If you've been following our Tacoma project you know we're working on a 2WD Prerunner with a 3.4L V-6 and auto tranny. We chose this model because we knew we'd be torching off the front suspension to install a solid axle. We also had ideas to use something other than a Tacoma transfer case. The Prerunners are available at a considerable cost savings as compared to a 4WD Tacoma and we knew of a solution for adapting the 2WD auto tranny to accept a transfer case.
Inchworm Rock Walkin' Gear manufactures a billet aluminum adapter plate that replaces the 2WD transmission tailhousing and makes the tranny output look like that of a GM TH350. From there, any number of transfer cases can be adapted.
We narrowed our choices to several configurations we considered. The first was a solution that kept the driveline essentially Toyota. Inchworm offers their Lefty Toyota transfer case conversion that utilizes the internals from the older style Toyota gear drive case to flip the front drive output from the passenger side to the driver side, as found on the Tacoma. Their unique conversion can also be supplemented with a second Toyota gear reduction and combined with low range replacement gears offering deep 11:1 gearing with four forward speed ranges. To build a full Toyota configuration we would also upgrade the output shaft to a chromoly replacement (comes on Lefty) to upgrade that one area of vulnerability in the Toy case, and we would add a twin stick conversion to allow for 2WD mode in any gear range.
Another option that came to mind was the use of an Advance Adapters Atlas II transfer case, or better yet, a 4-speed Atlas that could also provide us with plenty of forward gearing choices and deep gearing as well. In talking to Jim Christiansen at Inchworm, he was able to come up with an adapter scheme that would allow us to mount either Atlas behind the 2WD auto tranny. However, we found that the downside was that this placed the transfer case close to the tail of the transmission. Our desire was to keep the front output of the transfer case as high as possible and the shorter distance to the longer travel solid axle would result in a steep driveshaft angle at droop.
Our final choice combines a bit of each of the above parts lists. We decided to use a 2-speed Atlas II with a Toyota gear drive reduction box adapted in front of the Atlas. This setup will also give us four forward gear ranges combined with the superb durability of the Atlas. The Toy reduction is 2.28:1 and we chose a 3.0:1 Atlas for a low-low range gear ratio of 6.8:1. This should provide us with good gear choices for all wheeling possibilities from easy trails to snail's pace crawling.
The full Toyota setup is reliable and compact and Inchworm's Lefty case makes building a Tacoma setup easy. The cases use Toyota driveshaft yokes and the setup is clockable so you can get the front output tucked up and maximize ground clearance.
The cost of going the Atlas route is a little higher but we gain an essentially unbreakable transfer case and the ability to select front and rear drive outputs independently. The Atlas has proven itself over and over in many hard core rigs. The one issue that we knew we'd have to deal with was fitting the larger Atlas case under (or in) the cab of our truck, and getting it to fit in the confines took a fair bit of fab work. We'll give you a few details in the photos as to how we stuffed it in our Tacoma. We're sure we'll appreciate the stoutness and versatility of the Atlas choice.
With the transfer case installed we'll be looking to continue the project with the front suspension. Check out the next installment on Project Venture Toy to see how we get our Solid Axle D60 mounted to the front end.