The Stovebolt Site Forums The Shop Area Driveline Going into Overdrive

Ok, I'm just confused. somebody (with good knowledge) please help. I am wanting to go to the 3 speed with overdrive standard shift tranny. Every time I look at a different post, it tells me something completely different. At this point, I really have no idea what I'm even looking for. I understand that during the tri five era, Chevrolet installed in some cars a standard shift, three on the tree tranny with overdrive. I don't know what it's called. Well I thought I did, but that's when the problems started. Is it a Borg-Warner or a Muncie? Was is available in Chevrolet or just Pontiac? Is it a t85, t85n, 318,319, or 326? I do not want a 4 speed, nor an automatic. I just want more info on this transmission that I'm looking for.
Second, if anybody has a good working knowledge of this tranny, please pipe in. I want to retain the stock appearing look and feel of these old trucks with a period correct upgrade, but I want it to be functional too. I pull a small boat with minimal drag, and I would love to get on the interstate for about a 15 or 20 mile ride if I choose instead of having to map out the back roads before I leave the house.
If this helps, the truck is all stock with the 235. The only conversion has been a 12 volt conversion with a generator.
Thanks for any help.

What year is your truck?
What size is your truck?
What rear-end is in your truck?

Tim brings up some very valid information we really need to know to properly advise you. However, just for basic information there is some things we can tell you. Where some of the confusion you might be getting is that there are several different transmissions that were used with the overdrive unit, as well as 2 different overdrive units themselves. The overdrive part of the transmission was made by Borg-Warner, regardless of what the main transmission was. The 3 common transmissions it was used with were the SM318 (SM319 when used with the overdrive), commonly referred to as the 4 bolt Muncie, the SM 326, which is the 7 bolt Saginaw, and the Borg-Warner T85 (T85N with the overdrive. The first 2 are the ones used with GM vehicles, the T85N was primarily a Ford/Studebaker/Jeep transmission (the T85 was used in GM trucks as the HD 3 speed option, but not with the overdrive), though I have heard of some late '60s Pontiacs using them, too. The SM319 is the one used in the '55-'66 GM cars and trucks, and is basically the same as the standard 3 speed other than the overdrive unit itself. The SM326 was used in the later vehicles, is fully synchronized, and is a bit longer than the SM318/319. It also has the stronger R12 overdrive unit.
I would think you would be OK towing a small boat with any of these transmission, but you will want to bear in mind that with the overdrive active, you will not have any compression braking below 28 mph due to the over running clutch in the overdrive unit. You should also be aware that most of these overdrive units have speedometer gears in them calibrated for 4.10 gears, so you will need to recalibrate the speedometer, or drive off a tachometer like I do.

Pulling a trailer with the transmission in overdrive is a poor choice, at least with engines like the 216 or 235. Unless you have a very low ratio rear end gear, you'll be below the engine's usable torque range and performance and gas mileage will both suck. That's why there's a cable-controlled manual lockout that prevents the overdrive from engaging with the knob pulled out. The modern-day equivalent would be trying to tow a trailer with one of the small 4-cylinder 5-speed manual transmissions in 5th. gear- - - -the engine speed is too slow to get into the necessary power band for trailer towing. Flat ground running without any extra load is where OD is used. Climbing hills or trailering, not so much! It would be like installing a set of tires a foot taller on the drive wheels. The guys who run the monster mudder tires almost always have to install lower-ratio ring and pinion gears for the same reason.
Jerry

Jerry

If you want to pull a boat faster, gearing is not going to help. You need more horse power. I have been fooling around with overdrives for over 50 years and I like them. Just about all of them were behind a Y-block. But the tail section is the same Borg-Warner that GM used. I wouldn't worry about model numbers. A lot of people who are trying to to sell transmissions are clueless as to what they have. If it came out of a 55 and later Chev car or pickup and has a BW R10 , it behind it, it will work. The T85 T86 and T89 overdrive are strictly Ford application. They can be adapted to work, but its not worth the effort behind a 235. The Saginaw was used for a couple of years in the late 60's for high HP applications. They are so scarce that I wouldn't recommend looking for one. I found one several years ago at a swap meet and the guy was clueless as to what he had.

If you find a transmission that looks like what you have now except for having a OD tailshaft it will work with the shift cover you now have. If you have a closed drive shaft, you will have to go to an open drive shaft. That will give you a 3.90 ratio which is a good ratio for a 235. When you are pulling your boat, lock it in direct. When you are cruising without your boat, let her shift into OD and enjoy a little more speed.

As Tim posted it's always helpful to know what year vehicle you have. But since you indicated that it has been converted to 12 volts it could be pre 55. First series 55 trucks were 6 volt with an open driveline. Second series 55 trucks are all 12 volt. Since pre 55 trucks have an enclosed driveline an overdrive transmission cannot be used in your truck without changing to an open driveline rear end. An overdrive transmission conversion is a good option to make older vehicles perform better on the highway.
Fred

Sorry guys, the truck is a 1954 Chevrolet 3100. It has the 235 with the enclosed shaft and 390 rear gear. I know that I will have to change the pumpkin as to allow for the new trans and open shaft.
Thank you for the information, maybe I can find what I'm looking for. I live close to the water, so the boat tow is just a short shoot. The car shows however is a little further haul. That's where the interstate travels will beneficial.
Thanks again.

Here's some good OD info:
http://www.classictruckshop.com/clubs/earlyburbs/projects/over/drive.htm

If its an R10, the manuals are here:

http://devestechnet.com/Home/HowTo