[1stGenFB]

Thanks for all the input everyone. What I am hearing is the TH350 may be a good option after all. I could be motivated to keep it since the ole 69 bird is numbers matching which would make it better to keep it that way.

[Cliff R]

Nothing "weak" anyplace with a correctly prepared TH350, just another Internet myth folks regurgitate frequently.

I set one up for a Pontiac racing application about 15 years ago. At that time it was behind a stout 455 build with 330cfm E-heads running low to mid-10's. That engine was ill fated (lifter bores caved in) but lasted quite a few years. It was replaced by a 505 making 735hp and the car currently runs high 9's over 135mph.....the TH350 has been fine and only had one inspection in all these years and got a couple of direct steels/frictions as they had a few "hot spots" on them, otherwise no issues anyplace.

If the TH350 has a "weak" spot in comparison to a TH350 it's the case. We've seen a few crack or break clear off where the bellhousing area meets the case, which is not nearly as common with a TH400 trying to do the same thing.

Both transmissions "sore" spot is the intermediate sprag, and the TH350 should get a bolt-in center support and larger low roller clutch set-up from a 4L60 if it is going to be used in high HP/torque applications with good traction.....IMHO......Cliff

[amcmike]

Originally Posted by 1stGenFB

Thanks for all the input everyone. What I am hearing is the TH350 may be a good option after all. I could be motivated to keep it since the ole 69 bird is numbers matching which would make it better to keep it that way.

Even a standard rebuild should be more than sufficient for you application. Strange that you've had a bad experience previously as the TH350s are usually pretty strong straight from the factory.



You could also add an inline external cooler for extra insurance.


If it's just a pan leak and otherwise the TH350 is ok, now would be the time to add a Trans-Go 350 1-2 Reprogramming Kit. Should also increase trans life with higher line pressure and tighten up the shifts without being aggressive.

[geeteeohguy]

With your 2.56 gears, the stock TH350 is the way to go. Plenty strong, and properly rebuilt, will last for years and years. Funny how you think 2.56 gears are boring.....I used to get bored cruising 60 mph with my 3.55--3.36 gears. Now I get to cruise 80+ MPH with the 2.56's I put into my '67 GTO behind the TH400.....covering ground at high speed and sipping gas. Definitely the opposite of boring, for me.

[Skip Fix]

My 78 TA stock had a 180 HP 400 and a TH350 with a 2.56. Ran OK. Heck even in the 14s! but barely into 3rd gear! In fact at Texas World Speedway road course I did most in 2nd gear and it would only pull 4000 in high gear with that motor-but that was 130mph.
My 78 still has a TH350(different one) and has run in the 10s also.

I do think a low first gear(I think TA man has one) would help launch better but my standard TH350 did OK.

[Cliff R]

The key here to success with the TH350 (or anything else) is "properly rebuilt". Nearly all of these transmissions evolved thru the years and saw many changes. Some for the better, some not. The TH350 is simple, and strong. It has a few potentially "weak" areas just like a TH400 does. With a few minor upgrades the TH350 survives just fine in a high performance application. I ran one for many years in a street/strip 1965 Nova with a high-winding small block shifting at 7000rpm's and going thru about 7200 in high gear. Had another TH350 in a 1979 K-5 Blazer and logged nearly 200,000 miles on it after it's first correct rebuild and zero issues with it anyplace.

We do find and this is important to know that many TH350's will show excessive wear at the center support case lugs. This simply comes from zillions of loading/unloading cycles and the fact that the contact patch isn't that wide nor is the center support bolted in place. The aftermarket makes several types of "case savers" for them. I use and recommend upgrading the center support to the 4L60 type with the wider low roller clutch while you are in there for the rebuild. This is just a good idea all the way around and at least doubles the strength in that area vs just dropping in some sort of steel case saving device and having the center support "pound" the case in one or several small areas........Cliff

https://cliffshighperformance.com/Qu...ild-and-modify

[389]

With the 2.56, a TH400 with a 2.75 first gear set works nicely..

[chiphead]

I've used a built TH350 behind a stout 400 on spray. I figure it was making about 370 HP on motor and about 480 on spray. This was in a 67 Lemans weighing about 3700 lbs. It ran for about 5 years of street/strip duty.

When I pulled it down, the intermediate sprag was rolled, even though it had a hardened race. The bellhousing was cracked in half. Still ran and shifted well! It was likely my fault, due to pedaling the car at the strip with wheel hop issues.

[PONTIAC LARRY]

The 350 is more than adequate, I use on behind my 500ish hp 455 in my 11 sec Catalina and its not even "built" its used 50k original with a used shift kit swapped over. We have used th350 for decades behind 455's let alone a 265 hp rated 350 w 2.56 gears.. Its fine. fix the leak

[70 bird]

Originally Posted by 694.1

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

lol

[Cliff R]

"When I pulled it down, the intermediate sprag was rolled, even though it had a hardened race. The bellhousing was cracked in half. Still ran and shifted well!"

The aftermarket "hardened" sprag is NOT a fix for that set-up. Matter of fact I don't even use them much these days for high powered set-ups, we go straight to the 36 element set-up instead.

Below is a pic of the hardened outer race after apprx 70 runs behind a stout 455 running into the 10's at the track. The second pic is what we use and recommend instead for that sort of thing.

Breaking cases is more likely with a TH350 than a TH400, but poor pinion angle, using a solid mount on the rear of the trans, excessive chassis flex, and "wheel hopping" is quite likely going to break the case right off the front of any of them. Even with that said one of the TH350's we have in service has seen many hundreds of passes into the high 9's low 10's w/o issue in a very well set-up Pontiac powered car, so they will live just fine if the set-up is done correctly.......Cliff

[Cliff R]

Below is another somewhat "sore" spot on a TH350 behind the bigger CID engines which will see some drag strip time with good traction. I've pulled just enough TH350's down with this problem to recommend and use the improved input shaft for serious work........Cliff

[android 211]

There is less parasitic drag with a Turbo 350 than a Turbo 400. That equals out to better mpg and throttle response.

[77 TRASHCAN]

My 77 TA still has the original TH350 in it, about 150,000 miles on it. I installed a B&M transpak in it at about 20,000 miles. Jacked the car up on my Dad's backyard patio. Had no problems. The job was a little nerve racking, doing this at about the age of 21.

The trans still operates fine. It has acted up a couple of times, a strange deal. Both times I was just driving down the highway, and the transmission just went to neutral. I pulled off the road both times, checked the fluid (it was fine). Got back in car and it was fine. One of my co-workers had been a trans mechanic. He said something about my trans be high mileage that clutches with a lot of wear were causing some "O" ring not to seal, I don't remember for sure.

There are other GM transmissions with true problems like the TH200 metric, BUT those can be made into great drag racing transmissions, only takes about $5000.00 to do so...

BTW, the street outlaw guy that owns the Farm Truck ran a TH350 for years, without a problem. I talked to him at a track, right before they all became movie stars. He had just s itched to a TH400. I asked him why? He said really no reason nothing was wrong with the TH350, he just had a better peace of mind using a TH400 in his pickup....

[racegto65]

I have pretty much that combination in my '77 TA (TH 350,2.56 rear, slightly warmed over 400).
The trans is original,rebuilt it probably 25 years ago and still performs flawlessly today.

[Sirrotica]

Originally Posted by android 211

There is less parasitic drag with a Turbo 350 than a Turbo 400. That equals out to better mpg and throttle response.

The parasitic loss difference is only going to show up in a car that runs a 10 second quarter. The larger mass in the internals of the TH 400 does make it sap a tad more power than a TH 350, however the larger mass also translates into a stronger transmission.

A 9 inch ford also saps more power than a comparable 8.5 GM rear axle, the difference in strength makes the small sacrifice worthwhile by most peoples opinions.

The OP (350 2 Bbl) would notice zero difference between a TH 400 and a TH 350 in a daily driver as far as fuel mileage, or response from a stop light. The parasitic loss card is pretty insignificant, except in an all out drag car where .001 is something to be concerned about.

[punkin]

I won more bracket races when the Firebird had a ST300 in front of 2:56 gears. Ran the entire quarter mile in 1st gear and still it was good for around town and on the freeway.

[jimmy daniel]

Keep the T350 and add a 3:08 gear

[72projectbird]

A TH350 is far from weak.

Saying it is, is a laughable statement.