Propane is a way to go that I am also considering. I wish there were more places to fill up, but my travelling is more local these days, so I may consider it. From what I understand, propane will allow you to go considerably higher than a 10:1 compression ratio and will reward you in better efficiency. I guess if one has a dual fuel setup with pump gasoline, then you have to compromise on compression. Fuel aside, large intake runners give rpm & hp, but kill low-rpm torque.

 

Can you tell me more about the overdrive unit itself? Like torque rating, is it a separate bolt-on, how is it shifted, etc? I think I'm like 20 years too young to know about it...

 

I think the CCR for propane is around 12.5:1, but the octane rating can vary depending on your source of fuel. I've personally never been a fan of dual fuel setups because you loose performance from both fuels. Propane can take more timing advance than pump gas, and being a gas instead of fuel vapor, it can give the engine a lightling throttle responce time.

In theory, it might be possible to make up for the difference in BTUs between the two fuels by taking advantage of the superior combustion properties of propane.

 

I'll tell you what I can from memory - if you are really interested in more info, I can probably find some diagrams. If I assume you know gear systems better than you do, do not hesitate to ask me to explain in more detail. (Like-wise, do not be offended if I say things you already know - there are others reading that may need these details in order to follow along.)

First what is an overdrive? Originally (and still much of the time) it refers to an auxiliary transmission, which could be integral (attached) to the main transmission, or in a separate unit, either in front or behind the main trans. The word overdrive is used in gearing to describe a gear-train that steps up the rpm and correspondingly reduces the torque level in direct proportion. This means that torque times rpm of the input shaft (subtracting friction losses) equals torque times rpm of the output shaft. There are overdriven gear trains in many places that are non-automotive.

In the first half of the 20th century a lot of focus was put into developing manually shifted transmissions. (After that automatics became the focus of development.) The 3-speed transmission with a column shift became standard in the 1930's for automobiles and light trucks. Everyone who drove a vehicle was used to the system.

Borg Warner made a lot of transmissions for almost all the car-makers and they realized that some people wanted more economy and still needed a low starting gear - stock engines were then well below 100 hp. Trucks had 4- & 5-speed transmissions, but they were heavy and expensive, plus they all had floor shifts which were not desired in passenger vehicles. BW's solution was to produce a variation of their 3-speed transmissions that had an overdrive section behind the main section. They often made these transmissions the same length as the plain 3-speed, so that at the factory these transmissions could be installed with no changes in shift linkage, or driveshaft length. For the users, the column-shift stayed the same and they could leave the OD locked out - I suspect many people did. The overdrive was used almost exclusively as a road gear to be used out on the highway.

Lately (last 50 years) most vehicles are automatics, but since 1974, better fuel economy has been desired, so many over-drive automatics have been designed & produced. One point I would like to make strongly, is that it really does not matter much whether top gear is direct or overdriven, the overall gear ratio, including the rear axle ratio, is what is going to determine cruising rpm at a given speed and hence, economy. People associate OD with economy. You can still get the same good economy by changing your rear-axle ratio if you have a low enough gear for starting. This is what I would recommend anyone with a 4-speed (grannie-style) truck transmission. You will occasionally start in 1st instead of second and 4th gear will be only used outside of town.

This is enough for this post. I'll talk about the innards a little later . . .

 

By proper arrangement of the gears, any transmission design can have an output shaft that is overdriven (turns faster) than its input shaft. Most factory installed overdrives in both American (made by Borg Warner) and European (made by Haycock) cars were based on the same planetary gearset used by Model T Ford transmissions and all passenger car automatic transmissions. The planetary gearset consists of a sun gear in the center, a ring gear (with internal teeth) on the outside and three or four planetary gears that mesh both the sun gear and the ring gear. There are a number of advantages to the planetary design, including compactness, little need for support because all the components are self-centering, and ease of "shifting" via various means. (I put shifting in qoutes because only the earliest transmission designs actually selected ratios by literally shifting the physical positions of the gears.)

The planetary gearset can be used in a number of configurations, but for overdrives it is used by driving the planetary carrier (a block of steel that locates all the planet gears & drives them as a set) from the input shaft. The input shaft actually goes through a hole in the center of the sun gear. The output shaft is connected to the ring gear. For "direct" or a 1:1 ratio, the sun gear is locked to the planetary carrier and the entire assembly rotates together. (In this mode, it is almost impossible to "blow up" one of these units and only the bearings have any wear.)

In order to prepare to engage the overdrive ratio the sun gear is unlocked from the planetary gear carrier by actually shifting it a short distance - this shift is done by the classic Overdrive cable, usually installed under the dashboard of the vehicle. If the sun gear is no longer connected to the planetary carrier, it would rotate freely and the vehicle would be in a neutral gear. In the Borg Warner design, there is a "one-way" clutch made of 10 (I think) rollers that are inside a bore that is part of the ring gear assembly (connected to the output shaft). On the end of the input shaft there is a multiple cam that causes a wedging action when rotated one direction, causing the rollers to lock up with the bore mentioned above and drive the output shaft. When the output shaft trys to rotate faster than the input shaft the rollers are unloaded and there is what is called "free-wheeling". This mode allows the vehicle to coast without the engine holding it back. (Think of a ratchet action if you cannot follow all this talk about rollers. It would have the same effect, but would be noiser.)

Because some people failed to understand how to use their overdrives and had problems on steep hills, many states outlawed any transmission that free-wheeled and this type of overdrive went out of production in the early 1970's. All one needed to do when approaching a down-grade was to take up the slack in the drive-train by stepping on the gas and then to pull out the overdrive lockout cable.

 

We are finally ready for the shift into overdrive. If the lockout cable **** is pushed in, the overdrive unit will free-wheel when the driver lifts his foot off the gas pedal. At speeds over about 27 miles per hour, the overdrive has a governor that closes its contacts, energizing a relay, which sends current to the overdrive solenoid. This solenoid (& the governor) are visible, external components on the overdrive. The solenoid pushes in on a large square "pawl" - a block of steel which engages a pawl-wheel attached to the sun gear. This pawl is well supported in the case and absolutely prevents the sun gear from rotating. In this mode, the output shaft (attached to the ring gear) turns one turn for every .7 turn of the input shaft. There is a "blocker ring" that acts to only allow the pawl to drop when the sun gear is not rotating to avoid any grinding.

From the driver's point of view, when you want to use the overdrive, the cable **** is pushed in. At speeds below 27 mph, the overdrive free-wheels when the driver lifts his foot off the gas pedal. At speeds over 27 mph, the driver lifts his foot off the gas (as when starting to do a shift in the main transmission), but does not do anything with the clutch or any other control. Almost immediately there is a slight "clunk" as the pawl drops in and there is a little engine braking until the driver feeds in on the gas pedal again. With very little practice, this can be done with no noticable jerk. This clutch-less shift is satisfying - the engine rpm is low and you are ready to burn up some road.

If you come to a hill, or need to pass, you just press the gas pedal all the way to the floor, tripping a switch that kills the solenoid, dropping back into direct. (Actually this switch briefly kills the engine, removing the torque load on the solenoid pawl. The solenoid is not energised and a large spring pulls the pawl out of engagement.) At this point the engine starts to rev up, but is stopped by the one-way clutch in the overdrive, taking up the load. This feels pretty much the same as doing a kick-down in an automatic and has the same effect - you drop down a gear almost instantly by just pressing on the gas pedal. This gives you a two top gears (3rd & 3rd-OD) that you can shift up and down by just manipulating the gas pedal.

If you get below 27 mph, the solenoid is deactivated by the governor and you are in direct mode with free-wheeling. One important function is the reverse lockout that locks out the whole overdrive to allow for backing up. These units are pretty solid and reliable. It may seem complicated, but it does not take much time before using an overdrive becomes something you do without even having to think.

 

Chad,

I can answer any other questions you might have. I never heard of breaking even one of the three-planetary units. If you run in OD with no oil, you can burn the gears up. One can use 2nd-OD, but it usually is pretty close to normal 3rd. These units can be shifted manually with a toggle switch, by-passing the governor, but in very low gears one might be able break something, say by punching the gas in "grannie"-OD. I doubt you could break the unit when locked out - I have pulled some heavy loads (13,000 GCVW) up steep driveways with no problem. (Say 5,000 ft.lbs. through the OD.)

These overdrives are designed onto the back of a previously designed three-speed transmission. There is a different mainshaft in the transmission. It would be possible to machine up a double-bearing carrier to properly support the overdrive's input shaft - this would be bolted on the front of the OD and a yoke could be fixed so that the OD could be installed in the middle of two driveshafts. I personally prefer the simplicity of an integral mount.

There are also the Haycock overdrives made in Europe. They turned up in all sorts of European cars and, (would you believe?) in mid-'70's AMC cars behind a full-sychro Ford main trans; see:

https://www.ford-trucks.com/forums/5...d-euro-od.html

 

I still don't get why overdrives like this were not avalable through the 1980s. After reading all this, theres no excuse in my mind for trucks like mine that came with C6 transmissions.

Are these units hard to find?

 

Let me amplify . . .

I have driven MANY miles with overdrive transmissions and think that they are still very useful. One can get similar mileage gains by re-gearing the rear axle - easy to do with a 9" Ford rear end with a removable differential carrier, harder with the integral carrier axles, and a definite pain with a 4x4. There are also transmissions with more gears, if that is what you want. If you want to change only the transmission and want a 4-speed with top gear overdrive, Ford made one for you - a top-loader with top gear overdriven. (The only down-side, if you can find one of these rare transmissions is that, because top gear is not direct, the gears are running continuously under load and I have heard that sometimes the roller bearings between the mainshaft and the input shaft give problems. I would not recommend this nice trans for a heavy hauling vehicle, but otherwise, it is a nice transmission, but hard to find.)

I used to wonder why they suddenly stopped making the Borg-Warner OD transmission in the early '70's. In these FTE forums, I started finding references to OD's being "outlawed". I was an adult by the time the '70's rolled around and I did not remember any public debate about outlawing overdrives. The '74 gas "crisis" was a reason to keep OD's, not outlaw them, so I was more & more puzzled. Finally a post mentioned outlawing freewheeling and it all became clear.

Since the '50's & earlier, some states have had a law against "freewheeling". This freewheeling includes a transmission that is designed to freewheel (as the BW OD's) and coasting with the transmission in neutral. (Is there anyone out there who is not a criminal under this law?) I can remember studying CA laws in preparation for my first driver's license test and thinking: "What a stupid law! One more unenforcible, confusing, idiotic . . . ") The percieved problem is that when freewheeling or coasting in neutral on a downgrade, the driver will be unable to re-engage a gear in time to prevent a runaway condition.

I know how easy it is to take up the slack in a free-wheeling vehicle and lockout the OD. You ALL know that re-engaging a synchronized transmission is very easy. The only REAL problem dates back to non-synchronized transmissions, in trucks that had a small, low-revving engine, weak brakes, and a heavy load. These often did run away. Once an old truck picked up speed, the driver could not rev the engine up to a high enough rpm to double-clutch into a gear that would hold it back on a down-grade. To this day there are signs just before a downgrade reminding truckers to down-gear before the grade.

Unfortunately, the government is here to protect us from all dangers, real or imagined. I cannot quote facts at this point, but I believe that, after years of not seriously enforcing the anti-freewheeling laws, that one or more state officials informed Detroit that the law would be enforced and Detroit took the easy way out. AMC had an unusual response - the Haycock-built, de Normanville-designed overdrive, which was used on many European vehicles (see link in post #22 above), including Volvos, was spliced onto the back of a Ford top-loader three-speed. A Ford transmission that Ford did not use their own vehicles.

Thanks for letting me rant . . . (and please do not get caught coasting in neutral.)

P.S.: The BW OD (T-86 & T-85) transmissions are still in vehicles, both running and in junkyards. Some FTE members live in areas where the junkyards keep large numbers of old vehicles, so ask questions about locating an overdrive and make a "road trip" next time you have a vacation. I'll put my T98/T-85 in the FTE classifieds if anyone is interested it it. It is in good order, but I will not gaurantee you can't break it - I did once! I am moving on to my Roadranger 10-speed.

https://www.ford-trucks.com/forums/5...n-f-350-a.html

 

So it can be either cable shifted or pushbutton/solenoid shifted?

Doesn't quite sound like the best companion for my ZF 5 speed.

 

Chad,

When the cable is pulled out, overdrive operation to totally locked out. With OD locked out, the trans operates like a straight 3-speed in every way. When the cable is pushed in the combination of the governor & solenoid give a convenient semi-automatic operation. If you hook a toggle switch between the battery and the solenoid, you can shift almost manually. By this I mean when you are ready to shift, you flip the switch, lift your foot off the gas, and the OD shifts. To shift down, just flip the switch off, lift your foot, and the OD downshifts.

One tip: put a fuze in the line - at age 19, I found out that when a raw sheet-metal edge saws through the insulation of the wire, 3-feet of wire insulation makes more smoke than you would believe possible, with the copper wire glowing red hot.

All an OD would give you is one more higher gear. You probably do not need another gear to shift into. You could consider a different rear axle ratio if you need a higher road gear, say, for economy on the road. (This would entail raising all your gear ratios, so you would not have as low a bottom gear as you do now.) I do not know what your current rear axle ratio or the ratios of your ZF are, but I am sure other people on these forums have been in your position.

 

Bump, because I'm a stone cold techno-geek and this is some cool stuff. ;0

 

Concerning the 3 speed manual transmissions with overdrive, I just wanted to add that I was always told to put the car or truck in reverse when parking it. If the vehicle had an overdrive it could roll away forward if left in 1st gear to hold it parked, especially if the parking brake was not used.
Back in 1963 I was laying sod on a new yet to open highway in St. Louis, Mo. One of the workers took another worker's 1956 Plymouth to buy lunches for the crew. When it was time to go home the Plymouth was gone. It had been parked in 1st gear and had rolled away and down an embankment hit a culvert and rolled over. I felt very sorry for the guy with the Plymouth. It was still driveable but sure looked bad.
My Dad bought a new 1949 Ford car with overdrive. I was little but I still remember my Mom telling Dad to slow down. My older brother says Dad was doing 95mph!