So I have a built th400 transmission laying in my shop. I also have a 2006 Sierra 4.8 that I in the process of turbocharging. My current 4l60e won't take much abuse, and I'm being recommended to swap in the 4l80e. Besides 3 speeds, why not use the smaller th400 I already have? I currently have a 30.5" tire, 3.23 rear end. I would have to step up to at least a 3.73 rear to make driving more fun with the th400 final drive of 1.1. What's your thoughts? And what's all needed to make the speedometer and reverse lights work? People daily drove cars/trucks with th400s all the time in the past, I don't think I would hate 3 speeds instead of 4.

 

I think a Th400 would go great in your turbo build as it sits. If it is a toy then you don't need overdrive anyways. Being a turbo build the 30.5" tire and 3.23s will help you build boost and make for a nice setup and good cruising RPM. Plus as you know a well built th400 is much more economical to replace as needed.

 

thank you for the reply! You don't feel that I have to tall of a gear currently? I figured it would be pretty slow off the line.

 

It should be fine, but it's simple enough to change after the fact that you can wait see how you like it. If you feel like t doesn't move out then change them then. I believe 02camaroSS runs a similar setup. If you go towards something like a 3.90 or 4.10 I would suggest adding a GV overdrive.

 

Absolutely nothing wrong with using the TH400. For a nondaily driver the absence of overdrive is not significant in my opinion.

The more annoying thing is adding all the speedo stuff and getting it working. It will have to be manual shift too, I'm not sure if the vacuum modulators will work with boost.

The upside is that you can get a bulletproof TH400 for half the cost of a bulletproof 80e.

 

I found part# sen01128 through dakota digital which will convert the cable drive th400 to the electronic speedometer. Cheap enough at $113 . Next would be figuring out the shifter situation. Looks like my stock column shift uses a cable to select gears on my current 4l60e, I bet with some ingenuity I could get this to work with the th400. Would I need my computer reprogrammed for a manual shift truck? Would it start/drive without this programming?

 

Plug the vss into the abs and calibrate the revs per mile correctly


323 with a turbo truck and 30" tall tire isn't gonna perform as well if you had say 373 or 410s

 

As far as the speedo goes, late model 400's used a VSS. They were actually called 3L80's by then.