Rebuild or swap

[olemans85]

If i swap my th350c to a 700r4 is there noticeable fuel mileage increase? I'm considering to rebuild the th350c or swap it for a 700r4. If there's only 2-3 mpg increase, I'm not sure if a swap is worth the effort.

 

[PrairieDrifter]

what gear ratio do you have in your axle?

 

[Joe383]

What gear do you have and is it a 2 or 4 wheel drive truck? More than likely it will be only a 2-3 MPG on a heavy truck. On say my 3,300 LB Camaro it would be more like a 5MPG on the freeway. It is worth the lower 1st gear though. I don't care what people say about the 1-2 RPM drop it is worth it. I have grown up drag racing cars and mud trucks and even stock I prefer the 700R4 on anything over a TH350 or TH400. Only real problem is it takes some work if your pushing some power and can be a little expensive to modify. My 86 Camaro is making about 350 HP and slips the clutches at around 5K due to too low of line pressure. I have slipped the clutches about 4 times now at full throttle hitting both 2nd and 3d gear. If your pushing over 300 HP I would not just swap in a stock 700. Be carful what shift kit you install too. My neighbor is having a problem in his car with a 700 were it is grabbing 2 and 3rd for a split sec during the 2-3 up shift at full throttle caused by the intermediate servo not releasing pressure fast enough. If you do a shift kit do it with the transmission out of the car and if your making any power do install a kit. I say install the kit with the transmission out cuz you need to replace some parts in the pump to do a proper shift kit in these transmissions. Also don't get a transmission older than 87. If you do get an older transmission than take the pump off and open up the drain back hole in the oil pump with a 5/16"drill bit if nothing else to the transmission. The older transmissions would some times push out the front seal due to hole not being large enough and pressure building up behind it.

 

[olemans85]

My C10 has the stock 2:9?, but I can change that accordingly.

I'm in the process of putting together a 350-400hp 350. And I'm gathering this info. to stay with the th350c or change to a 700r4. The truck is only driven occationally, it's not a DD. I'm leaning towards beefing up the th350c with a 3:0? posi. I was considering a beefed up 700r4 with 3:55 swap. With the additional cost & modifications, the 2-3 mpg increase doesn't seem worth it.

 

[olemans85]

It's a 2:73 ratio not 2:9?

 

[MadOgre]

It's a 2:73 ratio not 2:9?

700R4s need 3:73 gearing or lower with larger than stock tires. 3:42 or lower with stock sized tires.

If you had say 4:10s with a 3 spd tranny the 700r4 would let you see a considerable increase in fuel economy like probably 6-7 + probably more like 10+ on the highway.

With 2:73 gears a 700R4 swap will only result in a burnt up 700R4 !!!

The high gearing that you currently have allows you to get probably the same or close to, fuel economy that you would get with lower gearing and a 4 spd OD transmission.

You would gain the ability to keep the motor a little more in the power curve with lower gearing and more gears. Although that will not be all that dramatic when going from a 3 spd to only a 4 spd. That's why you get new cars with low power engines and a 6 or 9 spd transmission. The low gearing with more transmission gears allows you to utilize less power. OR makes more power feel like even MORE power.

 

[jobberone]

If i swap my th350c to a 700r4 is there noticeable fuel mileage increase? I'm considering to rebuild the th350c or swap it for a 700r4. If there's only 2-3 mpg increase, I'm not sure if a swap is worth the effort.

First is better than 350 and only slightly less than 400 but better 1-2. Not sure that matters to you although should pull better and keep your torque thru 2nd. The decreased RPM is over the long run going to help with gas and wear and tear. If you have to buy the 200R or 700R and don't get a lot for your 350 then you'll have to drive it for awhile to get your money back. But the truck will drive better.

If you are interested in performance then a lock up OD tranny is going to help.

 

[MadOgre]

A Th350 or Th400 will shift better and smoother from 1-2 than a 700R4 !!! The 700r4 has such a steep 1st gear that it really jumps from 1st to 2nd gear. Its not bad if you get the TV just right. But still is not as smooth as a 3spd transmission just for simple math reasons.

A 700R4 still needs to have the engine reach sufficient RPM to make each shift. So it will not save much in the RPM game. Plus the fact that most 700r4s are used mostly as a 3spd anyways. Because they fail to reach 70 mph were the 700r4 is comfortable in OD. Mine rarely sees OD speeds. Then add the fact that 1st gear is steeper and you actually have increased RPM vs the Th350 transmission.

The only real way to properly utilize a 700R4 with its OD is to run 4:10 gears!

With out sufficient low gearing the 700R4 will lug in 4th (OD) anyways which is worse then higher RPMs.

A 700R4 can not be run with 2:73 gearing !!! It just cant !!!!

A 4L60 is a decent transmission. But a 700R4 is not. Both the Th350 an Th350c are better then a 700R4 transmission.

And both a 4L60 and 700R4 do not even come close to comparing to a Th400 transmission. The 4L80e is a Th400 with an additional OD gear. The Th400 is in another class all together!

 

[olemans85]

I'll be beefing up the th350c & save some dollars. Its a truck for cruzin, not a racer.
I'd be curious if anyone has a mild performance 350 engine with a th350 behind it's opinion.

 

[olemans85]

Also thanks for the replies, they've been helpful.

 

[Joe383]

I have the 700R4 in my Camaro with 3.73 gears and 28" tall tires. Even if you don't get much in fuel mileage out of the swap you will get better performance if combined with some gears and loose nothing in fuel mileage. Also although you might not see much in highway mileage you will also get a little increase in town too. One of the reasons GM went with such a low 1st was for in town mileage. Honestly I wish I would have went with 4.10 gears in the car instead of the 3.73s cuz I could have totally lived with it. Your truck with 3.73s and a 700R4 would be a different animal off the line than with 3.08 gears and a TH350. Just make sure you start with an 87 or newer transmission.

 

[smoothandlow84]

I just sit back and laugh at the guys that bash the 700r4 trans and even try to say that the th350 is superior....thats just someone who has a th350 and wants a 700r4, but can't do the swap. You can rebuild the th350 all you want, but it still wont be a 700r4. The 700r4 not only gains better mileage, is more reliable and will out perform the th350 when properly built. If you plan on installing a big block, then no, the 700r4 isnt your right choice. The overdrive is a plus right out of the box, even if you don't always use it, you have it. I personally like the shift points of the 700r4 versus the th350, especially from 1-2. If you plan on drag racing, or road coursing your square, then obviously neither of these transmissions will be sufficient. My build is for cruising and the occaisional daily drive. I have had 3 th350 transmissions in previous squares. The last 2 squares (both 2wd) had mid built 350's and the transmissions were worked, rebuilt and rebuilt again. Neither transmission ever shifted to my liking or held the power. Both th350's were built by the same transmission guy that has always done my work. With 35 years transmission experience, a former drag racer, fellow square owner I trust his judgement and extensive knowledge. After removing the th350 and swapping in a 700r4, I wouldn't even think of going back to one.

 

[350runner]

Same here.

Sent from the dust in front of you!