Gearing/trans questions

[BroncoManSam]

Hey everyone,
I'm in the process of choosing a new transmission, transfer case, lockers, and differential gearing for my '73 Bronco project and I have a general idea of what I want, but I'm looking for a second opinion (or two) about this tentative set up and my plan so far. 95% of the time, I'll be using the Bronco offroad, and that is what I'm building it for:

-Engine: Rebuilt 302, or stroked to 347 to put out *about* 300hp
-Transmission: NP435 4 speed
-Transfer Case: Atlas II Twin-stick
-Underdrive Unit: BlackBox-i (has a 2.72:1 low range, but also has the ability to multiply against existing transfer case low range offering 4 total low range options.): http://www.northwestfab.com/NWF-BlackBox-Underdrive_c_7.html
-Diff: 4.56 gears with ARB Air lockers front and rear
-Tires: 35x12.5x15r Goodyear Wranglers

I used this website: http://www.grimmjeeper.com/gears.html
to calculate my final drive and gear ratios with the setup above, and if I'm reading this correctly, my lowest gear ratio (transfer case low, underdrive low, 1st gear) would be 356.80:1 (I think?), which is insanely low, but I'm ok with that, as long as it's at least still drive-able at relative highway speeds (55-65 mph). Does this seem like a good set up for offroad use? Thanks in advance.

 

[Broncobowsher]

You know that you can get a 4-speed Atlas? Takes the place of the black box. Less parts to have to mate together.

But yes, sound good.might go for a touch less axle gear, 4.30s.

If you already have parts, keep them. But consider if you are still shopping.

 

[BroncoManSam]

You know that you can get a 4-speed Atlas? Takes the place of the black box. Less parts to have to mate together.

But yes, sound good.might go for a touch less axle gear, 4.30s.

If you already have parts, keep them. But consider if you are still shopping.

I’ve heard a bit about the Atlas 4 speed, they seem great, but from what I’ve found, the black box would take up a small amount of space, and combining the black box and Atlas II is slightly cheaper than just the Atlas 4, although I may yet consider the Atlas 4 because I haven’t purchased anything yet. Thanks for the info.


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[nvrstuk]

I wasn't going to say anything till you mentioned you didn't have all your drivetrain yet.

Have you seriously looked at the Titan/205? The Titan is much stronger than the Blackbox i and the 205 is stronger and about $2,000 less money than an Atlas.

I have run most of these doubler combo's and have put a couple of them together in a couple buddies rigs ....

I've personally run a Klune and a 203 doubler with Atlas's, D20's and 205's.

You can not get more gearing options with the strength combined with the least expensive option and the shortest than a Titan/205. Titan is only 5.33" long and appr $1800. The 205 is readily available for under 250 bucks. Twin stick it and you have the equiv of an Atlas with the toughness and parts availabilty of an OE and probably the best supported aftermarket upgrades in the 4 wheeling world.

You sure don't need a lower geared case than a 205 since you have the 6.6:1?? gearing of the 435. 6.6 x 2.7 x 1.9 x 4.56 and you have yourself some crawling ability and everything in between.

Good luck.

 

[DirtDonk]

Are you going to drive it to the trails at all? Or trailer it everywhere? What kind of trails and wheeling will you be doing?
Just wondering since with all that underdrive-ability you'll have, you can gear it totally for the street and still have your cake and eat it too. With those numbers you're talking about in low-low-low range, you'll probably be too low in most situations where you need to keep up at least a little momentum to overcome the lack of traction. A longer wheelbase can help there too, but with the Bronco's short wheelbase a slight momentum is your big friend on the rocks at least.
You might even be getting honked at on most trails when lesser geared folks are bunching up behind you.;D

Maybe it's not the same nowadays, but most "normal" geared wheelers used to get pretty frustrated coming up behind a Marlin Crawler Toyota or similarly low geared rig showing it's crawling prowess.
It's cool and all, but is anything over 200:1 really a benefit? Under what conditions I wonder? I've probably not wheeled what most of you do these days, so might not be in touch with what you're trying to achieve.
What have your combinations all netted you in their lowest gear nvrstuk? Maybe I'm off base with the 200:1 quote, but 356:1 seems a bit extreme.

Should be a fun setup though BroncoManSam! Certainly qualifies as LOW!

Paul

 

[BroncoManSam]

I wasn't going to say anything till you mentioned you didn't have all you drivetrain yet.

Have you seriously looked at the Titan/205? The Titan is much stronger than the Blackbox i and the 205 is stronger and about $2,000 less money than an Atlas.

I have run most of these doubler combo's and have put a couple of them together in a couple buddies rigs ....

I've personally run a Klune and a 203 doubler with Atlas's, D20's and 205's.

You can not get more gearing options with the strength combined with the least expensive option and the shortest than a Titan/205. Titan is only 5.33" long and appr $1800. The 205 is readily available for under 250 bucks. Twin stick it and you have the equiv of an Atlas with the toughness and parts availabilty of an OE and probably the best supported aftermarket upgrades in the 4 wheeling world.

You sure don't need a lower geared case than a 205 since you have the 6.6:1?? gearing of the 435. 6.6 x 2.7 x 1.9 x 4.56 and you have yourself some crawling ability and everything in between.

Good luck.

Hadn’t considered this option, seems like a good one. Cheaper is definitely better, but where exactly could I source an NP205 for that cheap? Junkyard perhaps? Online they’re going for about $1000-$1500 rebuilt, maybe I’m looking in the wrong places though. EDIT: I found that BroncoGraveyard is selling used NP205’s for $300, but they aren’t rebuilt (the bearing and seal rebuild kit is $120 or so), and they are listed under the ‘78-‘79 full-size Broncos, rather than an EB, but I don’t imagine there would be issues fitting the 205 in an EB? The Titan BlackBox also seems like a good option as well. So with the Titan/205 combo behind the NP435 my ratio would be about 161.4:1, if I did the calculations right? That seems pretty good in all honesty. Thanks for the info!


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[BroncoManSam]

Are you going to drive it to the trails at all? Or trailer it everywhere? What kind of trails and wheeling will you be doing?
Just wondering since with all that underdrive-ability you'll have, you can gear it totally for the street and still have your cake and eat it too. With those numbers you're talking about in low-low-low range, you'll probably be too low in most situations where you need to keep up at least a little momentum to overcome the lack of traction. A longer wheelbase can help there too, but with the Bronco's short wheelbase a slight momentum is your big friend on the rocks at least.
You might even be getting honked at on most trails when lesser geared folks are bunching up behind you.;D

Maybe it's not the same nowadays, but most "normal" geared wheelers used to get pretty frustrated coming up behind a Marlin Crawler Toyota or similarly low geared rig showing it's crawling prowess.
It's cool and all, but is anything over 200:1 really a benefit? Under what conditions I wonder? I've probably not wheeled what most of you do these days, so might not be in touch with what you're trying to achieve.
What have your combinations all netted you in their lowest gear nvrstuk? Maybe I'm off base with the 200:1 quote, but 356:1 seems a bit extreme.

Should be a fun setup though BroncoManSam! Certainly qualifies as LOW!

Paul

I can trailer the Bronco if I have to, and most likely will be traveling to other places besides Wisconsin to wheel (my home state), so I expect more rocks than mud, but I haven’t done a whole lot of wheeling because the Bronco’s been out of commission for the resto for so long. I’m not sure what’s considered “too low”, I’ve heard of guys that have a reduction of over 500:1 for extremely low crawling; I don’t think I need to go that extreme, but I’d like some capability for sure. What would you consider to be appropriate street gearing with a super low set up? One guy mentioned above that 4.30 instead of 4.56 would be good. Like you say, eating my cake and having it too would be great. Maybe I’ll go with the NP205 case and Titan BlackBox for a little less extreme low gears. Thanks for the info and questions.


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[Boss Hugg]

Watch Craigslist and facebook marketplace for the 205. I think a crawl ratio lower than the 162 I calculated from nvrstuk's numbers is already asking to break stuff.... In order to do the crawling that I expect that low number to be useful for is going to require some REALLY sticky tires. As in, stickies treated with chemicals to be even stickier.

 

[nvrstuk]

With 150:1 you will be so low geared you'll rarely use it-if ever.

Your main limitation in my opinion (worth .02 on a good day) is your trans choice.

You could also save some major coin (if you stick with the 435) and go with a 435/203/205 option. See used 203/205 combos up for sale every month.

Wild Horses has this adapter.

I just sold another 205 last spring for $250. Needed nothing-bolt it up and go.
Spread the word on Bronco lists or to local buddies.You'll find one that just needs the typical new rear seal and output brg. They are tough and unless they've been sitting in water rarely, if ever, need a full rebuild-just dont.

Like Paul said- so many gear choices... that's what makes a doubler unbeatable for year round wheeling.

Engage both those cases for nasty rock sections.
Engage the 2.7:1 when you are on a difficult trail.
Engage just the 205 when you need wheel speed in snow or sand and can't maintain it with engine power but the doubler can really help keep the rpms and wheel speed up. I used them extensively for this for 20+ yrs till I finally quit building stokers for others and put one in mine.

 

[BroncoManSam]

Watch Craigslist and facebook marketplace for the 205. I think a crawl ratio lower than the 162 I calculated from nvrstuk's numbers is already asking to break stuff.... In order to do the crawling that I expect that low number to be useful for is going to require some REALLY sticky tires. As in, stickies treated with chemicals to be even stickier.

I will do that, thanks for the suggestion, I’ve been doing some looking already. In that case, 162:1 would be a good choice then? What axle/diff. would you suggest? After looking at what people have said, I imagine I wouldn’t need to go any lower than that, in terms of gear reduction.


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[BroncoManSam]

With 150:1 you will be so low geared you'll rarely use it-if ever.

Your main limitation in my opinion (worth .02 on a good day) is your trans choice.

You could also save some major coin (if you stick with the 435) and go with a 435/203/205 option. See used 203/205 combos up for sale every month.

Wild Horses has this adapter.

I just sold another 205 last spring for $250. Needed nothing-bolt it up and go.
Spread the word on Bronco lists or to local buddies.You'll find one that just needs the typical new rear seal and output brg. They are tough and unless they've sitting in water rarely, if ever, need a full rebuild-just dont.

Like Paul said- so many gear choices... thats what makes a doubler unbeatable for year round wheeling.

Engage both those cases for nasty rock sections.
Engage the 2.7:1 when you are on a difficult trail.
Engage just the 205 when you need wheel speed in snow or sand and can't get maintain it with engine power but the doubler can really help keep the rpms and wheel speed up. I used them extensively for this till I finally quit building stokers for others and put one in mine.

I did see the 203/205 doubler option when looking around for info on the 205, that also seems like a great option as well. Would both transfer cases be twin stick, or single stick for both? What would my final ratio be with the doubler and 435? Also, could I stick with the 4.56 gears or should get 4.30’s instead? Thanks for all the great info and detailed responses, I appreciate it!


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[nvrstuk]

The doubler would only be "engaged" for gear reduction (single shift lever or cable) or "not engaged". The actual transfer case that would have the rear DS attached to it would/could be twin sticked.

 

[BroncoManSam]

The doubler would only be "engaged" for gear reduction (single shift lever or cable) or "not engaged". The actual transfer case that would have the rear DS attached to it would/could be twin sticked.

Ok that makes sense, so it would be the same amount of sticks if I had the 435, a twin stick case, and the underdrive stick.


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[nvrstuk]

Gearing is very personal. Many on this list seem to never want to have to shift down going up a hill on a County road.

Like Paul mentioned earlier. You will be so low geared for the trail you could run almost any highway gears. (maybe I added a little)

Engine power/torque choice and whether you don't mind shifting on a hill greatly determines diff ratios.

Also, will you EVER increase tire size? If there's a chance you ever will, then go with steeper gears.

I was extremely pleased with 4.56 gears with 37" tires and 351W. Also ran ZF/203/D20 so I had to shift out of OD on hills... got 14.5mpg on 2,000 mile trips, Qjet carbed with an accurate odometer.

I run a very strong stroker, 6r80, no doubler now, and only a 3.0 Atlas. For the first time ever, I don't need a doubler and I dont have to shift down on mtn passes any more either.

Soooo many options... have fun!

 

[BroncoManSam]

Gearing is very personal. Many on this list seem to never want to have to shift down going up a hill on a County road.

Like Paul mentioned earlier. You will be so low geared for the trail you could run almost any highway gears. (maybe I added a little)

Engine power/torque choice and whether you don't mind shifting on a hill greatly determines diff ratios.

Also, will you EVER increase tire size? If there's a chance you ever will, then go with steeper gears.

I was extremely pleased with 4.56 gears with 37" tires and 351W. Also ran ZF/203/D20 so I had to shift out of OD on hills... got 14.5mpg on 2,000 mile trips..

I run a very strong stroker, 6r80, no doubler now, and only a 3.0 Atlas. For the first time ever, I don't need a doubler and I dont have to shift down on mtn passes any more either.

Soooo many options... have fun!

That makes sense. I have no problem with shifting (or wheeling with a carb, but that’s a whole other story haha). My original goal was to run 4.56 gears with 35’s and a 302/347 with around 300hp. I will most likely get bigger tires down the road, so I’ll probably stick with 4.56. So far, I like the sound of the NP435 with the NP205 transfer case and Titan BlackBox underdrive, OR the NP435 coupled to the NP203/205 doubler. Again, thanks so much for the valuable info and advice.


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[Quick & Dirty]

Also, could I stick with the 4.56 gears or should get 4.30’s instead?

Not much different in the end result. 70mph on 35" tires is 3200 rpm with 4.56 and 3000 with 4.30. For mostly off-road I'd go with the lower gears.

If you were doing more highway miles or want to run 75 - 80mph, then the 4.30 may make more sense. But then, a 5 speed overdrive trans with 5.14 gears would be better.

 

[Boss Hugg]

I did see the 203/205 doubler option when looking around for info on the 205, that also seems like a great option as well. Would both transfer cases be twin stick, or single stick for both? What would my final ratio be with the doubler and 435? Also, could I stick with the 4.56 gears or should get 4.30’s instead? Thanks for all the great info and detailed responses, I appreciate it!


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To get your crawl ratio, you multiply the lowest gear ratios in each piece.

6.69 (NP435) x 1.96 (NP203) x 1.96 (NP205) x 4.56 (Diff ratios) = 117:1. This combo works for a lot of folks. I think 162:1 (Bluntly put) is stupid low. So there's the formula ^^^ You can insert the numbers from whatever combo you want.

Concerning the 203/205, it's gonna weigh about 6000lbs (maybe some over exaggeration there, it's all cast iron cases) when compared to the aluminum cases of the NWF parts and the Atlas case--just something to consider.

My dad wheels Hot Springs ORV park last weekend with NP435, Dana 20 and 3.50 gears on 35" tires aired down to about 10psi. the only real problems he had was the limited slip in the front. If it'd been a full locker, he' probably never would've had an issue. Also, his tires are just old out of date BGF ATs with no grip. So that combo gives him 6.69 x 2.36 x 3.5 = 55:1. He's pretty happy with the ratio. He only complains about the lack of a front locker and the poor pavement manners of Grizzly in the rear. It shifts the main drive from one rear tire to the other seemingly every time his shifts a gear or turns a corner. For mainly offroad it's a good choice tho.

 

[Broncobowsher]

You opened a whole can of worms when you said you have not bought stuff yet and asked about low gearing.

The 203/205 is generally a heavy poor mans way for gearing. In the end it isn't that low. Less than 4:1 combined. A 4.3 Atlas will out gear that combo Pack that behind a ZF 5-speed and drop the axle gears down around 5.13 or 5.38 and you just saved a lot of driveshaft length and have a lot of gear, and can still do some highway.

There are so many ways to do this.

If doing a reduction box I really like to see it as a different ratio than the transfer case. This gives options. Typical planetary setup of 2.7 gives a normal low range, 4ish is a deep low range, together is a really deep gear. A friend has a Rubicon that came factory with the 4:1 transfer case. He found that normal desert wheeling it sucked, too low. Needed something between. Got a Rubicrawler box (2.7:1) so he can have a normal low range. And when things get stupid, go for both.

 

[BroncoManSam]

Not much different in the end result. 70mph on 35" tires is 3200 rpm with 4.56 and 3000 with 4.30. For mostly off-road I'd go with the lower gears.

If you were doing more highway miles or want to run 75 - 80mph, then the 4.30 may make more sense. But then, a 5 speed overdrive trans with 5.14 gears would be better.

That’s sorta what I imagined as well, I think I’ll stick with the 4.56 gears; they seem like a good choice with my set up for about 95% off-road and 5% on road.


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[BroncoManSam]

You opened a whole can of worms when you said you have not bought stuff yet and asked about low gearing.

The 203/205 is generally a heavy poor mans way for gearing. In the end it isn't that low. Less than 4:1 combined. A 4.3 Atlas will out gear that combo Pack that behind a ZF 5-speed and drop the axle gears down around 5.13 or 5.38 and you just saved a lot of driveshaft length and have a lot of gear, and can still do some highway.

There are so many ways to do this.

If doing a reduction box I really like to see it as a different ratio than the transfer case. This gives options. Typical planetary setup of 2.7 gives a normal low range, 4ish is a deep low range, together is a really deep gear. A friend has a Rubicon that came factory with the 4:1 transfer case. He found that normal desert wheeling it sucked, too low. Needed something between. Got a Rubicrawler box (2.7:1) so he can have a normal low range. And when things get stupid, go for both.

Haha I’m starting to notice that, there’s probably a million options out there. I’m leaning towards the NP205 with the Titan underdrive, because I’d have the option to have crazy low gearing when both the T-case and underdrive unit are both in low, but I’d still have decent gearing when both are in high. Same thing with the NP205/203 doubler, just a higher gear ratio, less than 162:1. At this point, both seem like great options. Both are cheaper than the Atlas twin stick with the underdrive unit, I guess it’s just a question of if I want low low gearing for a more, or a little higher gearing for less.


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