"My goal is to get the vehicle to 30 mpg while retaining as much of its off-road capabilities as possible.  Are there any tricks to the injection pump or injectors that any one knows of that might help me get to my goal?  Any advise or experiences would be appreciated."

The best thing for mpg is advance the timing a little. If pump is all stock, then I'd say keep it at 14-16*, just as many 12 valve dodges get set at.  

what i would try in your situation. in the injector pump. there is a pin that is controlled by the boost pressure. it moves up and down, as the pressure increases it moves down that in return pumps out more fuel to get you on down the road. at the bottom of the pin there is a tapered off set upside-down funnel shaped thing cut in the pin. there is a little pin that is perpendicular that takes a reading off of (boost pin). not sure of real name of pin. what i did to give mine more power, you can do the opposite. (possibly increase mpg)


 (you may know this already) to access pin. disconnect the little tube that comes from intake. on top of pump there are four screws. take the screws out. (they seem very tight.) use a big screwdriver and clamp vice grips to it, the first time. a little one may slip, that will make it more difficult. take the cover off. you should see a rubber diaphragm with (that pin) in the middle of it. pull it out. you may have to rotate it slightly back and forth with upward pressure to get it out. you will see that funnel thing on the bottom of that. there should be a wear-mark on the funnel shape that goes up and down. you need to turn the pin and diaphragm so that the funnel shape is closest to the front of engine. if you wanted more power and black smoke you would turn it so the funnel was further from front. the factory setting is usually about 5/8 power. there is also a spring under the diaphragm that has a adjustable nut it rides on top of. if you tighten the nut it goes down taking resistance away. that would give more fuel quicker. do the opposite for your case. i cant tell you how much to move it. i dont know.

there is the fuel screw too. but adjusting it and the pin,  may make it lean. when you adjust it you have to adjust idle also.  

diesel power mag had how to give a 6bt more power on there rustbucket project. do every thing the same except turn pin the other way.

 just a sugestion, doing this may make it real lazy. but maybe more mpg. you may have to push the accelerator further for normal power.

 if i had the money i would buy a 9cm exhaust housing, install a 3200rpm govenor spring, and try to make as much power possible without the black smoke.  

I know the pin in the injector pump you are refering to.  I've played with it several times to get the best economy.

The engine does not blow any black smoke.....ever.  I did turn up the fuel screw one turn and the engine seemed to like that.  Still no black smoke and the fuel economy remained the same (just seemed to have more power at wide open throttle).  The only other change was I had to turn down the idle speed.

As for turbos, I'm running a turbo off a 1989 Dodge truck.  I replaced the 19cm exhaust scroll with a 12 cm exhaust scroll.  The engine really liked that!  The engine runs about 3 - 4 psi of boost while crusing down the road at 50 mph.  It will peg my 15 psi boost gauge when I crank it up to 2500 rpm's or go wide open throttle.

The exhaust manifold is also off a 6BT equipped Dodge truck.  I cut off the two outer exhaust runners for cylinders 1 and 6 and brazed small cover plates over each opening.  Then I turned the exhaust manifold upside down and installed it on my 4BT.  That gave me a perfect "mid-mount" turbo.

The stock 4BT exhaust manifold placed the turbo to low.  The turbo would be into the battery and the exhaust outlet was to close to the firewall.  Turn the original manifold upside down and it placed the turbo to high (about 1/4 inch from the inside of the hood).  That would have cooked the paint on the hood.

The rest of the exhaust system is 3 inches in diameter all the way to the back bumper with just a Flowmaster muffler in between.

As for the 3200rpm governor spring, that would certainly increase the top end speed of my Wrangler.  I think the engine has enough torque to give me 3200 rpm's in 5th gear, but that would equate to about 120 mph (33 inch tires/3.07 gear/.79 O.D. 5th gear).  Don't think that is any where a lifted Jeep needs to go (at least not with me in it).  I'll leave that kind of speed to the Corvettes and Vipers of the world.  

both of my 4bt's have automatics so i feel like the 3200rpm spring would be a help. what do you think?  

I don't know what a 4BT feels like at 3200 rpm's.  I've never felt the need to spin the engine that fast.  My engine seems to feel best at around 1700 to 1900 rpm's.  Maximum torque on a 4BT comes in pretty low.....I think it is all there at around 1500 rpm's.  So spinning the engine up to 3200 rpm's will only stress parts unnecessarily and probably shorten engine life.

I think what would really help your package would be an overdrive transmission (700R4 / 4L60 / 4L80).  GM offers a bunch of good transmissions that are calibrated for diesel applications.  Chrysler also has a selection of automatic transmissions that could work for you.  Both transmission makers have massive amount of support from the aftermarket.  You could put together a killer overdrive trans and eliminate the need to spin the engine beyond its design intent.

As for my Jeep, to be honest, I would much rather have an automatic trans in my Jeep than a manual trans.  In hind-sight, I should have converted the vehicle to an overdrive automatic when I installed the 4BT, but it is what it is and I'm not going to change it now.  

i don't think i will ever put the 3200rpm spring in. i  will possibly play with the turbo. i have a dodge overdrive tranny behind the 4bt's in both the dakota and durango. the top speed is like 100 mph in both so no problems really. the durango runs about 1900rpm at 70mph with the lock-in torque-converter engaged. i just need to keep mine from blowing so much black smoke. that problem is my heavy foot.  

One of the mistakes I made early on was thinking the 3200 rpm spring actually gives you 3200rpm.  Cummins actually never called it that.  I don't know where the name came from, but it will give you 2900 to 3000 rpms.  My jeep has the spring and the pump turned all the way up with an aftermarket fuel pin and the throttle stop screw out.  It made it much more responsive to drive.  Obviously, you shouldn't run down the road at 3000 rpms, but now the acceleration is similar to a v8 gas engine.  None of it lowered or raised my mileage.  When I dynoed my jeep, maximum hp was still at 2450 rpms.  

Hey I've got a 1995 YJ and want to throw in a 3.9L 4BT. I'm definitely very new to diesel anything and recently changed my mind from wanting a 5.7L small block chevy in my Jeep. But, the allure of diesel caught me and now I want that Cummins in my Jeep. What all can be done to improve performance on the 4bt? I've seen everything for the 5.9 but absolutely jack for the 3.9. I want to have more horsepower than stock and of course as much torque as I can get. Arizona had all kinds of awesome info, and this site has given me the most help so far. Any other help is greatly appreciated! Thanks!  

thumb through some of the dp archives, theres a 400 hp bronco that had a 4bt swapped in it, its pretty badass