[mikeyjd]

Originally Posted by eou_edu

So I have my project started. I now am the proud owner of a 2006 dodge ram diesel, 4x4 quad cab 6 speed manual ST. I bought the truck with a dead 5.9 common rail and had it swapped out for a 3.9 mechanical 4bt cummins. The engine was rebuilt and had a new cam swapped into it. So far I'm very happy with the results. The truck was built in Texas and I live in oregon. As of now it still is a work in progress.

First off despite what most people speculate it's not underpowered. These engines were in ford f350's in South America for many years. There also made it in several million 10,000 lb plus box vans. I did this project for 3 reasons. One it was actually quite a bit cheaper to swap this engine in than rebuild the cr5.9. I also wanted something for good MPG. A 3.9 vs 5.9, 300 less lbs, 2.0 less cid, and two less cylinders should get better mileage. Especially when you consider how little horsepower it takes to cruise down the road. When you aren't using that power to cruise, 2 extra cylinders is essentially just drag and weight. I also like the simplicity and reliability of mechanical engines. All in a truck that you shift your own gears, roll up your own windows, lock your own doors, no carpeting, and a pull to kill switch. It's kind of like an old 70's era pickup trapped in a 2006's body.

My first tank I was very disappointed with the results. 16.5 mpg! I thought I had wasted my time and every nah sayer was right. That is at or worse than what the 5.9's get! But 4.10 gears I was running hwy speeds and the thing was screaming at 75. No governor spring it was running for 400 miles right at the governor. Next tank I slowed down to 65-70 mph and returned 20 mpg. A little more happy with that but still about what the 5.9's get and running about 2250 rpms. The third and fourth tank I slowed down even more to 60-65 and returned 25 mpg..........I was much happier with this and no similarly equipped 5.9 I know of is capable of this.

My next mods on the list are for a gear swap to 3.42. Of course this won't give me all the benefited mileage I get from slowing down, but it will help. I am also planning to haul things. While the engine has plenty of power empty I would like a little more power for hauling (otherwise I'd be driving a geo). It won't take much to get at or above the power levels the standard output 5.9's put out in 2003. 250 hp, 460 ftlb torque vs my inline 4bt 130 hp and 390 ftlb. A set of injectors, water meth, timing, and possibly propane could be in my future. All of which should only improve on MPG and keep the benefits of the smaller 3.9. Already has the hamilton cam in it. I suspect some power improvement will also make the truck less sensitive to fuel mileage fluctuations by making work a little less hard to move the truck. One last benefit of 300 lb less up front is better handling. I noticed right away the suspension is better and it steers nicer with 300 lbs less up front. I also suspect it will be a lot more forgiving on front end, hubs, axles, and steering parts..............Pictures to come as soon as I find my computer cord.

5.9's can get <25mpg