Jake, I do not want that low first gear, it does not make the converter work as hard (multiply torque). My second problem is with the weight of the converter with the 4L80E, I dont want to carry the weight, I feel like driving around with an overdrive trans is like driving around with the parking brake on. And I do have back to back examples to prove my point. I would like your honest opinion, do you really think its a bad idea to put a turbo 400 in there with a 3.08 gear? Jake, please dont take this wrong, I think your serious about your work, allways trying to come up with new ideas, and I would have no problem reccommending you to person looking for an honest and good trans rebuilder. Keep up the good work.
We can put an 8" converter on a 4L80, we commonly use the 9.5" with them, same core as used with a TH400. Using the stock 4L80E converter would be a waste of time and nobody does it on a retrofit unless they are on a severe budget.
A converter will multiply torque no matter what the gear is, the gearing I would choose would depend entirely on the combo.
MOST combos don't pick up a bunch with the 2.75 ratio when used with a 3.55-4.10 rear but the heavier cars or milder ratios usually do pick up enough to be worth the cost.
We have one combo that went from a TH400 9.5" converter to a 4L80E, same brand 9/5" converter but setup with lockup clutch and actually picked up a couple of hundredths on 60' so feeling like the parking brake is on definitely wasn't the case.
TH400 with a 3.08 gear would work good in some combos, a 496 or larger cube BBC in a average or lightweight musclecar. It works very well in a turbo car.
However you will be giving up some possible ET in most combos and in any case you are giving up OD, you can run the OD with a 3.08 as well. In a performance car, a 3.42 is about as mild of a gear as I like to run, and even a 3.42 can get old without OD.
One of the biggest factors with OD for the last couple of decades is getting one to live.
The 4L80E is a direct descendent of the TH400. Many interchangeable parts, etc. There is a reason I don't sell 700-R4s/4L60Es, I don't BS the customer, and the results of the builders who do show up in posts like this one.