reverse transfercase- homemade overdrive
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Elder User
Joined: Jun 2008
Posts: 694
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From: Vashon, WA
reverse transfercase- homemade overdrive
I'm building a '59 f250. I've got it setup with a rockwell dually axle from a '60 f350. The ratio is a 5.83 which means I'll probably top out around 45.
Could I mount a transfer case in reverse that way when in 4hi/2wd I'd have a 1:1 ratio but when I shift to 4low I'd have an extreme overdrive.
I go to school in Montana which means a lot of long flat highway driving.
Bottom line is I need AT-LEAST a 2:1 overdrive, I'd really like a 2.5:1, and this seems like the easiest/cheapest way to get it.
Any flaws in this plan? Also what is the ratio on most tcases?
The truck is 2wd
Could I mount a transfer case in reverse that way when in 4hi/2wd I'd have a 1:1 ratio but when I shift to 4low I'd have an extreme overdrive.
I go to school in Montana which means a lot of long flat highway driving.
Bottom line is I need AT-LEAST a 2:1 overdrive, I'd really like a 2.5:1, and this seems like the easiest/cheapest way to get it.
Any flaws in this plan? Also what is the ratio on most tcases?
The truck is 2wd
Cargo Master
Joined: Jun 2001
Posts: 2,571
Likes: 22
From: Oxford, Indiana
To accomplish mounting it backwards, you'd have to have a divorced mount transfer case. One of the most common divorced mount cases is the NP205. It's low range ratio is 1.96:1. That would translate to an overdrive ratio of .51:1. Most normal overdrive ratios are in the range of .65 to .75. If you did this, you would have a final drive ratio of 2.97:1 in high gear with overdrive engaged. If you figure that an average sized F-250 tire is around 31" tall, then you're looking at the engine turning 1931 RPM at 60 MPH. I don't think you'd want to attempt using a transfer case with a low range any lower than that as you would be pushing awfully close to lugging it.
One thing to remember is that unless you're going to stuff a lot of torquey beef under the hood, that overdrive is going to be useless with a load. You can only shift in and out of low range in a transfer case at a dead stop. You would have no way to shift into overdrive if you were tugging something heavy enough that you had to start off in 1:1 in the transfer case to get it moving.
One thing to remember is that unless you're going to stuff a lot of torquey beef under the hood, that overdrive is going to be useless with a load. You can only shift in and out of low range in a transfer case at a dead stop. You would have no way to shift into overdrive if you were tugging something heavy enough that you had to start off in 1:1 in the transfer case to get it moving.
Elder User
Joined: Jun 2008
Posts: 693
Likes: 6
From: N E Ohio
Can be done with an NP205, but a few things to watch out for.
Lubrication can be an issue, depending on how you mount the thing. The best arrangement lubrication-wise is the original orientation, so you may have to reroute exhaust or some other things. For high speed operation you may have to add an external cooling loop and heat exchanger, becaues normally the low range gearing is at a lower output speed than input speed, and you are reversing this. Noise is another issue, these do tend to whine in low range, and back-driving teh gearing can aggravate this. Another item to address is parasitic losses. If your goal is better economy at speed, this setup is not too efficient. Any particular reason for the particular rear axle you are using? If you want DRW, a Dana 60 with the proper hubs or adaptors gives you a better selection of ratios, as will a Dana 70 or the Ford Sterling DRW axle.
Lubrication can be an issue, depending on how you mount the thing. The best arrangement lubrication-wise is the original orientation, so you may have to reroute exhaust or some other things. For high speed operation you may have to add an external cooling loop and heat exchanger, becaues normally the low range gearing is at a lower output speed than input speed, and you are reversing this. Noise is another issue, these do tend to whine in low range, and back-driving teh gearing can aggravate this. Another item to address is parasitic losses. If your goal is better economy at speed, this setup is not too efficient. Any particular reason for the particular rear axle you are using? If you want DRW, a Dana 60 with the proper hubs or adaptors gives you a better selection of ratios, as will a Dana 70 or the Ford Sterling DRW axle.
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Elder User
Joined: Jun 2008
Posts: 694
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From: Vashon, WA
reason for using this axle is because it was free, it's pretty much already a done deal.
The goal is not so much better economy at high speed but to be able to drive at high speed. The way it is now I'm pretty sure I'll max out around 45.
I want to be able to drive around 80 when I'm on the long flat stretches.
I realize I can't shift on the fly but if I have that big of a load then I'm not going to need to go that fast on the highway.
I'm running a 292 y-block.
I was planning on using the 205 but are there any divorced tcases out there with a lower low range ratio?
The goal is not so much better economy at high speed but to be able to drive at high speed. The way it is now I'm pretty sure I'll max out around 45.
I want to be able to drive around 80 when I'm on the long flat stretches.
I realize I can't shift on the fly but if I have that big of a load then I'm not going to need to go that fast on the highway.
I'm running a 292 y-block.
I was planning on using the 205 but are there any divorced tcases out there with a lower low range ratio?
Elder User
Joined: Jun 2008
Posts: 693
Likes: 6
From: N E Ohio
With a stock 292 and a 2.5 to one final drive ratio, installed in a truck with the aerodynamics of a cinderblock (I know, I have several) I think 80 is a stretch.
Another thing to look at is that you will be spinning the gears in that old axle faster then they were intended to operate for long stretches. With 31" tires, at 80 the tires rotate at close to 870 RPM. That means wth your 5.83 : 1 axle, the driveshaft is spinning at close to 5050 rpm. You will have to balance the driveshaft for those speeds, and possibly put a cooler on the axle to keep oil temperatures reasonable to keep bearing and gear life reasonable.
As to a transfer case with more reduction, look at the one used in the 80 and 90s Fords (Borg Warner 1345 and 1356), they have a 2.72 : 1 ratio. Also, some Dana 20 transfer cases had around a 2.5 : 1 ratio.
If you go ahead with this, keep us posted, I am curious how it will work.
Another thing to look at is that you will be spinning the gears in that old axle faster then they were intended to operate for long stretches. With 31" tires, at 80 the tires rotate at close to 870 RPM. That means wth your 5.83 : 1 axle, the driveshaft is spinning at close to 5050 rpm. You will have to balance the driveshaft for those speeds, and possibly put a cooler on the axle to keep oil temperatures reasonable to keep bearing and gear life reasonable.
As to a transfer case with more reduction, look at the one used in the 80 and 90s Fords (Borg Warner 1345 and 1356), they have a 2.72 : 1 ratio. Also, some Dana 20 transfer cases had around a 2.5 : 1 ratio.
If you go ahead with this, keep us posted, I am curious how it will work.
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Elder User
Joined: Jun 2008
Posts: 694
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From: Vashon, WA
well I think I'll just get a 205 for now. My tires are pretty much shot so I'm going to see if I can find 33's just to give me a little more traction as well as gears.
I've been running the numbers and I think with the 1.96 tcase I should be ok for now, I can always change stuff out later when I know how it all runs.
Not sure where to find them but I was reading that they also make 5.14 and 4.86 gears for my axle. Either way I'd like to have an overdrive.
Has anyone ever heard of a tcase being used as an extreme overdrive?
I've been running the numbers and I think with the 1.96 tcase I should be ok for now, I can always change stuff out later when I know how it all runs.
Not sure where to find them but I was reading that they also make 5.14 and 4.86 gears for my axle. Either way I'd like to have an overdrive.
Has anyone ever heard of a tcase being used as an extreme overdrive?
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Post Fiend
Joined: Jun 2001
Posts: 7,493
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From: Erie, pa
I have heard that some folks have mounted a standard shift three speed backwards behind the regular tranny - I never pursued the ratio math to see what the numbers would be - I would think that it would have to be a straight cut gear/crashbox style.
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Fleet Mechanic
Joined: Mar 2010
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From: Jamestown, IN
My grandpa had a 35 chevy pickup that he put a 331 hemi in in 1958. He told me that he put in a three speed backwards behind the other. He said if you could get it in first it would go 80 no problem, but you could not start off in first because the ratio being too high. He also said that if you put them both in reverse it would go forward!! He said he took it out when he tore out first in the main trans so he drove it as a two speed. He has some great stories.
Elder User
Joined: Jun 2008
Posts: 693
Likes: 6
From: N E Ohio
That sound like a better idea. An old 3 speed, something like the Ford 3.03, would work better than a transfer case. Helical gears can turn either way. And if you want to go extreme, a T10 4 speed would also work. To adapt the input shaft as an output, just go to a shop that does driveshafts and get a yoke with a pilot bore. Get it splined to match the shaft spline, and have retaining ring grooves machined on the input shaft to hold the yoke in place. The linkage should be no big deal, and with a synchro box, you can shift on the fly.
Still, driveshaft balancing and proper u-joint phasing will be critical for the speeds those will be spinning at. As close as possible to a zero driveline angle will also be necessary, and I would strongly recommend a driveshaft safety loop.
Still, driveshaft balancing and proper u-joint phasing will be critical for the speeds those will be spinning at. As close as possible to a zero driveline angle will also be necessary, and I would strongly recommend a driveshaft safety loop.
Hotshot
Joined: Mar 2001
Posts: 13,542
Likes: 76
From: Manhattan, KS
The overdrive sounds like a neat idea. An easier way to get higher top end, but you lose the low in gearing would be to just swap in a higher ratio dually axle from a F350. One from a '72 or earlier would just about be a bolt in.
New User
Joined: Jul 2011
Posts: 2
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Hello everyone, I am new here,but I just had to join to get some advice for my van about installing some type of an overdrive unit (and I don't have an extra $3 grand to buy a gear vendors unit-how long would it take for that to then pay for itself?). I have a 79' Ford E250 Econoline van with a Pathfinder 4x4 conversion. It has a 460 big block carbureated,C6 auto and a NP205 transfercase. But due to the rise in gas prices I need to find a way to install some type of overdrive to help save gas. I really like what someone said about mounting a 3 speed manual backwards-does anyone have any more info. on how to do this? Does anyone have any advice? Thanks!
Cargo Master
Joined: Aug 2010
Posts: 2,916
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From: Houston
I don't like the idea personally. Manual transmissions are designed for a specific gear rotation to lubricate the bearings. If you flip the ends you can end up running one or more of the bearings dry. Also, the transmission was designed to hold up to the torque put out by the engine, not the torque after being multiplied by another transmission. Your 460 puts out what, 400+ ft-lbs of torque? The torque converter and C6 will multiply this by at minimum 2.5 times in first gear. That means the reverse turning transmission has to hold up to 1,000 ft-lbs of torque coming into the wrong end of the transmission. I don't think it will last long. If you look at any of the purpose built manual auxiliary transmissions you will see the gears and bearings are very big to withstand the high torque input.
Posting Guru
Joined: Feb 2009
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From: victoria
back in the late 60s we put a standard 3 speed behind the cruise a matic in my 55 ford trying to get more speed ..2 slip yoke's back to back the the trans with a clutch center welded into a rear shaft ..it worked we played with it for a while till we found out the stearing and brakes were not up to the 390 in front..and something about god protects "children and idiot's "I don't know how long it would last on the highway ,,we never got more then 30 or 40 miles from home..the back drive shaft was about 2 foot long ..damm we were stupid....
Tuned
Joined: Sep 2010
Posts: 374
Likes: 3
Hi 
For many years I have heard of people wanting to try mounting manual transmissions backwards to obtain an overdrive. Personally I think it would work alright in off road or farm machinery but not the answer for highway use. "A" boxes have very short input and output shafts. Picture the long pilot shaft, which is normally supported on both ends, on a regular transmission with a yoke and drive shaft flopping around on the end of it. I don't picture it as being very safe or reliable. Sorry.
For many years I have heard of people wanting to try mounting manual transmissions backwards to obtain an overdrive. Personally I think it would work alright in off road or farm machinery but not the answer for highway use. "A" boxes have very short input and output shafts. Picture the long pilot shaft, which is normally supported on both ends, on a regular transmission with a yoke and drive shaft flopping around on the end of it. I don't picture it as being very safe or reliable. Sorry.