Not sure if this helps or hurts, but I did a tech a while back. I modded an Aux O/D off a Ford tranny to the back of a TH400. I needed a heavier duty tranny and wanted O/D but couldn't run the 700r4 because of a TV cable issue. Squirrel pointed out the the weaknesses of the old Aux O/D's but I never had any trouble. I have since sold the truck that I ran this in, but kept the drivetrain. I broke this thing apart maybe 7 months ago and checked the wear, and it was very minimal. And will be putting it back into a project i am currently chugging along on. http://www.jalopyjournal.com/forum/showthread.php?t=77856 '92 Chevy Truck TH400 out of a C3500 (1ton) this gets you the HD TH400 Case. The case is much heavier and stouter than the regular TH400 case. I also procured a mid '60s Ford truck 2 speed with the AUX. Overdrive unit that came stock from the factory. The Big 3 all had AUX overdrives on various vehicles up until O/D trannys became standard equipment. Look under trucks and vans. One thing to know GM in the '80s actually had some Camper Specials 1-tons that came with Gear Vendors units factory on a TH400 (The Holy Grail for this swap) Last junkyard part I needed was a longer tailshaft for the Chevy TH400. I went with a '70s model Cadillac TH400 for the tailshaft as its longer so I had enough room to respline. I suppose you could go with the tailshaft out of a BOP TH400 that has the switch pitch as I suspect its longer as well, just don't qoute me on that. Now find yourself a machine shop that will respline shafts for you. Mine charged me $180 to anneal the hardened Caddy shaft respline, then reharden it. You could CRYO if you want I didn't. You need to have your machine shop respline the Caddy Shaft to the same spline count as the AUX Overdrive that you have. Take your O/D unit with you to the Machine shop with your Caddy tailshaft and say you want them to match and you need a coupler to slip between the two. Since I was rebuilding my tranny I waited until I had my tailshaft and coupler back and rebuilt the TH400 with the new Resplined Caddy Tailshaft replacing the shorter stock Chevy unit. Now for the fun. I made a template of the rear of the TH400 housing and the front of the OD unit. Then con'd a friend with access to a mill into whittling out two blocks of aluminum to my patterns. Cost: Case of beer, and helping him down the road. He said it'd probably be about a $300 charge most places otherwise. Now bolt it all together. From top to Bottom; TH400 with Aux. O/D mounted TH400 Ford Tranny with Aux. O/D Fully mounted together, the aluminum adapter pieces The business end of a TH400 with the longer resplined Caddy TH400 tailshaft Coupler for hooking the TH400 tailshaft to the AUX. O/D Input shaft. Couple slid on, and note the factory lip on the TH400. Tranny end of the Adapter plates, note lip to accept the lip on the TH400 This is the end view of the TH400 with the Adapter bolted inplace. The O/D unit Adapter for the O/D unit, note the lips machined into to accept the O/D unit. O/D unit, note the two lips that the stock Ford unit had cast into it. This is a top view of the adapter plates. O/D unit slips right on engaging the collar. Tighted the bolts and viola.
Another thing you could do is find a divorved tranfer case off an old 4 wheel drive. Open the case and take the guts that would make the front output work. Then cut that part of the case off and weld it closed. Now turn the case around backwards and mount it up. Have an intermediate driveshaft built from your tranny tailshaft to the transfer case. Only problem is that the gearing would be really really steep. It could be way too much Over drive. Just something I've been toying with for years, and haven't done it yet.
interesting idea, but those transfer cases run something like 2.x to one gearing, so it would be way steep, and a heck of a drop in rpm. How about just getting a second job to pay for gas and forget about overdrive?
And that would just be running in highside on the transfer case, think how silly your numbers would get in Low Side. ha LOL Agree they are not super great alternatives but still possibilities.
I saw an old motor home (this was before they were "RVs") in late seventies that had a setup like that. Motor homes were not mainstrem and this kind of knowledge was passed among the cult by word of mouth. Seems like main tranny was an auto and the backwards manual they ran through the reverse gear to achieve overdrive.
Here's a link to the 4-speed swap article. I remember reading it when it first came out in the 70's. I've got the 3-speed trans. One of these days I'll come across a 4-speed cheap and give this a try. http://www.chevytrucks.org/tech/overdrive.htm
There are a couple of companies that build conversions for older British cars, using the de Normanville overdrive installed after the trans tailshaft, or even mid-car. This is a stand-alone overdrive, similar (very) to the Gear Vendors units and Volvo M46 overdrive units. Using these divorced-type overdrives would involve driveshaft fabrication, driveshaft tunnel mods and wiring.
I'm gathering some of the information here as we go along, but could someone explain this in more detail for me? I'm finding this an interesting concept.
does anyone have some pics of the difference between the saginaw 3 speed with OD and one without?, theres a guy who lives near me who crushes alot of cars and if i send him a clear pic so he knows what to look for he would save one for me, thanks in advance for your help.
I had planned on doing this to my Ultramatic in my Packard, using an R-11 O/D unit from a manual 3 speed O/D Packard straight eight. By the time I got the pricing for the new shaft and adapter to connect the two, I decided on another route to achieve O/D in my '56.... I'm putting a T56 in it.
Almost a four-year-old thread, I know, but I've been thinking about this recently. 1. Supposing that the gearbox is to be mounted in a "divorced" arrangement, wouldn't it make sense to mount the second 'box ahead of it? If you're looking for overdrive to reduce revs rather than increase top speed you wouldn't be running undue shaft speeds in either 'box; the second 'box would never be subjected to torque multiplied by the first 'box, so needn't be quite as stout; and the speedometer would read right all the time. The question is, how would one do it without the resulting assembly being unduly long? Perhaps a custom, double-ended input shaft running on a bearing in a mating plate? This assumes both cases have "ears" at the front like a toploader etc. 2. Would an overdrive on the second 'box be able to work as an underdrive "launch gear"? I've been trying to get my head around that one. Because the rotation would be reversed everything should work as usual, only backwards. I think the kickdown facility might do the trick if its action is quick enough, and if the sprag is strong enough. It might be necessary to use a fairly serious pawl return spring and an equally serious solenoid. The idea is for something to use with a tallish first gear, as one finds in a close-ratio gearbox, to restore some briskness off the line. It's usefulness would depend on an extremely fast and unmissable shift to direct drive. I see an on-off toggle on the dash, with a solenoid directly above it activated by a red button on the steering wheel. The drill is: select first gear; flick toggle up; feather clutch until you hear a clunk out of the underdrive; disengage clutch; bring rpm up; engage clutch. At the required rpm, press red button. That pops the toggle into the "off" position and, hopefully instantaneously, engages direct drive. Thereafter you grab second, third, fourth in the usual way. In other words, it's like having a Lenco for first gear, a stirrable four-speed for the rest, and then two overdrives on top of that, or seven speeds in all. Is it doable?
This is a photo of the Saginaw 4 speed with OD I built about 5 years ago.Very easy to do as mentioned in the article. 2.85 first gear that works out well in most situations.They are very rare for sure and the OD from the more common earlier 3 speed won't fit.. I adapted the stock handbrake for use in my 37 Chevy truck.The other tranny is a hybred Camaro V-8 T-5 with a S-10 tailhousing. I used the mechanical OD lockout mounted under the dash like a stock set up.The electric OD was on the shift lever,very handy.
I'm jot wrappiing my head around this I'm afraid and I'm sure that once I think about it it would make perfect sense. Are you thinking about ruinning them both straight forward and using it like a big truck tranny like say a 10 speed road ranger? The second transmisson being along the lines of a high and low range then adjusting your rear gears accordingly? I'm going to have to give this post some thought to get it right in my head. What we used to do was couple the tail shafts the way to shorten one up would be to use two identical trannys and make a stub shaft between them loosing the tailshaft housings on both. I don't recall seeing one that way what I have seen done is to couple the two using a U joint and two slip yolks. If you coupled them using a common stubbed tail shaft and bolt the cases together makes a lot more sense in a real world hot rod application. A little less backyard. It ends up being a heavy application but you could gut the second tranny using only 2 gears and lighten it up considerably.
What I'm seeing is, front gearbox is a three speed with OD, and it's mounted backwards. Reverse and attendant mechanism have been removed. Rear gearbox is a four-speed, mounted normally, complete with reverse, speedo drive, etc. In other words, it's like you said, only the backwards gearbox is slotted in ahead of the other gearbox. I don't see the OD - which would become an UD if it will work the way I hope - as a splitter but as a sub-first low gear, for acceleration off the line. That'd allow me a tallish first gear in the four-speed, which would make all four (close) ratios in the four-speed usable for slick corner-carving cog-swopping. But I don't know if an OD can be made to run backwards like that, and I don't know if the shift to direct drive could be made quick enough to make the whole set-up worthwhile.
I understand now I think. It seems doable but as you said making ths shifts quick would be almost impossible. Maybe with an air shift like they us drag racing. Even the way I had suggested it is not a quick shift setup. It is more akin to driving a big truck ith multiple sticks to deal with. Something that could be considered would be along the lines of a transfer case like a 4WD uses. They can be had dual range and with shift on the fly with later Jeep versions. You could loose the spliter from front to rear and only keep the high and low range, use low range like an under drive and gear appropriately. Transfer cases are easliy adaped to any number of transmissions they are cheap and readily available in most wrecking yards.
Hey, Ned; If you're thinking about using a BW R10 or R11 backwards, no it won't work. You'll destroy the pawls. = no OD, but lots of $$$ to fix. BW warned about *never* using the OD in reverse. As for the guys breaking them, were you using the R10, or R11? There is a difference in capacity. But these things were/are rated odd: like 20,000lbs. I'm guessing towing/GVWR? Never found a cross-rating for hp/torque usage. In the late 70s/early 80s', there was a guy that made bolt-on conversions for the TH350/400, C4/6, 727 using the R10. Or R11. Guy had a 6-71 blown 426 hemi mopar-something, & claimed no problems, even w/full throttle runs. Course, I doubt he was using the OD except as an OD for 3rd. I had gotten some plans for the conversion awhile back, but he still makes kits, if requested, last I heard. Article was in PHR, & also copied in a PHR Yearbook. Can't remember which one at the moment. Marcus...
I was more worried about breaking the sprag. The way I mean isn't really like OD + reverse gear, as all the gearing, sprag, etc. would be loaded the same way around as during normal operation. That's because the entire unit is turned back to front: it gets clockwise input looking at what's usually the rear of the unit. And the pawls would never be engaged at speed: to go from UD to direct you'd rather need to pluck the pawl out, whereupon the drive falls back on the sprag. And I reckon it should be possible to devise a way to pluck the pawl out with enough violence to effect a quick shift, with or without the ignition interrupt. Likewise the lock-up could be rigged to the shifter, so that any subsequent shift takes the torque off the sprag. But I'm not sure if I understand the gear/sprag thing properly myself ...
I knew a guy who did that on a 63 SS Impala. I never did see the unit and I do know there was some machine work involved. Pretty sure it was a T10 with a BW R10 od unit.
I gave it some thought and I'm sure it won't work. The reason is, any device where two ratios are in simultaneous engagement, like an OD or a Lenco stage, you can use a sprag to hold the lower ratio but not the upper. The advantage is that one doesn't need a separate mechanism to disengage the lower ratio. The principle is also used in automatic transmissions. The upshot is, one can't use a sprag to hold direct drive against a lower ratio. The only way my idea would work is to run a single-stage Lenco (i.e. 2-speed) without reverse or speedometer drive on the back of the engine, and then a divorced four-speed behind it - perhaps with an OD on that. Lencos free-wheel on all ratios except direct drive, which would be fine in this application, as I'd like to have engine braking normally but wouldn't need it in a launch gear. Well, so much for that. I still like the idea of a backwards gearbox as an OD, though. And I'm still wondering if a lighter-duty box would work if it sat in front of the main box.
Interesting concept, that, having an OD with a wide ratio spread so that OD+2nd represents a constant progression on 4th. Only thing is, how does the 4th-to-OD+2nd shift work? I've never experienced one of these 'boxes, but I can't think that it'll make for a quick or easy shift. And that is pretty much the middle shift in the progression, and presumably the one you're going to use most, up and down, when things get twisty. Hmmm. That's why my thinking was that a stout 4-speed's four ratios ought to be the middle gears, so you can go crazy upshifting and downshifting, and leave the unconventional shifts for the lowest and highest ratios.
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