Re: T-case as overdrive/Brabnod Mt. Gear


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Posted by Charlie on Tuesday, January 30, 2001 at 2:53AM :

In Reply to: Re: T-case as overdrive posted by Matt Wilson on Tuesday, January 30, 2001 at 0:01AM :

Read this long note if you want to know "what's out there"; if you're not curious, don't read it, but don't get mad at me just because "no-one needs this crap".

As I mentioned in an E-mail to Don, I have my concerns about the longevity of the reversed TC in terms of lubrication because the direction of rotation would be reversed and I strongly suspect that would mess up the oil flow. Also, I couldn't really make an assessment of its' strength with reversed power flow. I think lubrication would kill it before breakage. Other concerns: Gearing - 4th would be useless unless the truck is repowered with a strong diesel or a big V8 with final gearing of 2.98:1; the next gear would be about 5.07 overall. You'd be starting 1st. And, the ultimate problem: space. Where would you put the thing? If "divorced", it would require a minimum driveshaft of 7-8" on each side plus its' thickness of about 12" yoke to yoke. The "real" transfer case would be at absolute minimum about 14" further back. And one more consideration: ideally, a multispeed trasnmission should have CLOSE gearing at the top end (higher speeds, lower torque multiplication, each gear is a wider mph spread). My gear splits on my Clark (which is NOT an ideal example) are 1.72, 1.88, 1.86, 1.00, 1.25. A better example is the 5 spd in our diesel Landcruiser: 1.88, 1.83, 1.52, 1.18. The ideal transmission for a M37 or Power Wagon (especially with an industrial type 4 cyl diesel with governed rpm at 2800 or even 2500 in the case of the Cummins 3.9BTA but high torque output) is a 6 spd with 6th overdrive. The biggest problem with my 5spd is the jump from 3rd to 4th. A good way around this is to use your stock transmission with a Ranger overdrive in front of it (0.73 OD ratio, plus direct) which will give a 3rd-OD ratio of 1.24 which gives a 1.37 jump, then a 1.24, then another 1.37. The Eaton/Fuller FSO-6406A 6spd has ratios of 7.52, 4.13, 2.51, 1.59, 1.00, 0.78; NV5600 is 5.63, 3.30, 2.04, 1.39, 1.00, 0.74; ZF S6-650 (Ford and GM Duramax diesel, also GM 8100 Vortec) 5.79, 2.10, 1.31, 1.00, 0.72. All cost twice as much as a Ranger ($1500) and are 5-6" longer than the NP420/Ranger combo.
The Gear Vendors OD is mounted on the rear output of the transfer case. It costs over $2000. It only works in 2WD, uses planetary gears (they say they're "extremely strong" but I'm suspicious), has a built in pressure oiling system, has a smooth electric shifter with sprag clutching mechanism (allegedly shifts smoothly and quickly when under load just like an automatic). It's really designed like a little automatic transmission (0.78, 1.00) without a torque convertor.
One otehr interesting auxiliary gearbox other than a true Brownie. A Brownie is a big heavy truck or at least midrange truck sized auxiliary transmission which can be used as an up to 4 spd splitter. I think they make 3 and 4 spds. I don't think splitting a 4spd into 1.20, 1.00 and 0.85 would be terribly useful with something that weighs over 200 lbs and is huge. This other gearbox is the "Brabnod Mountain Gear". They barely have an internet site, it at least gives their phone #. He builds adaptors or compatible to NP205s for example and front adaptors or compatible to various 4spds. He uses big Eaton spur gears. It's nonsynchrobut he recommends an air shifter which is a good excuse to put in an air compressor. My F350 has a 4 gal front bumper and 125 psi from a York compressor and if I hadn't done the NP203/205 (much simpler, cheaper and "lighter") I would have done the Mountain Gear/205. The interesting thing is the ratios. He offers, for instance an overdrive of your choice, like 0.80 (increments or decrements of about 0.04, whatever it takes to allow nice "short" axle gearing), 1.00 of course, and two other gears, Like for example in one installed in a Bronco about 2.5 and 3.6. But he can go down to 7.9. So for example 7.9 and 3.8 along with the 1.96 of the 205 will get you low ranges of 1.96, 3.8, 7.0 and 15.5. So, with 5.13s instead of 4.88s and maybe a 0.88 overdrive I would have ended up with 454:1 ultimate low range instead of a mere 110:1, and I would be learning to shift back and forth between the close overdrive and direct to split 3rd, 4th and 5th. Top end ratios would be 1.00, 0.88, 0.76, 0.67. Nice for high speed driving with 5.13s and 11.00R16s. the split on 3rd would be a nice 1.42.
I don't think ANYONE needs this!!! I just want all of you to know what's out there.
Charlie



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