TH350C Transmission and a Mild 455

[1984Oldsmobile442]

I really have some homework to do. I will take some time to try and figure out what the best option is. I do not know a great deal about transmissions/ rear ends/ gearing. My learning curve will be steep. Hopefully I won't test everyone's patience.

Is a 350transmission mechanically the same as a 350C? I know the TH350C is supposed to have lock up capabilities. I suspect a standard 350 does not have this capability? I do like the potential lock up capabilities from the 350 C.

This assumes I understand how the locking torque converter works. My understanding is the torque converter locks up at a specific RPM after entering 3rd gear. This allows the transmission and engine to spin at a 1to 1 or near a 1:1 ratio. (I may be exposing my ignorance here.)

Is there a resource for comparison of weight and gearing ratios for various transmission units such as the TH400, TH350, TH350C, TH2004R, 4L80E? If not I can try to put something together for comparison. I also need to ID what gears are in my current rear end. If I'm going to take the time to do the research on weight and trans gears then I might as well understand how gearing in the transmission translates to the rear end. I may not have the cash to purchase what I want now. But I can at least make an informed decision. I'm going to ID the rear end shortly.

 

[1984Oldsmobile442]

The rear end is a 2AK code. 10 bolt, 7.5" Ratio = 2.14 Highway cruising should not be a problem with this rear end ratio.

 

[Sean1369]

Always replace the torque converter, it holds the majority of the trans fluid and if it was on a hurt trans its holding a lot of junk in that fluid that you don't want to run through your fresh trans. That being said I did a TH350 last summer that was behind a 455 olds and it was hammered when I got into it but she's still shifting fine now, it was all stock but also in a heavier 70 cutlass. The 350C from my understanding should have lock up capabilities, for fuel mileage at highway speeds in a non OD trans. I personally would stay away from the TH350 behind a big block, I mildly built TH400 would suit you better, heavier but stronger, its a trade off. A 700R4 is an option where you get a desirable low 1st gear and the benefits of OD and built properly should hold a mild 455. A lot depends on you budget. As for the driveshaft you will probably have to have one made, a local med/heavy duty truck place can usually build you one just tell them the length you need and they balance them too, I use fleet pride. I hope some of this helps.

 

[truracer20]

Years ago a stock TH 350 survived just fine behind both my 455 and my 425, stock pistons, 270* .500 lift cam, C heads, and Performer intake. I just added a decent size cooler then later on a shift kit using the street strip setup. It squealed the tires 1st to 2nd and 2nd to 3rd. That same car had a 7.5 posi from an SS Monte it also survived and I happen to have a really heavy foot and absolutely love burnouts... The 7.5 was replaced by an 8.5 only because the 8.5 had a better ratio. I did break a non posi 7.5 but it had a lot of miles and got beat up pretty good by a strong Olds 350 first.
I never messed with the TH 350C some say they are just the same as the TH 350 some say they are weaker.
I have no doubt that a quality built TH 350 will live behind a mild 455. Heat is the enemy. I recommend a good cooler and an extra capacity finned aluminum pan.
In a heavy car a like the 455 was originally used in a TH 400 is pretty much required.

 

[olds307 and 403]

The TH350C uses a unique pump, input shaft and slightly different valve body. All the other 350 tricks can be done for more strength. The Monte SS rear is a 7.625", has bigger, more evenly sized spider gears and holds up to more power.

 

[1984Oldsmobile442]

Thanks for the great information. I will make sure a new torque converter and trans cooler is used no matter what the set up. I am going to talk to local shops and see what type of pricing is available. I'm also going to troll some salvage yards for rear ends and frame bracing components.

I would love to try and re-build a transmission myself. I bought a Haynes Tech book a couple of years ago with the intent to learning. The manual scared the living daylights out of me! Maybe I need to look a different manual. (I may also need to let someone who knows what they are doing handle the transmission.)

Quick thought: does it makes sense to install new rear suspension springs at the same time as removal of the rear end? Are their any suspension or frame components that I should replace while the rear is out?

 

[spidereyes455]

My advise would be ditch the 350c you'll just be throwing money away on a mediocer trans that may or may not survive a 455 (keep in mind these engines commonly had 400-500 ft lbs of torque stock) also as stated before dont bother with a 700-r4 either the first gear is way more than needed with a BBO and its a chevy only bell pattern requiering an adapter to fit a BOP engine. I would opt for a 200-4R but not the garden variety you'll find in grandmas Delta 88 look for a performance calibrated one like an OZ, CQ, BQ, KZF, CZF or a BRF and have it gone through by someone who specializes in these trannnys, I myself would contact Dave Huesek for this he is extremely knowlegable with these trannys and is a great guy to deal with he is a member and paying vendor over on the turbobuick.com forums so it would be easy to get his contact info over there. I also wouldnt waste my time with a 7.5 for a BBO I've personally destroyed a few of them with a stock 403 Olds , instead look for an 8.5 from a GN, T type or H/O, or 442. I would also strongly recomend a set of rear trailing arm braces that tie the upper and lower control arm mounts together on the frame Hotchkiss makes nice ones for a G body but there are others.

 

[1984Oldsmobile442]

Thank you for the additional thoughts. I have found that finding an 8.5" rear end from high performance G bodies is quite a challenge. I'm going to try paths which are hopefully less traveled. Maybe I can find gold.

 

[spidereyes455]

Thank you for the additional thoughts. I have found that finding an 8.5" rear end from high performance G bodies is quite a challenge. I'm going to try paths which are hopefully less traveled. Maybe I can find gold.

They are out there its just whether you have the budget for one, avg price for a posi is $900-$1200 and even the one leggers are $450-$800. Good luck with the search.

 

[1984Oldsmobile442]

I found an 1986 Monte Carlo SS today that still had it's rear end. We were able to confirm it was a true SS by the VIN number. Unfortunately, all other numbers which I believe would ID the rear end as a positive track are lost from the car. The Axle code could not be found on any portion of either sided of the axle tubes. If I understand correctly, GM started installing the 7.65 (7-5/8" rear ends) in the SS after over drive transmissions started appearing? If this is the case then I am hopeful this unit could work for what I am trying to do.

Thoughts:
The unit still turns freely by hand. I don't believe it is positive track since one end would spin and the other remained in place. However, the other end was buried in the dirt.

What would it take to convert a 7.65 unit to a positive track if it currently does not have this capability?

This rear end has been in the junk yard for a long time. What type of work should I plan on doing to it while it is free of a vehicle?

Note:
This vehicle has the "V jounce Bar my car has one but maybe someone else on here needs one? (This assumes I go back and pick up the rear end.)

I will surf the forum this evening for additional info on the Monte 7.65.


Thanks again for all of your thoughts on this. It has been VERY helpful.