Th350 Lock-up /Non Lock-up

A lockup tranny basically is the short term used to describe a transmission with a locking clutch internally in the torque converter.

A non-lockup converter hydraulically locks at a certain rpm to create a drive source for the planetary gearset. together with gear reduction of different gears in the tranny, the converter provides an amount of torque multiplication to help get the car moving. it becomes unnecessary at highway speed for the converter to provide this added torque multiplication by slipping per say, but it still rotates a small amount under light load/cruising speeds. this eats away at your mileage

Iin the late 70’s is when we started seeing the first lock up clutch transmissions. it basically works just like a clutch in a manual transmission. At highway speed, usually around 45mph, the computer engaged the clutch in the torque converter to halt any and all excessive rotation of the converter to in effect create a solid drive source going to the tranny. obviously it can only be engaged at cruising speed cause if if didn’t ever release the engine would stall at the stoplight. also you need the added benefit of torque multiplication to get the car going. think of it this way…..a standard tranny has usually 5 speeds….an automatic for sake of arguement has 3. Since there is going to obviously be more of a gear/load jump the torque converter makes up the difference by creating torque multiplication. You can also start to realize how customizing how the converter is built and stall speed can really effect how your car performs can’t ya??

For derby use only, avoid running a transmission with a lock-up clutch. most of the time this clutch is already screwed up cause it came out of a car with years of abuse….especially something that was in around town a lot that cruised 45mph. the clutch dissintegrates and locks the torque converter up tighter than a crabs ass……and you’re dead in the water. even if you never engage the clutch it can still jam on ya as the tranny heats up.

When the Th350 first came out in the late 60’s, it is pretty obvious that they had planned on some kind of a lock-up clutch design in it’s future. In all there were 4 or 5 different versions of the th350. the the 250, 250c, 350, 350c, and 375B(somewhat rare). I believe the lock-up trannys started with gm in 1980 when the on-board computers came around, but i could be wrong. this line of transmissions underwent numerous changes every year of production. a dead giveaway you have a 250 or 250c is the output shaft is hollow….i avoid the 250 also because the clutch packs are pretty skimpy as it was a light duty tranny, along with a lousy gearset.

For derby use…..and for what i build the B-25 derby tranny’s out of, we want to use a non lock-up th350. Commonly they are longtail and have an input shaft that is fully splined to the end sticking out the front pump of the transmission. there are no electrical plugs in the case anywhere. Usually if it came out of a 70’s vintage tuna boat it’s a non-lockup.

700r4_establishing_bg A lock-up th350 transmission will look more like a 200R4 or 700R4. the input shaft sticking out of the front pump will resemble something like a manual transmission would have. instead of a fully splined input shaft there is a smooth arbor at the end of the shaft: narrower than the splines about 1/2″ long. this is the smooth arbor that engages the internal clutch of the lock-up converter. in addition, on the driver side toward the rear of the case there is a square electrical plug sticking out of the top of the case above the oil pan. if you find either of these things on a tranny, it’s a good bet you have a lock-up transmission. I have found that a lot of short shafts are prone to being lock-up trannys. now you can take the short shaft/tail housing out of a lock up tranny and put it in a non lock-up tranny, but other than that it is usually just easier to start with the right core to begin with. hope this helps.

Comments are closed.