Advice on 3.9 cummins!

[stover]

I have 2 chevy step vans with the 3. 9 cummins in them and am looking for a mid 80's chevy 4x4 short bed as a donor.

Now my question is what to look for? An automatic or manuel? If automatic a 400 or 700 transmission? The one truck has a 4-speed allison and the other has a 3-speed automatic. I would like to use the allison if I could but am not sure how it would work to hook up the 4x4.



My truck 95 Dodge 4x4 5-spd #6 tst 3gsk 4" turbo-back exhaust AFE Proguard7 dow pin fixed pillar gauges Oo.

 

[BCronk]

I think the most cost efective and easiest would be to use a gm transmission, that way you can use all of the drivetrain parts from your donor. Does your 3spd step van have a th400? if so that engine will bolt right to your donor vehicles transmission, as long as your donor has an auto. If your donor has a manual you will have to swap your flexplate for a flywheel. I would also use a 700r4 that has been built up some or a th350 or 400, without over drive you will want to run 3. 08 gears. I think you will have driveline problems if you use the allison, You will have to run a divorced tcase and that would put the output of the tcase to close to your rear axle. You will probably have severe driveline angles that eat up ujoints. That combo would probably work on a long box though. This is just my perspective after doing 3 diesel conversions.

 

[DKarvwnaris]

No matter which way you go, if you use an automatic, you'll need to either use one of the auto's out of the bread trucks or have an auto built to work correctly with the lower rpm speed of the diesel. Even the auto's designed for the GM diesel v-8's are too high rev for the 4bt's abilities. The automatic will not be in its correct rpm range behind the 4bt to shift right.



I would really look into a manual GM truck and just buy a flywheel from Cummins so you can run the manual transmission instead. Much higher chance of holding together than the auto's. Cummins still sells the 4bt GM flywheels, they are about $600 to $800 depending on your source for them. Not cheap, but better than an automatic rebuild, and rebuild, and rebuild. You get the idea, hopefully.

700r4's are not my preferred auto to use if I was to count on it day and day out especially If I was to having to get an auto built for a conversion, it would be a first generation or second generation Dodge automatic like the 46 RE or the tried and true 727's. They are designed for the rpm range needed, and you won't hear a bunch of excuses from a transmission shop when it comes apart or starts slipping. They are non ECM controlled so you can expect an easier time installing them. You can go with dodge core parts from a junkyard if you can't afford a new one and then just do an on the spot core swap with them for an in stock unit.

 

[BCronk]

I have a th350 behind my ctd 5. 9 It shifts fine & I have about 20,000 miles on it. I think these trannys are stronger than people make them out to be. Did I mention I have 35" tires , 3. 07 gears and full time 4wd? When I put the hammer down my tires don't smoke, I just go fast. I put the hammer down a lot, My transmission gets abused. I think the weak link is the torque convertor I am running a 13" low stall tc I think that is the only reason my transmission is lasting. My point is I think A th400 would easily hold your 4bts power. Just my two pennies again.

Driving A cummins powered chevy is a blast no matter what transmission you have!

 

[stover]

I'm thinking that if I decide to go with a 5-speed maybe go find one at a junk yard to put in. I did find a few trucks with the 4-speed stick but no 5-speed in those years. I could maybe get on of those trucks and it would be less hassle to go with stick than to buy an auto and convert it over to a stick.



But that allison auto would sure be nice!!??

 

[DKarvwnaris]

I'm thinking that if I decide to go with a 5-speed maybe go find one at a junk yard to put in. I did find a few trucks with the 4-speed stick but no 5-speed in those years. I could maybe get on of those trucks and it would be less hassle to go with stick than to buy an auto and convert it over to a stick.



But that allison auto would sure be nice!!??

To install the 5 speed in the older chevy bodied trucks, you'll need to go with a hydraulic clutch assembly. The way the bellhousing and clutch is designed in the newer trucks, it is alot of work to fabricate a mechanical linkage to the clutch arm in them.

A 1986 through 1990 manual truck will have a hydraulic clutch in it already. It is a reliable setup. I have used the master cylinders on these with a custom adapter line to work with the dodge slave cylinders in the 1994 and newer nv-4500's. There are some issues with the older 4 speed mechanical rod linkage's if you're planning any moderate off road use. The connector rod can pop out from frame flex. For that reason alone, I'd look into installing a hydraulic linkage, not hard, parts are readily available for it.

The Allison is a heavy transmission in the bread trucks I have looked at. Its really expensive to get fixed ,but will take alot of abuse if it is not worn out already.

 

[LAlessi]

ill post this here, for posterity sake.

the TH400 used behind a 4bt, as provided by cummins, required the use of a STRONGER pan.



now, this is just my theory, but, if a th400 case was not strong enough in its stock form to withstand being coupled to a 4bt... ... then IMO theres no possible way a 700r4 will be. bu hey, ive been wrong before.



btw, i can supply the part number for the strngthened pan.



keep in mind that off road i run 900rpm with a stick. the auto will probably NOT like that

 

[DKarvwnaris]

ill post this here, for posterity sake.

the TH400 used behind a 4bt, as provided by cummins, required the use of a STRONGER pan.



now, this is just my theory, but, if a th400 case was not strong enough in its stock form to withstand being coupled to a 4bt... ... then IMO theres no possible way a 700r4 will be. bu hey, ive been wrong before.



btw, i can supply the part number for the strngthened pan.



keep in mind that off road i run 900rpm with a stick. the auto will probably NOT like that

I rest my case. Was going to get more specific, but Lee summed it up nicely.

 

[stover]

I found a 86 chevy 4x4 short bed with a 4-speed manual that I think will work just find. I'm planning on having a friend do the conversion because I am not very mechanically inclined!! My friend has done a few conversions already. He is just finishing up a 89 Ford F250 4x4 5-speed with a 5. 9. And the previous on was a 98 Ford full size maxivan that he put an early 90's 5. 9 in with a 5-speed. Oo.

 

[daveshoe]

I have a '92 4WD 2500 five speed and FYO, the OD gear is way too tall for a gasser--even with a 350 engine. It only runs good in OD when well over the speed limit. This truck is in excellent shape with a brand new factory original bed that I re-installed after using a service body for work. It would make a sweet donor truck,but probably would be too pricey for a conversion.

 

[DKarvwnaris]

I found a 86 chevy 4x4 short bed with a 4-speed manual that I think will work just find. I'm planning on having a friend do the conversion because I am not very mechanically inclined!! My friend has done a few conversions already. He is just finishing up a 89 Ford F250 4x4 5-speed with a 5. 9. And the previous on was a 98 Ford full size maxivan that he put an early 90's 5. 9 in with a 5-speed. Oo.

Good solid donor for a 4bt. The lightweight chassis should make it a real fuel miser. If you have the GM adapter still for the 4bt, it will require little to no work at all. I have even reused the motor mounts out of the p-30 gm chassis bread trucks without any mods. Have fun with it. It will serve you well, so long as the truck you've located is in decent shape still.

 

[stover]

Donor on the way!

Well I finally found my donor truck. I am getting it from Vegas Nevada. 1982 chevy k20 with a new 6. 5 turbo diesel in it with 20k but with a broken crank. Truck has 103k original miles and was owned by the same family since new. Now for the good part it has original paint and upholstery, of course no rust, never been wrecked or had body work, power windows, locks, ac, banks sidewinder turbo, 4x4, 10k on rebuilt turbo 400 trans, limited slip 4:10, gearvendors 208 over-underdrive, twin 21 gallon fuel tanks. Truck is in mint condition. Can't wait to start working on it! Am thinking of just using the 400 transmission for now. The guy I'm buying it from said cruising at 70 he was taching around 2100 rpms. Oo.

 

[Morse]

That will be a great setup. . With the gearvendor unit, you already have your overdrive, and it's still got the strength of the 400. . With over 40 gallons of fuel, you won't be stoppin' at fuel stations very often. . Congrats. .

 

[DKarvwnaris]

As much as the truck has going for it, I would think twice about using the turbo 400 in it. Two reasons come to mind immediately, 1st being the 6. 5 in it has a cracked crank, it usually damages the front section of the transmission when this happens. 2nd, the turbo 400 in front of a 4bt will need upgrades to hold up. As Lee posted above, the 400's in the bread trucks have heavier basse pans, not to mention several heavier hard parts inside them. If you visit a transmission shop and talk to them about this, they will confirm this for you, so long as they actually know or have the specs on the Cummins application 400 trans.

I'd look around for a bread truck automatic transmission to use or borrow some hard parts out of it for the trucks existing auto, if its not damaged from the cracked crank.

have fun with the project, it will serve you well when you get it up and running.

 

[stover]

Actually I have 2 trucks with the 3. 9 in them. They are both the chevy p-30 one has the turbo 400 trans and the other the 3-speed allison. And so I could use the beefed up t-400 trans out of the chip truck.

The one truck that I have with the allison only has 42k on the transmission and reman engine. The reason I was able to get it was that the company was scared that someone would forget to pull the brake on the allison (These allisons don't have park) and it would go rolling. I talked directly to the mechanic in the garage who gave me this info. They had put this allison in brand new which was pretty expensive. I would hardly think that with 42k the allison would be worn out yet. Sure do love the way it shifts compared to the T-400 though!

I keep toying with the idea of this allison under the truck. I don't feel like I'm that mechanically inclined but I did a quick measurement of the t-400 and then the allison and it didn't seem that the allison was any longer. I guess the biggest thing would be hooking up the transfer case.

 

[DKarvwnaris]

Actually I have 2 trucks with the 3. 9 in them. They are both the chevy p-30 one has the turbo 400 trans and the other the 3-speed allison. And so I could use the beefed up t-400 trans out of the chip truck.

The one truck that I have with the allison only has 42k on the transmission and reman engine. The reason I was able to get it was that the company was scared that someone would forget to pull the brake on the allison (These allisons don't have park) and it would go rolling. I talked directly to the mechanic in the garage who gave me this info. They had put this allison in brand new which was pretty expensive. I would hardly think that with 42k the allison would be worn out yet. Sure do love the way it shifts compared to the T-400 though!

I keep toying with the idea of this allison under the truck. I don't feel like I'm that mechanically inclined but I did a quick measurement of the t-400 and then the allison and it didn't seem that the allison was any longer. I guess the biggest thing would be hooking up the transfer case.

If you have an allison actually designed for a retrofit behind the 4bt, this will be a good transmission to use. Most of the people who ask me about the Allison's are pulling them out of duramax equipped GM pickups. These model Allison's are worthless and have little to no chance of survival behind anything but the Duramax with integrated ECM controllers. Reason being, GM wanted the transmission and engine to work together to defuel the injection system if the transmission was getting overpowered. Without the defueling controller that GM designed, the GM pickup Allison transmission's will get torn up with no safety limits like the medium truck Allison's have in them.

If you're Allison transmission has the internal safety limits in it, then you'll be fine if its what you want to use. My biggest concern would be the same as the fleet you picked it up from, no park gear. Make sure you remember to always set the park brake. If your transmission has the tailshaft mounted park brake, keep it in addition to your pickup's OEM E brakes. You may need to look into a compact transfer case that will still allow for a divorced connection. Driveline angles could possible be an issue, but with the 4bt, you may be able to move the engine forward in the engine bay to get more room aft of it. You'll just have to measure alot before committing to it.

 

[stover]

My next step will be to pull the engines and trannys out of the bread trucks and then do some measuring and wait till my donor pulls up to make that final decision. I figured if I put the allison or the t400 in and it gives out on me down the road I will just put a nv4500 in from a Dodge.

This truck is just gonna be a town and weekend cruiser anyhow. Will probally park it for the winter and keep it out of all the salt junk. So it ain't like I'm gonna be pulling 10k down the road or anything.