88 Suburban build Tranny questions

[mcgall83001]

Hey guys, Just bought an 88 Suburban with a 350, TH400 and 4:10's. I plan to do a build thread some time after new years but need some help with a tranny question in the mean time. I'm getting a lot of conflicting info.

Anyway the truck has been awesome so far but mileage is running right at 10.5mpg. I would like to see if I can improve that some. First thing that comes to mind is putting in a tranny with OD. From what I have read a later year (post 88) TH700R4 is a good option. Does it sound like I am on the right track? Any problems with installation, (would prefer something that bolts right up) wiring, cross members, drive shaft length? The most disconcerting comment I got was that I would need new axles(???).


Anyway, any insight you guys can provide would be very much appreciated.

Mike

 

[Rot Box]

You will not need different axles.

I believe that the 700R4 is longer than the TH350/400 so you would need drive shafts but the rest of the swap is pretty strait forward. You will loose some strength swapping to the 700R4 as well. I don't have much experience with the 700 I do know that they could be had (88 and later I believe) with a lockup torque converter though and I would shoot for that. At speed it will lockup and help with putting power to the ground and also save fuel.

The ultimate would be running a 4L80E trans, but finding one and making the electronics work would require you finding a donor vehicle or a complete wiring harness to work with your engine (assuming you have the TBI 350).

Whatever you decide make sure to run a large aftermarket trans cooler and having a temperature gauge wouldn't be a bad idea either for peace of mind while loaded down with gear pulling the canyon :bike_rider:

 

[I Leak Oil]

If the TH400 is in good shape then I'd keep it. Great transmission, been around forever, easy to find parts or used units, and plenty of performance parts. You can get an aftermarket overdrive such as the Gear Vendors, which mounts between the transmission and transfer case but would still need to deal with the drive shafts.
Or just man up and go manual with an NV4500!
(Just kidding about the manning up thing)

 

[rparker762]

Post 86 700r4s are good just install a shift kit to prolong the life of the tranny. You will need the tranny to t-case adapter from a rig with a 700r4/208 combo. You will also need a new output shaft for the tranny and you will need to relocate the crossmember 3" rearward. Your cheapest solution would be to find a donor rig with a working 700r4, NP208, cross-member and driveshafts then it is just a simple swap. You will need a 12v hook up to hook up the lock up TC. Your best solution is to keep the th400 and keep your foot out of it. The th400 is dang near bulletproof compared to the 700r4. Also realize you are driving a tank anyways. I drive an 84 K5 Blazer that I am building now that has a 700r4/208 and I only get about 10 mpg. I am running 3.73's and 35" tires. Not the best for economy but it's what I have. I am installing an auxiliary fuel tank from a shortbed pickup for a total on board capacity of 49 gallons.

What size tires are you running? 4:10s are good for 33s but if you have larger tires 4.56s could help and no, you DO NOT need new axles. If you have any questions, you can PM me.

 

[bftank]

i'd second the vote to use the nv4500. but i am not much for suto's in the first place. in my experience manuals last longer, less problems, cheaper to rebuild, and better control in slippery conditions.

on the other hand it depends on what you want to do with the rig, beat around the woods, th400. expedition; diesel and a manual for better milieage and power.

 

[1leg]

For what it worth doing a convertion to overdrive will not increase you MPG that much maybe 1-2 MPG. I have owned a lot of diffrent chevy trucks over the years with alot of diffrent combination. A small block/th400 will be you most bullet proof combination you can have. If you look into the cost of rebuilding trannys the overdrive are the most expensive. If your stuck in some smalltown 1000 miles from home and need the tranny rebuilt to get home your going wish for a TH400.

If you must get a overdrive I have found these guys in my research.http://racetransmissions.com/store/index.php/cPath/1_25_56_111
I have not bought anything from them but i have heard good things. Make sure you get a "K" case from 89-92. You will need to do all the heavy duty upgrades to the internal parts. Make sure you set-up the TV cable right or you will burn the transmission up quicker then a drive to the store.

A gear vender will cost you around 3-4000.00 new and you will need new driveshafts and build a mount to support the extra weight.

1 issue with th400 in chevy 4wd is the tranny case can crack in the tailshaft area. keep an eye on it. I plan to built a support for the transfercase to help prevent any cracks.

My plan right now is to keep the th400 and learn to rebuild it myself. If i ever get an extra 2-3000.00 grand i'll get a monster 700r-4.

The Manuel trans is the cheapest option for an overdrive and easier todo. Won't work for me having only 1 leg.. :victory: go to ColoradoK5.com for info on how todo it.


Good luck

 

[mcgall83001]

May cooler heads prevail

Thanks for the help guys. I spent a week all over the web and ended up more confused than when I started. One message here and within 24 hours all my questions are answered.

Unless something just falls in my lap I think I will take the advise of the majority and just stick with what I have. The thing shifts like new and it would be a shame to get rid of something this reliable for a few mpg.

A few thoughts:

1. I like the idea of the Gear Vender and this sounds like the best compromise but the price is prohibitive.
2. I had not considered a manual, assuming it would be much more difficult. This seems like a good swap and there is plenty of info online (including a build in 4wd mag). I'll keep an eye out for a donor.....just in case.
3. I routinely make a trip to Jackson, WY to visit family. It's about 600 miles round trip. If a new tranny (or whatever) boosted me from 10 to 15 mpg it would save me $60 per trip (@$3/gall). While as extra $60 in beer money is nothing to sneeze at....it hardly brakes the bank.
4. If I pulled off the TH700R4 swap for $1K (which is hardly a given) it would take about 10k miles to pay for itself.
5. And most importantly...now I can obsess about other things like tires, bumpers, lights, dual battery setups.....the list goes on and on and on.

 

[1leg]

T
3. I routinely make a trip to Jackson, WY to visit family. It's about 600 miles round trip. If a new tranny (or whatever) boosted me from 10 to 15 mpg it would save me $60 per trip (@$3/gall). While as extra $60 in beer money is nothing to sneeze at....it hardly brakes the bank.
4. If I pulled off the TH700R4 swap for $1K (which is hardly a given) it would take about 10k miles to pay for itself.
5. And most importantly...now I can obsess about other things like tires, bumpers, lights, dual battery setups.....the list goes on and on and on.

Now that thinking. :wings:

Where are you located, General location not address....

 

[bftank]

and if you are going to stick with chevy i definitely second the advise to thoroughly search through the ck5 forum. i have an 89 sub build thread over there. and there are a lot of people that have already been where you are at. here's one that i got some ideas from.

http://www.reid.org/~dreid/89suburban/89suburban.htm

not mine by the way

1leg- it's all about feeling the gears and cramming at the right time

 

[ntsqd]

I've had a '79 carb'd 3/4t (350/TH400/NP208) and now have a '91 (350/700/NP208). Both have/had 3.73's and 285/75R16's. The '79 rarely ever got better than 9 mpg. The '91 has done as good as 15.6 mpg. Not a truly fair comparison since there is also the carb vs. TBI involved in the equation.
They are large vehicles, driving slower really pays off. With the OD in the '91 it costs me mileage to operate below 65 mph in OD.

Last that I knew the Gear Vendor does NOT install between the trans and transfer case, it hangs of the back of the transfer case. I've asked them for the in-between install several times at the SEMA show and every time the answer has been no. It has been some time since I asked, perhaps they've come around?

Something that I did not see specifically stated, the input for the TH400 application is 32 spline, the input for the 700R4 application is 27 spline. Therefore you would either need a different input gear or a different NP208. Frankly, if you're going for the whole t/c then I'd go looking from a right hand drop NP241. A tiny bit lower low range (not worth it by itself), a bit more HD (again...) and the ability to easily do an SYE. Something not so easily done with the 208 (needs Dodge parts, no aftermarket support).

My advice; drive slower and put a big cooler on the trans. Given your general location I would highly suggest using a Mocal oil thermostat to keep the cooler out of the system when it is too cold.

 

[mcgall83001]

etc:

1leg: I've been in SLC for about a year now. I spent January in the desert (hiking and slot canyons mostly) living out of the back of a van. Always liked the dessert so when I decided to go back to school I just moved on over to SLC. The X is still in Jackson with the kids.

bftank: I spent some time on ck5. Oodles of info there. Thanks. That link was a cool burb. Could not find your build though. Probably operator error.

rparker762: I'm running 235's that came with the truck. Would like to move to 285's. I think that’s about as big as I can go without a lift. My experience with my old Bronco and large tires on icy roads was not good though so I'm going to have to think about this. I had the 235's siped the other day (recommended) and Discount Tires had a set of winter rated 285 70r 16's that were fairly aggressive. Again, I don't really "need" bigger tires and will take fuel efficiency over clearance.

Rot Box: The rig has a towing package so the cooler is already pretty beefy.
“The ultimate would be running a 4L80E trans, but finding one and making the electronics work would require you finding a donor vehicle or a complete wiring harness to work with your engine (assuming you have the TBI 350).” I occasionally see some poor on craigslist that spent 6 months begging the wife for a new tranny just to have the engine go south a month later. I’m going to continue to look for a “deal’ in that regard.

 

[mcgall83001]

etc:

1leg: I've been in SLC for about a year now. I spent January in the desert (hiking and slot canyons mostly) living out of the back of a van. Always liked the dessert so when I decided to go back to school I just moved on over to SLC. The X is still in Jackson with the kids.

bftank: I spent some time on ck5. Oodles of info there. Thanks. That link was a cool burb. Could not find your build though. Probably operator error.

rparker762: I'm running 235's that came with the truck. Would like to move to 285's. I think that’s about as big as I can go without a lift. My experience with my old Bronco and large tires on icy roads was not good though so I'm going to have to think about this. I had the 235's siped the other day (recommended) and Discount Tires had a set of winter rated 285 70r 16's that were fairly aggressive. Again, I don't really "need" bigger tires and will take fuel efficiency over clearance.

Rot Box: The rig has a towing package so the cooler is already pretty beefy.
“The ultimate would be running a 4L80E trans, but finding one and making the electronics work would require you finding a donor vehicle or a complete wiring harness to work with your engine (assuming you have the TBI 350).” I occasionally see some poor on craigslist that spent 6 months begging the wife for a new tranny just to have the engine go south a month later. I’m going to continue to look for a “deal’ in that regard.

 

[bftank]

my build is here:

http://coloradok5.com/forums/showthread.php?t=249133

for good winter tires i just bought a set of 235/85r16 discover cooper atr's that have tons of siping. we got 18-20"s of snow here in rifle, co a couple weeks ago that i had to drive through in my stock dodge in order to get home, never got stuck and the ice that came later didn't seem to phase them either. you might check them out.

33's should fit, might require minimal trimming.