My Bronco desperately needs lower t.case gears, but how low is too low?

Almost all of my fourwheeling experience was in my old CJ5, so bear with methat all of my figuring is with that as a baseline, I've got to build off whatI know. The CJ and my Bronco both have a crawl ratio of 34:1. In the Jeep Ialways kind of wished for lower gearing, but the 302 in the Bronco doesn't idledown as low as the 258 in the Jeep so it needs lower gearing even to crawl aswell as the Jeep, and I'd like it to be better than the Jeep. I’m figuring I’d like the Bronco to have the equivalentof at least one and preferably two gears below its lowest now. So that would put my desired crawl ratio atleast 54:1, and preferable closer to 87:1.

I’ve bought an NV3550 trans that’s going in the Bronco this winter. I’m planning on putting an Atlas t.casebehind it and now I’m trying to figure out what gears I want in the t.case. 3.8:1 gearing with the NV3550 and my 4.10:1 axles would give me a crawlratio of 62:1. That’s into my “acceptable”range, but not very close to my desired 87:1. 4.3:1 gearing would put me at 71:1 which is sounding pretty good to me.

But my concern is what happens in "normal" fourwheeling? I mostlytrail-ride and rock crawl. I don't expect to be doing much high-rpm hillclimbing and no serious mud. But trail running in the Jeep I was usually usingan overall ratio of 21:1 or 12:1. The 4.3gears would give me 14:1 in 5th gear low range. I’m thinking that will be OK, but I don’twant to end up hunting back and forth between low and high range a lot.

Going to 3.8 gears wouldn’t change things much. I’d be at 12:1 in 5-low. Going to a 4 speed Atlas would solve it,letting me trail ride with 2.72 gearing and have the really low gears just whenI need them, but that’s a lot more money.

What do people think? Are 4.3:1 t.case gears too low?

 

I liked the doubler I had, I ran 44s with 5.38 gears, a C6 and a 203/205 doubler. It wasn't very often I used the 4ish:1 low low range. Usually just ran in low.


Also, my experience with manuals, I ran a NP435 with a 2:1 low range with 4.56 gears on 37s. The 1st was like 6.69:1 or so I can't remember. But that was plenty low for crawling around.

 

Yeah, I know I wouldn't need the 4.3:1 very often. But I can always upshift the trans to get into a more normal range, that is, until I hit 5th gear. My question is whether people with 4:1 gears who don't also have 2:1 gears end up regretting that choice.

That gives a crawl ratio of 61:1, about what I'd have with 3.8:1 t.case gears. I don't think I want that because I don't think it'll be enough better in "easy" trail riding than 4.3 gears, and while it's probably low enough, I don't want to end up wishing for more a few trips down the road.

 

While the 302 doesnt like as low an rpm as the 258, the 302 has quite a bit more power at the middle and upper range, i doubt you will ever find that the 4.3 gears will be a liability.......as you stated, you dont mud bog, or do a lot of high speed hill climbs.

 

I overlooked some earlier. That rig I ran 4.56s with 37s, was a 74 bronco. 302, np 435 and dana 20. It was great, wasn't too low, and not too high. It worked great for a lot of stuff.

 

If that was a Bronco Dana 20 it had a 2.46:1 low (Jeep had 2:1 low Dana 20). That would give you a crawl ratio of 75:1. Even "correcting" for your 37" tires (I have 33s) it's 67:1. Not a bad place to be on the low end.

But I want taller gears for the highway too, plus I want an easier shifting trans. I put a lot of miles on either an NP435 or a T-18 in my first pickup and on the T-18 in my Jeep. But now I want full synchros and an overdrive in my Bronco. I'm going with an NV3550 which only has a 4:1 first gear, so I need to go lower than 2.46 in the t.case. I could have gone with a ZF or an NV4500 to get a lower first gear in an overdrive 5 speed. But I didn't really want to go that heavy and I didn't want to mess with a hydraulic clutch for the ZF.

 

Another way of looking at it is that with 4.3:1 gears, in 5th gear I'll be going 18 mph at 2500 rpm. How likely is it that I'll want to go over 18 mph when I'm in low range? If I want to go faster than that for very long I'd obviously stop and shift into high range. But if I'm constantly stopping to shift to high, and then back to low, and then back to high I'll wish I had geared it differently.

Going to 3.8 gears only brings it up to 20 mph at 2500 rpm, not enough to be worth it. Going with a 4 speed Atlas would give me 2.72 gears as well as 4.3 (and 11.7!). The 2.72 gears get me up to 28 mph at 2500 rpm. I know that would be good, but it's a lot more money when I kind of think the 18 mph of 4.3 gears would be good enough.

(With a 4 speed Atlas I wouldn't see actually using the 11.7 low-low, the advantage would be having both 2.72 and 4.3 gears)

 

It sounds like youve chosen a logical combo, it will do everything you want off road.....but still be a nice easy driver that you could take anywhere, with great road manners.

 

don't forget the price of the adapter w/the Atlas.
Now you're up well over $3K, no?

Again, there's the lower (than stock) gears to rebuild the D20
even tho pricey ($800) may still be the option for us cheapskates...
(read: fewer mods - inc. none to DS w/the 3550)
Unfortunately, I don't see the ratios now.

 

What adapter? From what I've heard the Atlas will bolt right up to the 3550. Although I guess pretty soon I'll find out if that's true.

Crawler gears for a Dana 20 are 3.15:1 if I'm remembering correctly. That, along with my stock 3 speed would only give me a 44:1 crawl ratio. Not nearly enough for what I want. So if I went the lower geared Dana 20 route I'd need to either also go to a granny low 4 speed or lower axle gears or something to fix my low end. For what I'm looking for in this Bronco I don't want to compromise the on-road performance and/or easy shifting. So I'm kind of stuck with either an Atlas or some sort of doubler. The doubler probably wouldn't be much cheaper than an Atlas, and it would get a lot heavier.

But yes, this ain't gonna be a cheap project.

 

What doubler(s) would U use?

Still lighter and less expensive than Atlas 4?
Might be...

I don't wanna B the 1 to give U an excuse 4 THAT purchase.

 

I haven't looked into doublers close enough to have anything specific in mind. But I figure any doubler setup is going to end up with essentially 2 cast iron transfer cases, so it's going to be heavy. And while there are probably doublers that work with some of the lighter cases, since the double goes in front of the "main" transfer case, the main case needs to be pretty darn strong. I'm not sure I'd want to put a doubler in front of anything less than an NP205, and now you're getting really heavy.

Besides, the only thing a doubler gains me over a 2 spd Atlas is that I'd get a "normal" low range (around 2:1) in addition to the creeper low (around 4:1). Since I'm kind of landing on thinking I won't need both ratios I'm not really considering a 4 spd Atlas anymore. But it'd be cool!

 

I was thinkin of the dodge doubler. I understood it was just the 'range box' (frnt 1/2 of a tranny) so thought it might B lighter than an Atlas 4 speed. No sure of all that tho. Glad you'll skip the 4 speed now ($).

There's over and under drives too. Go after all that (Tcase) but are v e r y heavy, I think & don't help w/the frnt drive I guess.

U found an online caculator to figure the ratios? Hope the formula's got a variable for the tire sz too. Did it fall into your sought for specs for the other Atlas or will you do something even different from that?

 

"...Dana 20 are 3.15:1..."
T shift (that's till '72) low position 2.46:1
J shift ('73 and beyond) low position 2.38:1

I like my T because it has 5 positions and is easier to adapt to after mrkt prts. I often hafta stop, go in reverse a lill - to switch gears tho.

Good luck, Bob

 

I'm assuming the "Dodge doubler" is using the low rang gearing off an NP203 t.case. Dodge used a lot of those, but so did Ford and Chevy in the '70s (don't know if all work as doublers though). I know the NP203 is about the heaviest transfer case ever made (at least for 1 ton and lower trucks), so I doubt that's a light weight option.


Yeah, I'm looking for more options in 4wd, so anything that goes behind the transfer case won't help me.

It's just basic algebra and I'm an engineer. I figured out the formulas myself. And yes, I took tire size into account (although it's not much of a factor in my comparisons because the Jeep and Bronco both ran/run 33s)

The Atlas 4.3 gears are what I'm thinking about. There's still a little overlap between 5th gear low range (13.8:1) being just a little taller that 1st gear high range (16.4:1). More overlap might be nice, but I'm thinking that'll be OK. By the way, those both include the 4.10 axle gearing.

3.15:1 are replacement crawler gears if you want to go lower than stock. My '71 has the T shift as well, so it's the 2.46 that I've got now.

By the way, I do have the trans now (NV3550 from a '03 Wrangler). I need to get some house stuff done before I do any more right now. I'll start posting in my build thread on the Bronco page once I have anything to report.