[1stGenFB]

Hey all -
I've begun thinking about fixing up a 69 Firebird convertible 350ci 2bbl for my wife.

It has a TH350 trans in it now that leaks like crazy. I know from experience how weak the TH350 is and don't want to go down the path of rebuilding it.

The car will be stock horsepower and more of a cruising machine. The 2.56 gears work great to save gas however boring.

What are some sturdy transmission choices I can bolt up without modification?

Thanks
FB

[JLMounce]

A 700r4 would be a decent choice. I'd do a few things to it like an upgraded input shaft to get rid of the snap ring and a corvette servo. On a stock 350 in a first gen bird, that should do it. You could go further with a beast shell, kevlar band and upgraded frictions and steels if you wanted a bit more room for growth.

the 700's steep 3.06 1st gear will really help out that car with the 2.56 in it off the line. I'd also keep a short 26" total wheel/tire height.

The only issue I really see with the 700, is that with the 2.56 gears, your overdrive gear becomes incredibly tall. Too tall IMHO. I think the car would ultimately benefit from having a 3.08 put in it at the same time, or as funds are available. Do a posi at the same time. Going to a 3.08 with the 700 gets you a starting line ratio of 9.42 which is going to be a lot of fun. Then, the over drive with the 3.08 ends up at a 2.156. It would be more fun around town stop light to stop light while also being great on the highway.

[1stGenFB]

I have heard good things about the 700r4 but have the same concern about the overdrive gear making it a slug on the highway with 2.56 rear end gear ratio.
Not sure if I want to change out rear end gears.

Would TH400 be a good option? I know they are pretty sturdy

[694.1]

That TH350 will never last behind a 350 2bbl. I'd go with an Allison.

[b-man]

The TH400 is very strong. However there’s nothing wrong with a properly built TH350 either.

[JLMounce]

The TH400 has taller gears than your TH350, while it's certainly a sturdy transmission, it's actually going to slow the car down over what's in there now.

The TH350 isn't a slouch either, it's far from a "fragile" transmission. They can be built to handle quite a lot of power and their gearing is better for performance cars.

You could have the th350 rebuild and do a low gear set in it. That would give you better around town performance and make no change to how the car currently performs on the highway.

TCI makes a 2.75 low gear set for those. That would gain you a bit more than half a point of added starting line ratio.

https://www.speedwaymotors.com/TCI-3...st,333738.html

[Grand73Am]

th350 isn't weak, and is more than fine for your Pontiac 350 2 bbl. with 2.56's in a Firebird. I have the same combo in my 75 Lemans Sport(although I converted it to a Q-jet), which is a bigger, heavier car than your 69 Firebird. The transmission is original with 170K miles on it and still working fine.

[surfsama]

Originally Posted by 1stGenFB

It has a TH350 trans in it now that leaks like crazy. I know from experience how weak the TH350 is

I had a TH350 in a 1/2 ton van that was anything but weak and I believe ta_man here on this site went 10 sec with one for awhile.

The TH350 2.52 vs a TH400 2.48 first gear also compliments the 2:56 a little better. No doubt the TH400 is stouter but it's also heavier and longer so you'll have to find a different driveshaft or shorten the one you have.

If you're not interested in overdrives and the engine is stock/cruiser car a TH350 from a reputable builder is the easiest path. Otherwise you can't really go wrong with a TH400.

[Grand73Am]

Also, with 2.56's, you don't need an overdrive. If you want stronger, a th400 would be the choice.

[1stGenFB]

I guess it's because I have really bad memories from back in college with the TH350. I went through 2 or 3 of them with my heavy foot. 2nd gen Firebird with 2bbl 350 in it.

One time I had traveled from NY to Virginia for a wedding and the transmission let loose. had to get a ride home then go back and pay for a rebuilt transmission.
That one probably lasted 1.5 years. The others were pretty bad too.

I like the option of 700r4 with rear end gear change the best.

[JLMounce]

FWIW if you do decide to do the 700r4 with a gear swap, my personal opinion is that doing a 6 pinion planetary upgrade on the 700, then going to a 3.36 gear would be the money option.

That planetary upgrade changes the gear set in the transmission from 3.06 1st, 1.63 2nd to 2.84 first and 1.55 second. It makes the gear spread between 1-2-3 closer and eliminates the huge gap between 1st and 2nd. Going to the 3.36 rear gear gets you to a 9.54 starting line ratio and then your OD ratio ends up at 2.35, which is going to be great on the highway without lugging it.

That would be a fun little car, even with the stock 2bbl 350.

[ta man]

Originally Posted by surfsama

I had a TH350 in a 1/2 ton van that was anything but weak and I believe ta_man here on this site went 10 sec with one for awhile.

The TH350 2.52 vs a TH400 2.48 first gear also compliments the 2:56 a little better. No doubt the TH400 is stouter but it's also heavier and longer so you'll have to find a different driveshaft or shorten the one you have.

If you're not interested in overdrives and the engine is stock/cruiser car a TH350 from a reputable builder is the easiest path. Otherwise you can't really go wrong with a TH400.

Ive only had one th 350 fail in the last 17 years..I had a second one that needed work , that the case cracked because of a driveshaft failure. A th 350 is plenty strong for the OP.

[stevep]

Originally Posted by 1stGenFB

I guess it's because I have really bad memories from back in college with the TH350. I went through 2 or 3 of them with my heavy foot. 2nd gen Firebird with 2bbl 350 in it.

One time I had traveled from NY to Virginia for a wedding and the transmission let loose. had to get a ride home then go back and pay for a rebuilt transmission.
That one probably lasted 1.5 years. The others were pretty bad too.

I like the option of 700r4 with rear end gear change the best.

The 350 with the 2.56 is your most economical option.
What RPM are you turning @ 70 MPH now?
A 700 with a 3.36 may give you a similar cruising RPM after you spend thousands.

I rebuild transmissions and that 350, if done properly is fine.

[ZeGermanHam]

The TH350 was never known as being a weak transmission. It's more than up to the task of handling a grocery getter 350.

Sent from my Pixel XL using Tapatalk

[leeklm]

"You could have the th350 rebuild and do a low gear set in it. That would give you better around town performance and make no change to how the car currently performs on the highway."

Although cost is a little higher, this is a great option for what you are doing with your 2.56 gears

Sent from my SM-G965U using Tapatalk

[Half-Inch Stud]

2:56:1 is a fun rear gear with a Posi, in a GTO.

[Sirrotica]

The reason there is such a disparity with peoples experiences with TH 350 is because there were a lot of variations the Hydramatic division built. They came in everything from a 231 V6 in an H body with 2 clutches in the stack, to 3/4 ton trucks that had 5 clutches in the stack. Lots of variation in how they were built for each application.

If the transmission was built like it was going in a 3/4 ton truck, (which a competent builder can do easily) it's going to be fine behind a 350 bbl. Pontiac.

I prefer the TH 400 in my builds, but I tend to overbuild my cars for the utmost reliability. A friend of mine put a mild 428, in front of a mostly stock 350, 73 Esprit Firebird, (had a shift kit in it) and he never blew it up. If he couldn't blow it up, the average person would never have trouble with it.

After 1975 all the Trans Ams came with TH 350s in front of the 400 4 Bbl engines without a lot of warranty issues with them, but they were built for that application from Hydramatic

['ol Pinion head]

In the mid 80's thrashed the original T350 behind my modded '80 Turbo 301, had under 35K miles on it. From the car's measured performance would bet mid 400's ft lbs of NET torque. After swapping in a '70 PQ Turbo 400, had the original T350 rebuilt with 36 element sprag assembly & hardened race. The modded turbo 301 car actually slowed down with the PQ Turbo 400 in it & had to be manually shifted as 51-5200 rpm was too high to automatically shift under WOT.

[mgarblik]

With the 2.56:1 rear gear you have stated you would rather not change, your most economical and plenty strong enough option would be a TH350. I like the idea of a low first gear set to help get the car moving from a standstill. You don't need overdrive with that rear gear. A properly built TH 350 will be plenty strong enough for a stock 350 2-bbl. No other modifications needed either. A TH 400 is much stronger but just not needed for what you have described here. Good luck. Another advantage is a TH-350 is super easy to rebuild yourself, much easier than an overdrive like a 700. Parts are very reasonable as well.

[A.W.Dille]

I have a Turbo 350 in my Grand Prix (see sig) with 2.54 gears and have no complaints. Nothing wrong with the Turbo 350. With 275-60-15's on the rear lighting them up is no problem. Get it rebuilt and don't worry about it.