Swapping TH400 to 700R4

[HotRodPC]

I don't know a whole lot about transmissions, but the transmission guy I spoke to about a 700r4 said that he takes 700r4's out of trucks and puts TH350s in them because they're so much tougher.


One thing that surprised me reading this thread is that if I'm cruising at 70 mph in my TH350 equipped C10, I could be overheating my tranny. They recently increased the speed limit between my place and my parents' from 65 to 70 on a 10+ mile stretch that pulls a pretty big hill. I only have idiot lights in my truck, so I have no idea what temp I'm running at.... the truck will do 85 mph with no trouble, but I don't sustain that speed for more than a few seconds at a time....


So my question is: Should I not sustain or exceed 70 mph pulling that hill, or anytime for that matter?

I should also note that my truck is my toy, which gets ~1,000 miles per year, but the transmission has 194,000 miles on it.

Yes, the Th350 will tolerate more abuse for sure than the 700r4. But someone is bound to pop in and say a 700r4 is stronger, or a PROPER built 700r4 is better. Hands down, the Th350 has better longevity no matter what upgrades you put in it. Being a half ton, you might have 3.08, 2.76 or even 2.56 rear gears and NO OD really needed. I've got by with 3.73's and 31in tire running 75 with no problems. 4.10's ehh, might want to keep it more like 65ish to 70 but motor will be screaming a little bit, but not gonna hurt it.

 

[Jrgunn5150]

I don't know a whole lot about transmissions, but the transmission guy I spoke to about a 700r4 said that he takes 700r4's out of trucks and puts TH350s in them because they're so much tougher.


One thing that surprised me reading this thread is that if I'm cruising at 70 mph in my TH350 equipped C10, I could be overheating my tranny. They recently increased the speed limit between my place and my parents' from 65 to 70 on a 10+ mile stretch that pulls a pretty big hill. I only have idiot lights in my truck, so I have no idea what temp I'm running at.... the truck will do 85 mph with no trouble, but I don't sustain that speed for more than a few seconds at a time....


So my question is: Should I not sustain or exceed 70 mph pulling that hill, or anytime for that matter?

I should also note that my truck is my toy, which gets ~1,000 miles per year, but the transmission has 194,000 miles on it.

That depends on gear ratio, tire height, condition of engine, etc etc.

My C10 has a TH350, 3.08 gears, and I was able to maintain 65-75 mph for the duration of my trip from Little Rock to Michigan, it ate about a quart of oil.

My F100, with a very tired 360, would have blown every drop of oil out of the valve covers in about ten miles lol. While that one was together, I kept my speed down to 55.

 

[HotRodPC]

At 1000 miles a year, even if it had 4.10's I'd keep the Th350. You just don't drive it enough to justify the conversion expense, and being a toy I'd hate to give up durability and dependability.

 

[mtnbiker727]

Yes, the Th350 will tolerate more abuse for sure than the 700r4. But someone is bound to pop in and say a 700r4 is stronger, or a PROPER built 700r4 is better. Hands down, the Th350 has better longevity no matter what upgrades you put in it. Being a half ton, you might have 3.08, 2.76 or even 2.56 rear gears and NO OD really needed. I've got by with 3.73's and 31in tire running 75 with no problems. 4.10's ehh, might want to keep it more like 65ish to 70 but motor will be screaming a little bit, but not gonna hurt it.

Where can I find the gear ratio?

 

[HotRodPC]

Where can I find the gear ratio?

If you have your RPO label, either in the glove box, on 76 I think it was on the inner fender well under the hood. If the axle has been swapped it could be wrong.

You can also jack up the rear wheel(s) and calculate it which is what I do,
http://www.gmsquarebody.com/forum/showthread.php?t=3860

Or you can find the code on the axle tube and look it up. Again, if someone has swapped the gears in the past, it could be wrong.
http://www.gmsquarebody.com/forum/showthread.php?t=4185

 

[mtnbiker727]

If you have your RPO label, either in the glove box, on 76 I think it was on the inner fender well under the hood. If the axle has been swapped it could be wrong.

You can also jack up the rear wheel(s) and calculate it which is what I do,
http://www.gmsquarebody.com/forum/showthread.php?t=3860

Or you can find the code on the axle tube and look it up. Again, if someone has swapped the gears in the past, it could be wrong.
http://www.gmsquarebody.com/forum/showthread.php?t=4185

It's a KAA, so according to the second link it's from 1974-1979 and has 3.07 locking. So what does that mean for me?

 

[yevgenievich]

The 85 that i used to have had 3.08 gears and th350c(lockup did not work). It would hold 70mph with no problems and often drove 85mph for hours. Was spinning around 3200 rpm. The downside was that it used oil runnng constantly at high rpm.