Datzenmike, can the lock up torque converter be used in an older transmission (3N71B or L4N71B) that did not originally have a lockup torque converter?
Unfortunately the locking torque converter tranny is also different inside the transmission itself. You can't just swap on the one part. BUT you could get a lock up tranny from a 720 truck and because the front bell is removable, you could swap the L series bell onto it. Voila'. Oh yeah the torque converter is larger and the flex plate is also different shape to fit this. So you would need those parts too. This is a picture of a locking and a regular flex plate from the engine side:
The L4N71B is a 4 speed transmission and was never equipped with a locking converter.
How does the L4N71B do coupled to a stock L20B?
What gearing would you suggest for my 520 with an L20B & 3N71B?
What gearing would you suggest for my 520 with an L20B & L4N71B?
Well that is a very tall overdrive ratio but at least it is an automatic and a certain amount of slip is built in.
For an empty truck I think 4.375 is fine with the 3N71B automatic and even with one converted over to locking converter. The RPM drop is only a couple of hundred at 40-50 MPH I think. Don't forget both trannys have the same first three gear ratios but the L4N71B has about a 30% RPM drop in 4th. This means a 3000 RPM cruise on the highway at 60? in third, would drop down near 2000 RPMs in 4th. Will an L20B have the power at 2,000 RPMs to push the truck through the wind at 60 MPH? Don't know. At least on hills or windy days the 4th gear can be switched off manually.