12valve@heart

OK Gents & Ladies, will it work?

I've been wanting an additional overdrive for quite some time in addition to the fact that the spacing of the gears in the Getrag 360 is less than ideal for towing. I've looked at the GearVendor Over/Underdrive units and they seem like what I need but I hate to put that kind of money into an auxillary transmission SOOOOO, I got this idea from a streetrodder buddy of mine.

Back in the late '40s thru the late '60s, car and truck manufacturers ranging from Studebaker to Ford to Chevy used three speed manual transmissions with an electrically actuated overdrive. The idea that my buddy suggested is to replace the carrier bearing with a 3-speed transmission. If you shift it into high (direct) and wire the overdrive to a switch, you end up with the potential for direct and over in every gear just like a GV. If you add linkage, you have the potential for direct, over, and two underdrives in each gear; something that might be handy for backing a trailer.

My question is, will an automotive or light truck 3-speed manual transmission handle the torgue of a Cummins and does anyone see any specific problems to this idea? Does anyone know of any other transmission that could be made to fit behind the Cummins that has a double overdrive?

Let's hear it!

edwinsmith

On the chevy 3 speed with OD the overdrive unit was a planetary gearset added in the tailshaft housing. It had a lever and a solenoid to activate it. It also had a overrun clutch when in OD so it wouldn't be any good for using an engine brake. Those trannys were really light duty units used in the passenger cars and 1/2 ton trucks. Not hefty enough for a Cummins IMHO.

What would be nice would be an over/under rearend which I have seen on some medium duty trucks. They were electrically controlled from a push/pull switch on the gearshift. My dad had one in his big rig when I was a kid.

59FORD

It will never take the CTD torque.
I don't believe you will make it through the gears without failure.

You might look at older medium duty trucks, lots of them had aux trans
with big gasser engines.

Hemi Cat

What ever trans or OD unit you put back there would have to be able to handle the torque output in low gear.If your eng puts out 600 lbs of peak torque, and low gear is 3:1, you get 1800 lbs of torque out the main trans. You need a pretty hefty aux trans or OD unit to take that much torque. You definitely need to look in the med to heavy duty trucks.

Jim Lane

I always wanted to install a 2 speed either electric or air shift rear axle from a larger truck, say maybe 2 1/2 ton it would definately handle the torque. You would just have to figure out the shift pattern because because of the spread on the ratios, also it would be a trick getting a larger pumpkin under the bed.

Check this clean setup.
http://www.drivetrain.com/new_overdr...g3spdauto.html
The old dragsters in the 60's and 70's used to run an external planetary overdrive that was air actuated but I can't remember who made them.
Jim.

wannadiesel

I'm with Hemi Cat on this one.

Those electric overdrives were really weak. Checker offered them up until the early 70's, but would not equip 327 or 350 powered cars with overdrive because the high torque would rip the stop pin for the ring gear out of the housing. Only 283, 307, and 6 cylinder cars were available with overdrive. The basic 3 speeds weren't that strong either. When the 350 came out they stopped offering the manual with it after 1 year. It seems the countershaft would punch out the bottom of the case under heavy throttle in 1st.

A trans like you're thinking of is known as a brownie. If you have a heavy truck junkyard near you they might have something for you. The problem will be finding one small enough that you don't need to cut a hole in the bed.

Alec

Actually, there will probably be plenty of room for the auxilliary transmission under the bed, but ground clearance might be a problem.

This would probably be your best bet: Spicer 5800 Description And you can buy them from: Goodys

I have wanted to make a three speed transfer case that would go on the back of an NV4500 for years, but I don't think there would be any demand . . .

Transmissions used to be rated by their maximum output torque. This combined with their highest numerical gear ratio would give you the maximum input torque they could handle. HP rating may also necessary, as the transmission has to be able to shed the heat lost from internal friction, which is directly proportionate to the horpower it is transmitting.

None of the little OD units from cars and pickups would be appropriate for application with the 6bt.

dix39

Hi again Jay, I also would agree that the auto/light truck units referenced would not be strong enough. I'm with Alec, there were a lot of aux overdrive units built for larger applications through the '60s and 70's. These ranged fron 2 to 4 speeds with top gear generally being overdrive. They were sold as Browning (Brownie?), Watson, and probably other names, but from my experiance with a 2 speed Watson, I believe the parts in them came from Dana/Spicer. The problem I had with the Watson, which provided a direct and over or direct and under, depending on which way it was installed, was that the gears finally wore out and new ones weren't available. This was probably one of the smaller of the aux units and was designed for up to 1 1/2 ton trucks. Even it would be "light" for the Cummins. I used it in a F 250 4x4 I converted to Diesel in the early '60s. The Ford had a 6 354 NA Perkins, International 5 speed OD main trans, and the Watson behind that set up for direct and over. This ran through the stock 4.10 diff gearing, and ended up with about the same top gearing as a 5 speed 3.54 Dodge. I hauled a 11.5 foot camper and towed all sorts of things all over the US with it from about '82 'till '94 when I bought a '89 Dodge Cummins. The Dodge has done the same thing the Ford did up to now, with a heavier camper, but it isn't limited to 65mph on the level when loaded because of power. There have been a few times I would have liked a "splitter" with the Dodge, but I'm really pretty happy with it the way it is. I do plan to install the intercooler from a '91 that is currently in my front yard causing marital problems. Enough rambling.

I think where I was going with all that is, that I gave the Ford to my son after I got the Dodge and he found a 3 speed aux that he plans to install in place of the worn out 2 speed Watson. These things are available, often for good prices and sometimes in strange places. I bought the Watson at a garage sale. If Jay's truck is a 2 WD, and it sounds like it is from the description, installing one of these shouldn't be a problem. One thing to be aware of, is that adding another overdrive will place additional loading on the main transmission overdrive. Also, when I was putting the Ford together I did a lot of number crunching to determine the combinations of gearing would provide the RPM/MPH combinations I wanted, before I started looking for the gearboxes. Just some things to think about.

Sounds like a fun project, good luck with it.

Steve

12valve@heart

Well, I guess I'll have to start keeping my eyes open for a brownie to tuck under my bed. Wonder if I'll be able to break 25MPG running empty in double over?

timeagain

I have a US Gear underdrive unit for my truck, they also have overdrives. All you do is hit the switch and you have OD with a bang. they take the torqe, simple and easy.

Vulcan

I'd look around for an old rig with twin sticks thats parked out in some farmers field or something. I bet you could get an aux tranny cheap and those would definitly handle the torque. I know they came in 2, 3, and 4 speed in the big trucks. Not sure how well they would fit under a dodge though.
Vulcan

Begle1

Just get a lift kit... Problem solved.