Got a 6.2 diseasal... should I put a 700r4 or a Turbo400 behind it?

[Firehound]

Howdy!

I've got a 3/4 ton 85 Burban with a 6.2 in it... It has a turbo 400 behind it with a cracked case. I'm parting out a 1/2 ton 85 burb (with a 350) that has a 700R4 in it...

Here's the quandry: Fuel mileage is a very important consideration. I know the 700r4 will get better mileage with the OD... BUT I've heard lots of nay-sayers telling me that it is a weak transmission to put behind a diesel. Endurance is also an important factor, I dont wanna be rebuilding my tranny every year...

Usage: This rig will be my DD AND my Search and Rescue Rig. Cruisin and 4x4in... when/if I tow, I'll probably only be towing my 3/4 M101 Army trailer (basically an 8' pickup bed with a tarp-top)

In my quest for fuel mileage, how much difference are we talking anyway? 3-5MPG, I may as well keep the heavy duty Turbo400... 10+mpg... I guess I'll go with the 700r4?

Thanks y'all!

[zaccaglin]

I have an 85 3/4t 4x4 burb with a turbo 400 as well and have been wondering the same. Who thinks a 700r4 would be a better choice??
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[68Timber]

I have the 6.2/TH400 in my military blazer. These trucks have 3.08 gears. My speedo cable needs to be replaced so I don't know my mpg. But other CUCV owners regularly report 20+ mpg hwy. I really doubt that is at 70 mph though.

Here's my take on it. The TH400 is hell built for stout even in stock configuration and if you choose to rebuild that transmission, it'll likely be the last time its rebuilt while you own it. It will also be the least expensive route. The TH700R4 earned a bad reputation from the first and second gen models in 1982 through 1986 model trucks. But they can be rebuilt to hold insane power now - all bugs have been discovered and remedied. The burden is yours to find a transmission guy that can do it right. They've been around so long most any trans guy worth a crap knows how to do this.

I'd rather have the overdrive. It's not going to deliver 3-5 mpg, maybe 1-3. But you'll be able to cruise at 70 or 75 and still get 20+ mpg.

[rcbildr]

I would stick with the TH-400. I have an 85 diesel burb with the th-400 and it has 4.10 gears with 35" tires and I easily get 18-20 mpg. I managed to break a 700R4 in my diesel truck (trucks gone now tho) and the tranny was supposedly built to the hilt. A while back I also had an 82 2wd diesel burb with 4.10 gears and stock tires and was getting 20-22 mpg. I would think with your gears, the mileage would be decent. I wouldn't use the 700R4 in a truck/burban I would always be wondering/worrying about how long it would last.

[72longhornTX]

I myself would go with the 700r4. You may not get 5mpg more but every little bit counts. I have seen people use them behind the 6.2s and 6.5s all day and never have an issue. As long as your not out there racing it and tearing it up, they tend to last.

[special-K]

There's more to an overdrive transmission than fuel economy. Running your diesel at a lower rpm on the hiway will make it a happier motor. My '85 Jimmy came with a 6.2/700r4 combo and as far as I can tell everything is original and doing just fine. I have had a good many of these trucks and never broke 20 mpg w/o overdrive. My Jimmy now has 3/4t axles,35" tires,and 4.10s. It gets 22+mpg. my '91 Suburban that the axles and tires came off of got 22mph hiway with 6.2/4L80E. Al I know is these things love rolling all day long on the hiway in O/D,just like a big rig. My first 6.2 was an '84 K30 make in the '80s. I added a Banks Sidewinder kit and Gear Vendors to it and it also was 4.10 w/35" tires. I think that's a great combo for these motors. I think about 33" tires would be ideal.

[jjzepplin]

Pre 87 700r's are not a good trans.7 vane pump instead of ten vane etc. When I was looking for one I had a free 85 700r. I got a kit from B-M to rebuild it and found that they wanted me to replace all kinds of parts with post 87 parts. So I called them and they said this was not even possible.....I had already pulled this thing apart and could see what they meant. So I returned the kit and went to the junk yard checking #'s. Found one in a pile that the guy said was cores. Bought one for $50. Put the new completely different B-M kit in it with Corvette servos and yadda. Drove from Cali to Florida no prob. Then boom. TCI torque converter blew up. Had it cut open and wouldn't you know it..Totally stock unit ???? Had one custom built and replaced the pump. Ground pounding wheel spinning 4wd monster!!! Thank you Tampa Converter! Moral = free sometimes is not good AND no more expensive TCI converters!

[68Timber]

Originally Posted by special-K

There's more to an overdrive transmission than fuel economy. Running your diesel at a lower rpm on the hiway will make it a happier motor. My '85 Jimmy came with a 6.2/700r4 combo and as far as I can tell everything is original and doing just fine. I have had a good many of these trucks and never broke 20 mpg w/o overdrive. My Jimmy now has 3/4t axles,35" tires,and 4.10s. It gets 22+mpg. my '91 Suburban that the axles and tires came off of got 22mph hiway with 6.2/4L80E. Al I know is these things love rolling all day long on the hiway in O/D,just like a big rig. My first 6.2 was an '84 K30 make in the '80s. I added a Banks Sidewinder kit and Gear Vendors to it and it also was 4.10 w/35" tires. I think that's a great combo for these motors. I think about 33" tires would be ideal.

Tim, that's a good point. The 6.2 isn't exactly known for its stout bottom end. Any revs you can take away from it make it a happier motor.

[csa daddy]

700R4 will be fine newer versions of the trams are the best
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[Edahall]

If it where mine, I would forget about both transmissions and go with a NV4500 5 speed manual. However, if you do go with a 700R4, make sure to get an 87 or newer. My 82 Suburban 6.2 ate through several 700R4's in short order until I went with an 89 700R4. And even then, it had to be tweaked several times in order for the torque converter to hold in 4th gear. The 6.2 had enough torque to cause the torque converter to slip in 4th gear.

In my 1990 Suburban, I went with a manual transmission and couldn't be happier.

[cal30_sniper]

I'm dropping the TH700 out of my 90 parts car behind my 454 3/4 ton as soon as possible. I took a freshly rebuilt trans and added a B&M shift kit and traveller torque converter, along with a +3qt pan and Hayden transmission cooler. Many happy miles of towing my boat in overdrive and not a problem. The trick to the TH700 is keeping it cool, and keeping the tv cable adjusted. The overdrive will make a big difference I the rpms you see on the highway. May not net you much gas mileage, but it'll make your engine last a lot longer.
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[Firebirdjones]

When you say 6.2 diesel I assume you aren't talking about a turbo diesel? In which case I wouldn't worry about the 700 behind it. Build it right and it will last you many miles. A turbo diesel however I'd probably look at other options. I don't feel using overdrive while towing is an option with the 700 so if that's what you are looking for Firehound you may want to rethink. I'd personally recommend what I'm going to do (mentioned in post below) and that's take my tow rig, keep the 400, and put a gear vendor unit behind it. Then you can use the overdrive while towing anything you wish to hook behind it.

Although I wouldn't (and don't) use the overdrive while towing with my other overdrive vehicle, I built my 4L60E (electronic version of same thing) with billet parts, 5 pinion planet, beast shell, alto clutches with coleen steels etc....that is rated to hold over 600 hp. The same can be done with any 700R4. If built in this manor and it still breaks, then it's probably time to step up to a 4L80E.

I shoved mine behind a stout 6.0 LS (which GM never did) with plans to throw a maggie on top. I'm not really worried about it.

[Firebirdjones]

Originally Posted by cal30_sniper

I'm dropping the TH700 out of my 90 parts car behind my 454 3/4 ton as soon as possible. I took a freshly rebuilt trans and added a B&M shift kit and traveller torque converter, along with a +3qt pan and Hayden transmission cooler. Many happy miles of towing my boat in overdrive and not a problem. The trick to the TH700 is keeping it cool, and keeping the tv cable adjusted. The overdrive will make a big difference I the rpms you see on the highway. May not net you much gas mileage, but it'll make your engine last a lot longer.
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I wouldn't recommend towing with a 700 in overdrive. With light loads it might be doable. I assume your boat isn't overly heavy? Obviously you've been getting away with it.

My tow rig is a 79 1 ton now with a 502 (originally 454) and the original 400 turbo. Since I'd like to tow in overdrive I've been looking at alot of options. What I'm doing since most of your transmissions don't recommend towing in overdrive is installing a gear vendor unit on the back of the original 400 turbo. Not only can I use the gear reduction to tow with, it's also a gear splitter assentially making the 400 a 6 speed, keeping the gears close and rpms in a sweat spot while pulling those heavy loads from a stop light. Similar to what the Allisons do in the turbo diesels.
Since I pull a 10,000 lbs. trailer I figure the gear vendor is the best option for me. I actually have a 4L80E here and decided against it.

[Edahall]

The 700R4 is definitely very weak in overdrive and even more so behind a diesel engine that puts out lots of torque at low rpm's. It was never designed for heavy pulling in that gear. If the engine is racing in direct drive (3rd gear), it's better to select a different axle ratio that drops the rpm's to a respectable level than to tow in overdrive. My 1982 diesel Suburban with 3.08's is happy at 1900 rpm's in direct drive towing at 60 mph. On the hills, I simply downshift to 2nd gear. Overdrive is only used for solo driving and I can go as fast as I want in that gear.

[Firebirdjones]

Originally Posted by Edahall

The 700R4 is definitely very weak in overdrive and even more so behind a diesel engine that puts out lots of torque at low rpm's. It was never designed for heavy pulling in that gear. If the engine is racing in direct drive (3rd gear), it's better to select a different axle ratio that drops the rpm's to a respectable level than to tow in overdrive. My 1982 diesel Suburban with 3.08's is happy at 1900 rpm's in direct drive towing at 60 mph. On the hills, I simply downshift to 2nd gear. Overdrive is only used for solo driving and I can go as fast as I want in that gear.

I agree completely. The only nice thing I can think of with the 700 is the 3.06 first gear and a 1.62 second, which still makes taking off nice even if you run a 3.08 rear gear. So you can still cruise in 3rd at a reasonable rpm for towing,,,,just as you mentioned.

[Edahall]

Originally Posted by Firebirdjones

I agree completely. The only nice thing I can think of with the 700 is the 3.06 first gear and a 1.62 second, which still makes taking off nice even if you run a 3.08 rear gear. So you can still cruise in 3rd at a reasonable rpm for towing,,,,just as you mentioned.

Exactly...... I've tried 3.73's and 3.43's but ultimately, 3.08's ended up working the best. It does fine towing my 26ft Airstream trailer in 3rd gear and unloaded, I can go as fast as I want in overdrive and enjoy good fuel economy. Another nice thing about 3.08's is I can tow going up a grade at 50-55 mph in 2nd gear.

[68Timber]

I read somewhere this weekend that even with the 10-vane pump used in the 87-up trannys, the 700R4's don't move adequate fluid at lower rpm. So it's not that the 6.2 is some kind of powerhouse but makes its peak torque at a lower rpm.

[Edahall]

Originally Posted by BRUISER

I read somewhere this weekend that even with the 10-vane pump used in the 87-up trannys, the 700R4's don't move adequate fluid at lower rpm. So it's not that the 6.2 is some kind of powerhouse but makes its peak torque at a lower rpm.

This is exactly correct. The 700R4 is at best is marginal behind a 6.2L diesel. Yes it can work and even be reliable but the transmission needs to be set up correctly and the driver behind the wheel needs to know what they're doing (such as keeping it out of overdrive when towing and watching the temperatures).

[cal30_sniper]

Originally Posted by Firebirdjones

I wouldn't recommend towing with a 700 in overdrive. With light loads it might be doable. I assume your boat isn't overly heavy? Obviously you've been getting away with it.

It's only an 18' jet boat, so about 2500 lbs with the trailer. Add to that another 3000 lbs of stuff that I had in the back, and that's what I was hauling down here. Never saw the temperatures get up, and nearly 5000 miles of towing hasn't been a problem with it yet. I don't know, maybe I'll rethink if I shell it behind this 454, but I'm going to run it for now because it's what I have.

I originally was going to go with a TH400 (already behind the 454) and a Gear Vendors, but there are a couple of large drawbacks with the gear vendors that made me choose otherwise in the end. One was cost, good luck finding a setup for less than $1300 bucks. Two is the lack of a lock-up converter. They don't make one for the TH400. The lock-up converter on the TH700 really helps with gas mileage during unloaded highway cruising.

-cal30sniper

[Firebirdjones]

Originally Posted by cal30_sniper

It's only an 18' jet boat, so about 2500 lbs with the trailer. Add to that another 3000 lbs of stuff that I had in the back, and that's what I was hauling down here. Never saw the temperatures get up, and nearly 5000 miles of towing hasn't been a problem with it yet. I don't know, maybe I'll rethink if I shell it behind this 454, but I'm going to run it for now because it's what I have.

I originally was going to go with a TH400 (already behind the 454) and a Gear Vendors, but there are a couple of large drawbacks with the gear vendors that made me choose otherwise in the end. One was cost, good luck finding a setup for less than $1300 bucks. Two is the lack of a lock-up converter. They don't make one for the TH400. The lock-up converter on the TH700 really helps with gas mileage during unloaded highway cruising.

-cal30sniper

Actually my gear vendor was nearly $2500, that includes making a new $400 driveshaft.
I prefer not to have a lockup converter in my tow rig. Nice for my daily drivers for sure, I agree. But for my tow rig the 400 turbo is a must. Towing with lockup converters (just from my experience) they kick in and out all the time when you hit any type of grade (lots of those in arizona ), and gets annoying after a while.
For my tow rig, the gear vendor gets the rpm down about 600 rpms and gives me a little extra mileage while still having bullet proof durability. 6 speed auto = quicker acceleration, while towing it's less strain on the engine, and I can use the gearvendor while towing, (unlike an overdrive trans) so I get a gas mileage benefit while towing as well as empty.

I agree the price is a drawback for most people, but I can't think of any other drawbacks. Actually I was going to spend just as much to shove a 4L80E in there (trans, controller and harness, driveshaft, was actually slightly more money) and I'm told they still shouldn't be used to tow in overdrive with the kinds of loads that I pull (10,000 +), so I saw the gear vendor as having the advantage there, as well as gear splitting while going up through the gears.

[Ramadi09]

NV4500 5 speed manual with a 205 transfer case and a Centerforce clutch kit.

[79Betty]

Originally Posted by Ramadi09

NV4500 5 speed manual with a 205 transfer case and a Centerforce clutch kit.

Gets my vote right there lol. I'd run this or if funds are tight a sm465.
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