I am looking into the reasonable feasibility of swapping a complete Cummins 5.9 and Allison 6spd into my 1985 D350, replacing my gasser 360.

I have about $4K into the D350 (6 pack cab, dually, 2WD) as it sets and I am using it to pull heavy box trailers. I averaged 7.5 MPG this last trip towing fully loaded trailer and truck.

The 360 in it runs strong and pulls fine, but it certainly knows it has a load. The TF727 in it seems to be doing fine so far on the 750 mile round trip through Missouri hills and turns I just got back from.

An opportunity to purchase a Cummins 5.9 (only 108K miles) and Allison 6spd has been presented to me. It includes the complete wiring harness and push button tranny control. They only want $3K for it all. It came out of a School Bus setup, the bus was totaled due to frame damage from the rear when some vandals pushed it backwards down a hill into concrete pillars. The guy bought it to put into a Chevy, but soon found it wasn't heavy duty enough to handle the extra weight and I believe there were some clearance issues inside the frame.

I am soliciting some realistic feedback on whether or not this would be a worthwhile swap. Would I see any real mileage savings? I get 7.5 now.

My fuzzy math (7.5 MPG vs. 12 MPG) says it would take me ~16K miles to recoup the initial investment, not including all the extras needed to complete the transplant.

Someone talk me off the roof or onto it...LOL...

 

i don't know if i have ever seen a 5.9 cummins and 6 speed transmission in a school bus. i guess if you got the time money and skill anything is possible. may want to try posting on DTR or cumminsforum. they might be able to help you better.
http://www.cumminsforum.com/forum/89-93-powertrain/

http://www.dieseltruckresource.com/d...opics-f93.html

 

Time and Money are always at odds with one another...while the setup is a great deal, I just can't pull the trigger. I can put several gallons of gas in my beast as it sits and be happy I suppose. A person really can't cry over 7.5MPG with a fully loaded truck pulling a heavy loaded trailer up and down Missouri hills. I've driven worse (Chevy 454...4MPG loaded pulling).

 

if you don't got the time to do the swap then maybe you should just buy a truck with a cummins already in it. you can sometimes get some great deals on a truck.

 

You will absolutely see an increase in mileage.

There is a guy on Cummins Forum that is doing this exact swap. There's also a lot of hypothetics in the swap, but not much concrete evidence regarding the use of a six speed Allison behind a Cummins. The Allison TCM communicates with the GM ECM to control torque management and shifts, so it's a pretty involved deal stock. Let me dig up a link to get you started, running a TCM from a GM 8.1L gas application since it seems to be better suited behind the Cummins.

Question: what Cummins is it?

Your next option is a 12v and overdrive trans, manual or auto. It'll blow that 7.5mpg out of the water, every day of the week. 20mpg is a cake walk.

 

Thank you all for the feedback. I have considered just buying a setup truck just right-out...but everyone in my area is VERY proud of their vehicle and prices are stupid-high for diesels.
I may just live with the mileage for now and not make as many longer trips and just stay closer to home-base. The ol' truck isn't a beauty queen, but she has character (or at least one driving her...LOL).

 

ok. you didn't say it was a crew cab. if you told us that i would have said you should go for doing a cummins swap. if you search long enough you might find a good deal on everything you need to do the swap.

 

Just a quick note here: If you get a Allison get the old industrial style 6spd auto, not the newer computer controlled ones. The old style has a hydraulic governor and will last waaaay longer behind a Cummins.

 

This is the newer one with computer unfortunately...are they that bad? It isn't pulling a school bus anymore, but just a truck...and I don't plan on twisting the snot out of the 5.9...pretty much going to leave it stock...at least that is the plan.
On a good news side of things...my wife agrees it would be a good idea!!! How cool is that?!

 

If you are not planning on turning the Cummins up it should be fine. Just be sure to change the tranny fluid and filter religiously at 15-25K miles. I have seen countless numbers of electronically controlled Allisons break down, for two reasons. 1) They didn't change tranny fluid and filter strictly at 15-25K and 2) they used O/D while towing or turned the engine up.

Otherwise they are a okay tranny, a 6spd auto behind a Cummins will be sweet

Just remember (this applies to ANY auto electronically controlled tranny), any electronically controlled auto tranny is more sensitive to dirty fluid (not changing often enough) or reduced fluid flow (dirty filter, even if it looks clean to the naked eye it will be dirty) than hydraulic governed autos. Auto trannys always leave little bits of clutch material and normal wear in the fluid, this is harmless, and with a transmission wearing at a normal rate the factory clutches are designed to provide enough wear to last for several hundred thousand miles. Your job is to keep removing those little wear particles. If you do that by changing the fluid and filter properly, it will keep the tranny free of those little particles which is what will really make it last long.