Trying to do the logical thing.
 

I am debating a 700R4 swap for my 71.

My truck has a crate 350 with a nicely built TH350 transmission. I recently had to rebuild the rear end, so......I changed from a 3.08 open to a 3.73 with full case detroit locker :smoke:

So, basically, the truck is great right now and doesn't need a transmission. But, at 70mph I am at 3100 -3200 rpm. With a 700R4 it would be 2200rpm. I drive it about 10 gal of gas a week.

So, logic says:
-A 350 chevy can run 3200 rpm all day no problem.
-At $2/gal the cost save on gas would be $6/wk = $300/wk.

Is there any reason I should move forward with a 700R4 swap other than the sound it makes on the freeway?

Re: Trying to do the logical thing.
 

So is the object here to keep spending money? With the 3.08s you were right where you should be. Swapping to 3.73 created the need for an overdrive. Trans + the rebuild + a shortened driveshaft could be upwards of close to 2k. Then you've got to get the geometry 'just right' to avoid burning up the new trans and hopefully it will last long enough to justify the difference in mileage. Sorry to be so blunt but I have a lot of disdain for the 700. Even the 'built to last' 700s really do not have a lot of longevity

Re: Trying to do the logical thing.
 

Putting in a constant pressure valve body in the 700 makes the tv cable adjustment not as critical. I'm currently gathering everything for a 700 swap in mine w/3.08 gears currently and have done a bit of research in to the swap. Kwmech is right as the 700 rebuild won't be cheap.

Re: Trying to do the logical thing.
 

Ran my 700 for about 20 yrs. before it grenaded requiring a rebuild. Everything wears out. Now time for a new engine, the old having been put in at the same time as the trans.

Re: Trying to do the logical thing.
 

Sorry to butt in but I been mulling over a 700r too, which I know where I want to buy it from and I have an idea where to get it put in (not my thing to install and it must be done right).

Anyways, I swear I read if you have a 3.08 rear end (stock I believe with a 350/TH350?) you need to change that out, say for at least 3.73 rear end? Which has kind of put a dampener on any of plans to swap out to a 700r.

Re: Trying to do the logical thing.
 

I don’t see an upside with the amount of miles you do other than saying there’s a 700 in it . I contemplated putting a 700 in when my tb350 went. I put about 12k on the 72 a year . I had the 350 rebuilt , my truck has 4:11s

Re: Trying to do the logical thing.
 

The benefits to an OD transmission (in general) are lower rpms at the motor which means less wear on the motor, less noise, and better gas mileage. If you have the time and money...most people like the OD upgrade. I have converted a car to a 5 speed manual and really like it, but it was not necessary. The previous auto OD conversion that was in it was a problem. I could not keep it from burning up the clutches, but I cannot speak about the 700 (My car was a F@rd BTW).

Rich - I have only researched OD transmission upgrades to a small degree and most say that the 3.73 gears are very good and are as low (numerically) as you should go with the occasional person saying they like ones at the 3.5 range. 3.73 and 4.10/4.11 are the most popular. Too low, like 3.08's, and the motor will lug down once it gets into 4th. 4th gear likes a higher motor RPM, certainly above 1500 and 3.08's just don't cut it. If you do the math or run the online calculators, you find that highway speeds with numerically low rear gears will drive the motor just above idle in 4th...just not enough horsepower at that rpm to keep the vehicle moving along with out downshifting excessively.

Back to CJ - My 72 C10 has had every opportunity to get the OD conversion, but I am staying simple and affordable and sticking with the th400 and 3.08's (for now). If the 3.08's are too wuss for me...I'll go the the 3.73's and drive bit slower on the highway.

It's all about desire, time, and money!

Re: Trying to do the logical thing.
 

If it's a daily driver...OR....you put 8k+ miles a year on it.....DO IT. You're already set up with the gears. From what I've seen, the people that hate the 700r4, have had bad experiences. A lot of times it's an expensive rebuild by an incompetent mechanic that doesn't last long. Then you get a bad taste in your mouth for them. I swapped one into my daily driver and drove it for 16 years. I'll be putting them in my next 2 projects as well. If you drive less than 5k a year then why bother? But otherwise, it will pay for itself in fuel savings. My old daily was a mild built smallblock with 9.5:1, 268 Crane cam, headers, dual exhaust, aluminum intake, holley street avenger carb, 3.42 gears, and the 700r4. It weighed across the scales at 3950#. It knocked down about 15mpg everyday mixed driving and 18/19mpg on a road trip.

Re: Trying to do the logical thing.
 

I just realized I read cj847 bits about the rear end gears backwards... silly me. Though thank you for your response sick472!

Re: Trying to do the logical thing.
 

I installed a 700R4 in a '72 SWB I had a bunch of years ago. It had 3.73 gears. I also installed cruise control from a '70s van in it. That truck was so nice to drive, especially on the freeway. The low first gear was fun around town. My current truck has 3.73 gears and a TH400. It is quite loud and unpleasant at speeds above 55mph. I'm planning on installing a 700R4 in it. For me, it's all about the driving experience.

Re: Trying to do the logical thing.
 

.

Don't forget that huge jump from 1st to 2nd compared to the 350 and the 400...

gear ratios:
th700: 3.06/1.62/1.00/0.68
th400: 2.48/1.48/1.00
th350: 2.52/1.52/1.00

So the jump from 1-2 for both the earlier trannys is 1.0 whereas on the 700 it's a massive 1.44, which after a near redline in 1st brings you down to a low rpm 2nd gear start.

A replacement planet is available that changes these gear ratios in the 700 to a more respectable 2.84/1.55/1.00/0.68 but it is expensive:

https://www.sonnax.com/parts/3734-2-...ut-carrier-kit

...just some additional thoughts.

So...when my 700 blows, I'm either going 4L80 with all associated expenses or rebuilding the 700 with this sonnax part ( or something similar ).

-klb

Re: Trying to do the logical thing.
 

I have a '92 700R4 with 3.08 gears and love it. It was rebuilt with a new older valve body version and a hydraulic lock-up kit, so no solenoid needed. On the interstate I can run 70+MPH at under 2000 RPM. If I'm tooling around locally (<50 MPH or so), I just drop it into "3" to keep it from going in and out of 4th. I have a spare rear end housing that I would like to build as a posi, and may lower the 3.08 gearing to 3.42.

Later 700R4s are recommended as they have better 30-spline input shafts and other improvements over earlier versions.

Re: Trying to do the logical thing.
 

Mistake #1: trying to make a truck decision that is logical. If we apply logic, we'd all be driving electric vehicles :) (Not happenin)
Your change to 3.73s when you had to rebuild the rear end shows you clearly wanted more acceleration in 1st.
If you can live with the highway rpms as they are (and keep in mind, these rpms were the norm back in these trucks' era), then you're done! Drive and enjoy your 3.73s.
If you can't live with the highway rpms as they are or you just want an "ideal" combo that works at both ends (with even more accel in 1st than your current setup), install a (beefed up) 700R4 (or even consider a 2004R) if your budget allows.
Another option is to keep an eye on CL ads, etc for a used GV o/udrive unit. They can be had used for 1500-2500, but brand new they run about as much as a built 700R4 (in the $3K+ range). But it makes your TH350 a 6 speed or 4-speed o/d, depending on how you want to use it. Requires d/s chop, but that's not very expensive.
If you must be logical - by your math you could pay for your 700R4 upgrade in 8-10 years in gas savings, so it "could" be approached with that mindset (i.e. if you're trying to convince the spousal unit)
All depends on your budget, tastes, and goals. No wrong answer and it's certainly ok to keep exactly what you have right now. If the TH350 ever gives up the ghost, then you know what to do :)

Re: Trying to do the logical thing.
 

if you are saying that you are planning to keep the 3.08s and install a 700R4 - depending on your tire size, you may be unhappy with the results. The O/D + 3.08 combo can set you up to lug the engine rpm at lower highway speeds (50-60ish). Just something to think about. If you share your tire diameter, I can tell you precisely what to expect - but below is your scenario for a 30" tire. Note 1300-ish rpm at 55 mph. Not really where you want to be when you have to be at 55 for whatever reason.

Re: Trying to do the logical thing.
 

I didn't say I was going to keep the 3.08 but will have it for a while. I have a friend that has the identical setup in his truck and loves it, only difference is his tires are 26 mine are 28 tall. I figure I will be ok until I can change the gears to what I want which is 3.42.

Re: Trying to do the logical thing.
 

There is no need to utilize OD @ 55mph w/a 30" tall tire & 3.08 gear. You can leave it in drive/3rd & cruise along @ a very SBC rpm friendly 1897. Even getting up to 65mph in drive/3rd you'll still be @ a SBC friendly 2242rpm.

The OD/4th gear will be nice for longer highway driving but the engine combo needs to be tailored for the 1800rpm which most carb fueled SBC's aren't & why computers/EFI made it work better.

Old-school SBC's seem to be out of the 'lugging' range @ 2k rpm or over.

Re: Trying to do the logical thing.
 

As someone already posted, with a 3.08 diff and the lock-up final drive on a 700r4 being .696, your engine rpms are probably not going to be high enough to prevent the transmission from constantly searching for 4th/lockup. Which in my experience very quickly = burnt out lock up solenoid.

I have a 1989 Suburban 4 x 4 with 3.42 diff. I run 31" tires, which run 48 revs/mile less than the stock tire size. At 55 mph my engine is turning over at 1427 rpm. In my case, I have a freshly rebuilt stroked 383 with a mid-range cam, so it makes enough torque that the engine is happy with this. Not sure it would like it much if I had a 3.08 rear end.

And as has been stated previously, the 700r4 is really a dog of a transmission. GM's answer to late 1970's oil embargo and high gas prices. I have had 4 of them in my truck, only one of which got ruined by someone (not me) doing something stupid. I've had bargain-basement franchise built ones and custom shop built ones with the heavier duty parts, none of them got much past 80,000 without issues. And I take care of my truck. Just not a good transmission.

If the current one ever craps out, I will bite the bullet and find a later electronically controlled unit and buy the wiring/ecu adapter kit.

Re: Trying to do the logical thing.
 

I look at it this way, your mileage may vary. BUT
When g.m. had Both a 350/th350 carb truck and a 350v8 /7004r carb truck.
The city mpg was the same and highway was 1 mpg difference.
So pick your poison .
The O/P already stuffed 3.73's in the rear, so at this point a 7004r might make sense, I think his math is suspect. His daily drive is not going to be all in 4th gear at a steady rate. So I think what he THINKS he will save in fuel cost, is um, well. you know.

Re: Trying to do the logical thing.
 

Oops. I read it wrong. Yes, now has 3.73. I had the wrong way around. So yes, with a 3.73 diff it makes more sense. But I sure would not go to that kind of effort unless I was putting a lot of miles on it on a regular basis.

And if I was starting from scratch, or with the budget I have now, I certainly would not pick a 700r4. Too many better, more reliable choices.

Re: Trying to do the logical thing.
 

Do tell? I've gone thru a th350 and 2 th400's in mine that's why I'm looking to go 700r. If your suggesting a 4l60 they're scrap to begin with and I can't afford to rework a 4l80.

Re: Trying to do the logical thing.
 

You do understand that a 4l60e IS the 7004r , only one is mechanically controlled and the other electronic sensors.
same guts, and internal parts.

Re: Trying to do the logical thing.
 

I am certainly no expert on transmissions, so I should qualify that I'm basing this off personal experience. And my experience with 700r4 transmissions has not been good. I did have one built by a custom shop with upgraded internals that worked great right up until it didn't, at about 80,000 miles. That same shop built another one that lasted 1000 miles. They are extremely finicky, want the cable adjusted very precisely or will grenade on you. I have also found not many places that seem to know what to do with one. If you ask 3 transmission shops the same set of "test" questions and get 3 different answers, it does not inspire confidence. I think partly this is due to the age of the design, and a dwindling pool of transmission shops with direct experience with them.

My most recent venture, to get the truck running again, I found a retired guy who built me one, seems to be OK. Even at that, 500 miles later I had to replace the new lock up solenoid because the part was just bad from day one. My cam/carb/intake set up seems to not make enough vacuum for the vacuum switch to work, so at the moment I have the lockup switch wired for manual control while I wait on an adjustable vacuum switch to arrive.

Again, no expert, but I have had a couple of people say the late 4l60e is the one to go after, i.e one from about 1999 to 2000. The major advantage there being dropping the finicky cable adjustment, letting the ecu shift and not worry about amount of vacuum, etc.

I misread your early post about diff gearing. If you are driving highway speeds a 3.73 gearing would probably benefit from overdrive. Just can't say I recommend a 700r4. For the hassle if you are only using 10 gallons a week it will take a long time to benefit from extra fuel economy. That being said, I myself find it a sort of personal affront to not get the best mpg I can, no matter what car I am driving, so I get it.

Best of luck.

Re: Trying to do the logical thing.
 

I have to wonder if the reliability issues have to do with improperly adjusted TV cables, which is a very common problem. Or were they known to be unreliable even as installed by the factory?

That said, I'm considering a 2004R for my 69 C10. Low gear isn't crazy low like the 700R4, and the driveshaft doesn't need to be shortened. I'd go with a non-lockup converter.

https://www.cpttransmission.com/catalog.htm

Re: Trying to do the logical thing.
 

One of mine was ruined by someone I let work on my truck thinking it was basically a Th350 with overdrive, and drove it without the cable connected. Toasted it. One I had built by a professional shop where the "old guy" who owned the shop, and had seen a lot of them when they were common on the road, did ok. The next one, built by the same shop, but by then where younger guys did all the work, they did not tighten the cable clamp at the carb down tight, and it burned out literally on my drive home. I made them build me another one at their cost, they did not drill the hole in the pump correctly, got a few hundred miles on it, started cavitating to beat anything, would not stay in lockup and was always shifting in and out, leaking fluid, etc.

So yes, the improperly adjusted cables are an issue. Supposedly, the early 700r4's when new were very problematic, and by about 1986 this had been improved with better parts, which supposedly are all one can get nowadays for a rebuild.

But I've had two others that ran for around 45,000 miles then would quit staying in lockup, or burnt out 4th gear, or just got some kind of gremlin in them without any sign anything was wrong until they just quit working. Conversely, my wife's van, which had the same transmission (4l60e) but ecu controlled was still going when the engine finally gave out.

Had an old Blazer Chalet I wish I had kept with a TH350 that seemed bomb-proof. But other people have had bad luck with them too.

So yeah, maybe it's possible to find the right shop to do a good job, but they are so picky. If this one gives out I will never have another one.

Or maybe reincarnation is real, and in my former life I crushed cars for scrap and they are paying me back for it in this life.

Re: Trying to do the logical thing.
 

Definitely a better 1-2 gear ratio split but it still requires the same focus ensuring the TV cable is installed & adjusted correctly.