If Cost Is A Factor...

    BS: I currently have a 350ci mated with a TH350 in a 1965 with a 355 rearend 10 bolt. I love everything except the high rpm's on the freeway. I was thinking of getting a 700R4, but a transmission guy showed me a TH350 with a overdrive-type torque converter. He said they made these for several years prior to the 4-speed automatic coming out.

    Anyone know how good these units are? The transmission guy likes it better then the 700R4 because he said it's cheaper (which I really like), stronger, and will help me out on the freeway rpm's.

    S_: The guy must have been talking about a TH350C. This is a fairly standard 350; same gear ratios, looks the same, etc. The torque converter has a clutch in it that locks at about 45mph and eliminates converter slip. It's cheaper than a 700R4 and probably stronger, too. (I'm from the "old school" that thinks that if you can't tear a 700R4 to bits in a week of driving, you must have three cylinders that don't run). Back in the early-to mid-eighties, the 700R4 was a total piece of junk. I'm told they're better now.

    The 350C should be an easy swap. You'll drop some rpm's at highway speed, but it won't be drastic. I can't remember what activates the converter clutch, but I think that it was driven off of the on-board computer. You will need to rig up a switch to activate the clutch; no doubt somebody makes a nice kit to do that.

    DS: The TH350 is a fairly good transmission, but a 4L60/THM 700R4 would be a great swap. Later 700R4's (around 1987-present, except the 4L60-E) are the best bet for a modification. These transmissions have internal upgrades, and a computer is not necessary for controlling converter lockup. A typical 700R4, if modified, can withstand 500ft. lbs. of torque.

    This might be the least expensive swap for an automatic, because the 4L80-E (the overdrive version of the TH400) might be too expensive for a swap. This kind of transmission would require a computer module and a throttle position sensor, in which a swap might run as high as $3000.

    I would prefer a 700R4 as the least expensive swap, but there are other technical issues that would apply to a 700R4 swap:

    (1) The 700R4's overall length is the same as a TH350 with a 9" extension housing about 30 3/8" from the face of the block to the tailhousing,

    (2) The driveshaft must be shortened 3 inches, and

    (3) there is an extension housing for the 700R4 that allows the transmission to be mounted in the same place as the TH400.

    Remember to position the crossmember back, since the frames on all GM A-bodies are drilled for a specific transmission mounting position.

    Also, remember to use a TH350 slip yoke if swapping from a TH400.

    Archivist: Tom Wilson

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