Tech article by Rick Halle (SaddleUp)
The history behind my choice: (Skip this if you just want to know what parts were required and how I did it)
The following article describes the C6/203/205 doubler assembly I installed in my 69 Bronco along with my reasons for choosing this setup over the other options available. The main reason I’m writing it up is that I’ve answered the same questions many times both on and off the message boards regarding the details of my setup.
To start with I’ll go through a bit of the history behind my choice. Basically I knew that the C4 in my Bronco was slowly dying and therefore I began looking at different transmission options. Not that the C4 isn’t a decent transmission, but rather that it just wouldn’t live with the way I was abusing it. My primary goal when building up my rig has always been that whenever anything breaks I try to upgrade it to something better that will not break the next time around. My preference is to do it once and do it right. With this in mind I began my search.
To start with I began looking at direct bolt in options with better gearing choices. This pretty much left me looking at a transfer case change at the same time unless I purchased an expensive adapter. I pretty much decided right from the start to go with a NP 205 transfer case because their strength is legendary. The main issue with them is that the low range is only 1.96:1 which did nothing to help me in the low gearing department. To get around this I began looking at transmissions with more gearing options. First I looked at the AOD transmission using the wide ration gears. I dismissed this option though due to reservations I had about it being strong enough. Next I looked at the ZF transmission which I almost ended up using. At one point I would have bought one if the clutch parts would have been on it. (At that point I had already decided on the ZF and was just finding parts to do it) By that point I had also run across information on doing a 203 doubler and I was giving it serious consideration.
At about the same time a discussion was also going on regarding automatic transmissions and taking the torque converter into account when figuring crawl ratios. Since my primary reason for choosing the ZF over the others was the crawl ratio I quickly ran some searches to dig up what information I could on the subject of crawl ratios when using torque converters. The information I turned up both surprised me and also made complete sense. It answered the gnawing question I had as to why auto transmissions had higher gears than manual ones. Basically it boiled down to the torque converter which slips when it is at speeds lower than the stall speed which in essence creates much lower gearing than the actual gears in the transmission. The actual conversion numbers vary from one torque converter to the next, but they usually fall between 1 1/2 to 2 1/2 times the low gear in them for calculating the crawl ratio. A quick calculation in my head put the 2.46 ratio in my current C4 at about 5:1 for calculating the crawl ratio so it became quickly apparent that the ZF wasn’t going to be a big advantage there and that it would sacrifice the convenience of the auto transmission. Back to the drawing board I went.
Specifically I looked at the C6 at this point because it had the same gearing as the C4 I was already using and because it is also known for its legendary strength. This time though I took into consideration that I could use a 203 doubler at the same time. A few calculations later and I came up with the following: The C6 low gear is 2.46:1, the NP 203 range box low range is 2:1, the NP 205 has a low range of 1.96:1, and I was running 4.56 gears. This gave me a crawl ratio of 44:1 before taking into account the torque converter. After taking it into consideration though it put my crawl ratio at somewhere between 66:1 and 110:1 with a median calculation of 88:1. I figured that my crawl ratio would be somewhere in the neighborhood of 90:1 using this setup.
Some of you may be wondering at this point why I didn’t look at the Atlas. Well, there were a few reasons. First is that they are hard to obtain if they break. I.E. You can’t run to the local wrecking yard for parts. Second is that they cost a lot of money. I figured approx. 3 or 4 times the cost of doing the doubler setup instead. Last is the gearing options. Basically you are given two choices; Either go fast or go really slow. Nothing in between. With the doubler I get the options of running 1:1, 2:1, and approx. 4:1. I could take this a step further and have the gears in the 203 swapped out for a 3.1:1 low range set as well to give me even more options. Doing that would give me 1:1, approx. 2:1, 3.1:1, and about 6:1. Some day I might go this route along with the low gear set I plan to put in the C6 which would give me a crawl ratio of about 160:1. At a cost of a cool grand though this will wait for awhile.
Since I brought up the low gear set for the C6 I’ll briefly cover it. Basically it uses a wide ratio gear set from a newer Ford transmission (There are two option) which basically drops the low gear enough to make up for a normal gear swap of say 4.56 to 4.88 gears. Another word with it installed my low gear would feel like I was running 4.88 gears instead of 4.56 gears. They are also supposed to hold up well in the C6 in contrast to the C4 ones which are not very reliable.