyway. Based on how I use my current tow rig, I'll probably only put 4k miles per year on it at the most, and the majority of it is towing (I daily drive an old accord, not the burb). We only use the burb for a few trips each year not towing (like to the OBX, or if we go to the mountains in the winter). When I'm not towing I run 35s on it, So I don't think I'll miss the overdrive.

I don't *think* I need the deeper first gear of the 700 when towing. In fact I don't like the wide 1-2 ratio spread of the 700 in my current tow rig when towing

I think I've talked myself into the th400, but am I overlooking anything that would be a drawback to the th400?

I'm trying to keep this low budget. I have no plans for a nv4500 or 4l80e. I already have both the 700 and 400, so I can choose between them.

Really just looking for input about the trans comparison, so please no comments about cooling, hitch, brake controller, springs, etc. I've got all that covered. :)

Thanks!
Richard
 
Both of those loads are well within the 700s relm of pulling but the 400 stock is as strong as a built 700 if you run 35s empty you will have no problem on road trips my 88 k5 has a tbi350/700 i drive around in 3rd till i hit 80 on the hwy then thats the only time i use od
 
for towing the lower first gear will be advantageous.
From your described usage the th400 will provide no benefit and will cost you the lower take off and the OD
 
Thanks offroad cowboy.

Hey Ron! Haven't talked to you in a long time. My current tow rig is a different 90 burb with a 6.2 diesel. It really has trouble with the wide 1-2 ratio of the 700r4. If I'm starting out on a big hill, I can redline it in first, but it still bogs down bad when shifting into 2nd. That could just be the 6.2 though - it really has no power. Maybe the tbi 350 will handle that ratio spread better? Since the 350 can rev out more in first, when it drops into 2nd it will be at a higher rpm than the 6.2 would have been. I've got a spreadsheet to compare rpm vs speed based on axle, trans ratios, and tire size, so I'll take another look at it today
 
My 88 k5 handles 6-7k no problem even with 35s and 3.73 gears. i really dont have a bog problem from 1-2 my friends 6.2 has that "bog" also
 
I think the 6.2 makes peak power right up top so dropping it down to the next gear probably feels like quite a bog. I will tell you that my 6.2 is absolutely no powerhouse and requires precision just not to be run over some times.

Personally, I would rather have a properly built 700r4 in your scenario. Even if you didn't need overdrive for towing, you have the option when on the flats or driving empty. Even though the TH400 is built like an anvil and super reliable, it straight up sucks as a transmission. Tall gearing, no OD, no converter lockup, and saps the little power I have to spare. Since mine has less than 15k miles on it, I'm sure it won't fail for probably another 30 years but a 700r4 or NV4500 is on the horizon because this thing worked great...back in the 70's...
 
Thanks for all the feedback. I had talked myself into the th400, but sounds like I should stick with the 700.
 
Just put a big auxiliary cooler plumbed before the fluid goes into the radiator and a good shift kit like transgo hd2. Take advantage of having od when you aren't hauling and just manually downshift it and hold it in gear on the long hills so it isn't shifting like crazy. I would use the 700 for the loads you are describing.
 
Might want to put a manual lockup switch on it so you can lock the converter in 3rd while you're towing. It'll help keep it cooler and keep more power going to the ground.
 
After wearing out three 350 motors in my 84 suburban with th350 trans I always thought I would have saved a lot of gas and motors with overdrive. Go do a test with and without a trailer over the road with overdrive and then with trans in 3rd. You want to keep that overdrive.
 
The parasitic losses with the TH400 are marginally higher compared to the 700R4, and the lower 1st is to your advantage for towing. I'd rather have the OD trans any day of the week. For a TBI 350 power plant you will not kill either transmission if you keep them cool.
 
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