View Full Version : OD unit or built 700r4?
Have a TH400 in the burb with the 3.73's in diff. I could really use an overdrive and have been looking hard for a 700r4 to build, but just thought I'd ask for some thoughts on keeping the TH400 and getting an OD unit. I pull our pop-up camper which weighs in at 2500 lbs with water and gear. The 6.2 is N/A, stock motor.
Thanks
Edit: been surfing during lunch, The OD units are very expensive ($2500). I've been told the TH400 is a very stout tranny, and that the 700r4 can be if built right. Haven't researched the cost comparisons yet, so anyone have a ballpark figure for rebuilding a 700r4?
I had one built for me by www.fourthgearinc.com
It was built very strong.
I am very pleased with it.
Paid somewhere $1600-2000, can't remember exactly, with new converter
Why dont you pick up a 4L80E trans which is a T400 with O.D.
It will drive the same but give you an overdrive.
You can bypass the electrical by installing a trans controller.
The controller should set you back about $1000.00, plug and play.
I picked up a rebuilt 4L80E at the junk yard with drive shaft and cross member for $75.00
You can go to this site for controller information. http://www.compushift.com/
With a rig that has a 400 I would opt for the OD unit.
A 400 can't be beat (other than an Alison)
The 700R can be upgraded BUT "It is a nice LITTLE tranny"
The components are small and even a good rebuild is not really enough to make this tranny a long term solution to heavy use.
To build a 700R for HD service is a $2500 Plus task
The 4L80E needs a controler to run it.
The 4L80E has issues too that the 400 does not. A 400 thats in good shape with a good cooler and a shift kit will last a bazillion miles and take just about everything you can toss at it.
I would add an OD unit to the rig if it were mine.
The only thing you wont have is the lockup converter and that is IMHO not worth the cost to convert to the electronic 4L80.
I have been into all these boxes and built a buttload of 400's, 700's and a couple 4L80's
The 4L80 is nice when its working but there is just too much complexity in there that you simplly dont need to make your truck perform very well.
The 700 is just too small in regards to the internal gear train, clutches and such. They are not even as beefy as an old TH 350.
We had an 86 Burb with a 6.2 and it had 4.10 butt gears. drove that thing for a lot of miles (many hundred K) before selling it and it always got 20 plus on the hwy and abourt 16 around town.
Good luck and keep us posted
Robyn
I too have been thinking about adding OD or a 4L80E with the controller. I have a 23’ motorhome that I drive around 57 to 62 mph now. I’d like to be able to run 65 with out running to engine so fast. Plus I don’t want to have to down shift at every molehill. I don’t know how long I’d be able to stay in third over, though it would be nice to be able to split the gears. I’d like to get the MPG up a bit too. I do 10-11 @57 mph now, if I could get to 15 mpg @57 mph I’d be a happy camper.
Anthony
arveetek
04-05-2008, 18:41
have been looking hard for a 700r4 to build, but just thought I'd ask for some thoughts on keeping the TH400 and getting an OD unit. I pull our pop-up camper which weighs in at 2500 lbs with water and gear.
I have a 700R4 in my '81 that I originally purchased used in 1996. This truck is a heavy duty 3/4 ton that has seen a LOT of miles pulling heavy loads. I have yet to rebuild the tranny. The lockup TC is just now starting to give me fits (replaced it once and it's already out again). The 700R4 can be a wonderful tranny that will last a long time in severe use.
THAT being said, the biggest drawback to the 700 is also it's best trait. What I mean is that it has the steepest overdrive ratio on the market. That means it drops the RPM's way down, which is excellent for cruising, but bad for towing. My problem is that when I'm towing heavy loads at highway speeds, the engine is revving too high for my tastes in 3rd gear, and lugging the engine in 4th. I can hardly ever tow in 4th gear, and I have 4.10 gears. If you want an overdrive gear you can actually use while towing, an OD unit like GearVendors is your best choice. If you swap in a 700, you may never get to use 4th gear while towing.
Casey
What has been said about the 700's OD ratio is very true.
The 700 was designed in a time when GM wanted there CAFE numbers up and the way to that goal in the SUV market was 3.42 gearing with the tall OD gears.
The K5 Blazers with a 6.2 would regularly see 24 MPG hwy if driven reasonably, same for the 1/2 ton Burbs and pickups
The 700 is simply put, Not a great choice for heavy service.
The 700 can and will work great for a Blazer (K series) and the 1/2 ton pickups and Burbs with a 6.2 and even a 6.5 if the tranny is built well.
Motor home and heavy towing is just not an arena that the 700 is going to shine in at all.
The GV unit behind a 400 or the Tcase is the way to go.
The 400 can stand just about any tough assignment you can toss at it and with the OD unit you can split things up some too if you like.
1/ 1OD -- 2/2OD -- 3/3OD If you really want to make the best of things get a special low stall HD converter built that has welded blading in the pump, turbine and the stator (Unless its cast)
The addition of a "TIGHT Converter" will help eliminate the loss thats normally seen at higher speeds (That the lockup was meant to stop)
The lockup eliminates any loss in the converter once the clutch applies.
A tight converter however will not totally eliminate this loss but greatly reduce it over a stock unit.
A 4L80 is a nice tranny but even this unit has issues when used under heavy towing. The lockup converter should be replaced with a custom HD unit to get maximum reliability.
The OD is not really up to the task of towing heavy in stock form either.
Short heavy hauls are no sweat but towing a large RV OR?????? can and will eventually cause troubles
The addition of the Allison in recent times was a long overdue step in bringing the GM pickup line up to the task of heavy service.
Good luck
Robyn
In my 87 suburban I took out the 400 and put in a 700r with a gearvenders. That was a slick setup until the 700 went gunnysack. (it had a bad temp sensor in it and the torque would not lock up until it was super hot.) I believe if the right outfit builds the 700 it will perform well and last. After I already had overheated it I had mine setup with a toggle and could lock up the torque converter in 2nd through 4th. I also had an exhaust brake on it. I am a firm believer in a locking torque converter. Properly setup the transmission will run cooler. I still have the suburban but will probably part it out. I'm planning on putting a manual in my 87 GMC pickup with the gearvendors. The 400 is a good transmission, but it is terrible for fuel mileage and you can't effectively use an exhauast brake with it. I highly recommend Bowtie overdrives for a built 700. Mine was built by some crooks in Georgia or some place near there. If I only had more time and not so many toys. tpitt
If the 400 is set up with the proper converter its is still a very good choice.
The big problem with the 700R is the size of the components.
The shafts,clutches, sprags and such are all very small compared to the 400.
The clutch packs can be modified and more plates and better steels added and the sprags can be upgraded. This only goes so far though.
The 400 behind a 6.2 is not going to cause terrible mileage.
As I mentioned earlier I have had 2500 4x4 Burbs with a 6.2 and a 400 get 20+ on the Hwy and this was with 4.10 gears.
The lockup is nice but not absolutely necessary.
Another issue can and does arrise with using the converter lockup in the lower gears (2)
The lockup clutch is a very narrow band of friction material around the outer rim of the lockup plate in the converter and this is pressed against the front face of the turbine housing by internal oil pressure.
This clutch was meant to hold modest torque loads under cruising conditions.
Continued use under more severe loading or "braking" will cause the clutch to fail.
Other styles of trannies such as the Allison have a multiple plate clutch that is designed to hold much higher torque loads and will handle the lower gear lockups as well as the exhaust braking activities.
The 4L80 lockup clutch is very little better as far as being able to handle high torque loads.
TCC failures are common when the system is reworked to allow use of the clutch other than under cruising conditions.
A well built 400 converter is a lot heavier and has much larger internal components far more capable of handling heavy service.
I have even had a special non lockup converter built for a 700.
These are a pecial unit that has much larger internal components (Pump ,Turbine and stator) than a factory 700 converter.
These require some changes inside the VB to allow their use.
Now there are many things that can and are done with the 700R. Some work, some dont. The overall picture for heavy service with a 700R is marginal at best.
They are well suited for the smaller rigs like the K Blazer (Tahoe 2 door)
and 1/2 ton rigs used for passenger service.
Towing a light boat or small utility trailer will be fine.
A heavy rig with a large RV or use in a Motor Home is not going to be a workable combination that will last.
Just some pertinent info
Robyn
Thanks Robyn, it looks like I’ll be going with the gearvendor.
I put about 100K on a gearvender towing a 9K 5er & you Wont regret using one. 2nd over is a awsome hill pulling gear.
Stock Allison only has a single clutch in the torque converter. The allison has a better case though and don't flex as much. I personally have a billet converter in my allison, but it is only a single disc. It will hold anything I will ever put to it.
The problem with my 700r was I overheated it pulling 8300# out of Las Vegas with outside temp of around 112 degrees. If the temp. senser in the pan was working properly and the torque converter would have locked up when it was supposed too, I dont think I would have had a problem. It is not a problem locking the torque up in 2nd through 4th if you do it right. I used this mainly for the exhaust brake application though. If I had more time and didn't have two Volkswagen TDI's and a Duramax I would have Bowtie build me another 700 for my 87 pickup. I would have a properly built 700 over a 400 any day of the week. Believe me I've had everything from powerglides on up.
Overall I think you will be happier in the long run with the GV unit.
Get your little diesel in good shape too if you are looking for top power and mileage.
Worn injectors and IP will cause a great loss in power and mileage.
Just a fresh set of injectors can make a big difference.
best
Robyn
Ah ha! How about putting the powerglide as a gear splitter behind the TH400 with, like 2.73 gears.
:rolleyes:
DmaxMaverick
04-08-2008, 09:10
Ah ha! How about putting the powerglide as a gear splitter behind the TH400 with, like 2.73 gears.
:rolleyes:
It could work, but you'd have to omit the torque converter (on the powerglide), and supply pump pressure from the first tranny. And, a reverse/reverse would give a hellacious deep low forward. Sounds like a fun project, but probably wouldn't be practical in the end. Before I'd go that far, I'd just toss the tranny altogether, and install a full hydrostatic drive system. I've been tossing around that idea for years.
...And, a reverse/reverse would give a hellacious deep low forward...
ROTFLMAO! ;)
Speaking of powerglides, my dad had a '67 Olds Cutlass with a 330 4bbl. If you held your foot on the floor the lifters would pump up at about 79 mph and the engine would quit before it shifted... (Don't ask me how I know this...)
I've been away for awhile and it looks like I missed alot. For many of the reasons listed previously, I have decided not to go with the 700r4. Helping me with that decision was my subaru blowing a head gasket. Rather than fix it (and other stuff), I dumped the car as is and we bought a 05 Passat TDI. Along with the Passat comes a car payment, so truck projects are on hold for awhile.
I have been getting 15mpg with mixed driving, and 17mpg w/o a load on the freeway. For now I just have to go along with the "if it ain't broke, don't fix it' way, until I can afford an OD unit, or maybe a NV4500 conversion. So for now, as much as I would like to cruise around 70mph with good economy, I can only chill at 55-60 mph with OK economy. More time to soak in the scenery, I guess.
raunch,
You'll love the TDI. I have two of them. I hardly drive my pickups now with the price of fuel.. If you don't know about it tdiclub.com is a very good source of information.
tpitt
I have a 700R4 w/a GearVenor and pull a "lightweight" 30' travel trailer. Combined weight is somewhere between 13,000 and 14,000 pounds. Rear end has 4:10s in it.
Drove to Las Vegas a few year back, 2500km both ways, and averaged 23 Canadian mpg (no trailer).
I had the tranny rebuilt last year cost $2600, but it had over 800,000km on it, it was apart at least twice before, but original case. I updated it with a 10 vane pump, and a few other things for pulling. I also pull in OD if the EGT are reasonable.
Wouldn't trade it for nothing.:)
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