Looking for recomendations for transmission change

[fast_n_low86]

Hey all.

I am currently running my truck with a th350c, modified 5.7L 350, and 3.42 ratio gm 8.5" rear end. Kept motor carbureted and don't want to start messing with all this new electronic crap when replacing my Trans in my street machine. I have been thinking about going with a th700r4 but am open to other ideas. I of course will either rebuild it (get someone to do that one for me) or find an already stacked one.

 

[SirRobyn0]

@fast_n_low86 I have 700R4 in my truck and really like it, My truck is a C20 with a stock carbureted 305 and 3.42 rear end. I would pick a well built 700R4 to the TH350 or TH350C any day for the lower first gear and the added fuel economy of the overdrive. I have hit 16 MPG running on the highway without a load. Some people will say the wide spacing between 1st and 2nd gear is a problem, but I'd disagree. Yes, it's there but even when I'm towing a heavy load it's not really a problem and the lower first gear for getting a load moving is very helpful. Again speaking for me. I'd really like to be running 4.10's or even 4.56's in the rear end. The reason being is that makes the first gear even lower, and makes the overdrive even more useful. I find with my truck on the highway in overdrive at 60 is the worst but even at 70 the RPM is so lower it's out of the power band so ever small hill it unlocks the converter and / or down shifts.
The lock up is something to think about as well. Most of the 700R4's I've been in contact with that don't have a computer running the lock up (like yours and mine), run the vacuum controls for third gear and sometimes 4th, but also some guys will wire the 4th gear so anytime the truck is in 4th it's locked up. Whatever you do you HAVE to have a working lockup with a 700R4 (unless it's been modified not to need one), otherwise it'll run hot and live a shorter life. If the increase in fuel economy is one of the attractive things about the 700R4 then you wouldn't wanna do away with the lock up anyway.

Guys are going to come along and say the 700R4 is junk, but it certainly hasn't been my experience. I think most of the guys that have issues with the 700R4 either had a bad rebuild or out them behind an engine that produced a lot of power. Which is definitely something you'll want to let your builder know (what your 350 makes) when you have it built.

 

[Turbo4whl]

Welcome to the forum! I will also agree with Rob, the 700R4 is a fine choice over the TH350C. Since you have the 350C, the the truck has what you need for the locking converter. Slight change to the wiring and the trans bolts right in. Need to move the trans cross member back. The 700R4 has a different cross member but the 350C cross member will work. Need to cut a small notch in the cross member to clear the 700R4 trans pan. You also will need to drill 2 holes in the frame as the two cross members have different hole spacing.

Most important is changing the cable to the carb. The 350C has a kick down cable, 700R4 has a TV cable and it works differently to adjust the pump line pressure. Not hard, just need to get it done correctly.

 

[Ricko1966]

I'll 3rd the like for 700r4 I think the biggest killer is incorrectly adjust tv cable, I like the deep first gear it helps taking off with a heavy trailer. I don't believe they last as long or are as durable as th350 but the overdrive and the deep 1st gear advantage out weigh the short comings. Just my opinion. FWIW I abuse things and I know it I'm relitavly certain all of my transmission failures have been my own fault,manuals automatics doesn't matter I can break it. One more little tid bit that I like about 700r4 is you can run a looser converter without screwing your drivability or milage. Not stupid drag car loose but looser than stock.

 

[Matt69olds]

If the trans is built right, I wouldn’t hesitate to run a 700 behind a big block. There is zero reason a 700 wouldn’t live behind a stout small block.

The BIGGEST killer of a 700 is the TV cable. It HAS to be right, close isny good enough. While it’s true the TV cable travel needs to be maxed out at WOT, the cable pull at part throttle is just as important. There are lots of linkage kits to get it dialed in.

 

[SirRobyn0]

If the trans is built right, I wouldn’t hesitate to run a 700 behind a big block. There is zero reason a 700 wouldn’t live behind a stout small block.

The BIGGEST killer of a 700 is the TV cable. It HAS to be right, close isny good enough. While it’s true the TV cable travel needs to be maxed out at WOT, the cable pull at part throttle is just as important. There are lots of linkage kits to get it dialed in.

I'm just curious do you have a linkage kit that you prefer?

 

[Big Ray]

If your 350 has held up good and not exploded from horsepower, a WELL BUILT 700r4 is a good choice. It's a bit longer than a 350, so driveshaft(s) mods may be necessary.
A 700r4 is basically a 350 with overdrive.

 

[Turbo4whl]

If your 350 has held up good and not exploded from horsepower, a WELL BUILT 700r4 is a good choice. It's a bit longer than a 350, so driveshaft(s) mods may be necessary.
A 700r4 is basically a 350 with overdrive.

The long tail TH350C is only 1/2" different in length from the 700R4. Driveshaft will work. Square body 2 wheel drive pickups from the the 1980's all would have had the long tail TH350C if they were built with the TH350C.

I'm just curious do you have a linkage kit that you prefer?

The carb linkage on the factory Quadrajet will not work right on some aftermarket carbs. You have an Edelbrock, correct? Edelbrock should have available the correct linkage parts.

On a Holley the vacuum secondary carb would be the one that can be set well with the Holley bracket. I believe if you were to use a double pumper Holley (mechanical secondaries), then they cable pull rate would not be a very good match.

 

[Rickf]

Have you considered a gear vendors or another brand OD unit? They can be pricey but, if there is nothing wrong with the th350 keep it. I use a GV OD behind a th400 in a 1-ton flatbed, I just needed to have the driveshaft shortened and minimal electrical wiring.

 

[SirRobyn0]

Have you considered a gear vendors or another brand OD unit? They can be pricey but, if there is nothing wrong with the th350 keep it. I use a GV OD behind a th400 in a 1-ton flatbed, I just needed to have the driveshaft shortened and minimal electrical wiring.

Just my 2 cents on that. I'd run a GV unit behind a TH400, but I'd swap a TH350 for a 700R4, the reason being is the high cost of the GV unit, and expense of driveline modifications for the installation of the GV unit. With the TH350 and GV unit your not gaining much advantage over the 700R4, unless for some reason a guy doesn't want the low first gear, or wants the ability to split gears like when towing or hauling.

 

[mrdap]

My two cents...
I've had nothing but trouble with 700R4s.
The throttle valve cable is problematic, although it can be eliminated with some additional work inside.
I would stick with a TH350, TH400 or 4L80E.
Of course the 4L80E is an expensive option.
But it sure is sweet!