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BigTorqueCJ 03-25-2008 09:16 PM

Official Cummins 4BT Swap Thread for CJ's
This is the Official Cummins 4BT Swap Thread for the Jeep CJ's

Okay, should I call this another 4.0 swap? Nope, it's a 3.9L and it smells like diesel! Before I get too far in depth with this thread, I'll first run through the highlights of my 4BT swap:

The Engine
I'm sorry to be so blunt, but anybody with a brain that does any research will always come back to Cummins when they are looking for a strong diesel engine to put into their Jeep.
My 4BT (4=4cylinder, B=B series, T=Turbo)
is factory rated at 105hp and 265ft/lb torque with a 17.5:1 compression ratio weighing in at about 700lbs.
The 4BT is the exact same Cummins engine as the first generation Dodge Ram Diesels 6BT except with the front two cylinders chopped off. Therefore all the Dodge Ram Cummins 6BT parts are interchangeable with the 4BT.

As far as I know, there is nobody out there that makes an aftermarket 4BT to CJ frame engine mount kit. I constructed my own and they can be seen here:
I used polyurethane AMC V8 motor mounts and built the brackets myself.

This is where it gets fun: everything here is built for the 6BT, so what I'm implying is that it is all huge and super heavy duty because it was built to handle the 6cyl with twice the power of the 4BT.
As far as transmissions go I have always preferred my manual trannies, so as Auto trannies go: I won't be much help.
I was fortunate to have a Dodge NV4500 in my Jeep previous to this swap. All I had to do to connect it to the 4BT was buy the following from Dodge:
1. Flywheel Housing
2. Flywheel
3. Clutch
4. Pressure Plate
5. Bellhousing with clutch release bearing
6. and all the small parts and bolts to put them all together.
I did have to put a heavy duty input shaft on my NV4500 to match the massive clutch.

I already had my Dana300 with 4:1 mated to my NV4500. Nice case, so no upgrade needed here.

I use my stock (heavy duty) radiator for an AMC V8 and have no overheating issues. I also have a 14inch Mr. Gasket electric cooling fan mounted to it.

I towed my CJ to the exhaust shop and had them bend up a nice 3in exhaust with nice Magnaflow muffler.

The alternator that comes with the 4BT is a Delco-Remy SI series High-output alternator that is significantly larger than the D-R alternator that comes with the CJ originally. I had mine rebuilt and it is super nice and powerful!
I have a starter for the Dodge Ram Diesel; so as you can imagine, I expect it will last for a long time.

Taking for granted that you can wire up a starter and an alternator (two things you should already have anyway) all you need to run the 4BT is one wire! The diesel has a fuel shut-of solenoid that must be powered up to open the fuel system to the pump. Therefore a "key-on" power wire (your existing "ignition" wire) is necessary to start and shut-off the engine.

Brake Booster
Since you lose the vacuum capability with running the Diesel engine, I decided to go Hydroboost.
I bought a hydroboost setup from a Dodge Ram Diesel. I had to make some custom brake lines to attach to my proportioning valve, but boy is it powerful!

I'll continue to add to this thread to provide some more complete info on my swap. Meanwhile, enjoy some videos that I made:

For an unlimited pool of info from many many people that have done 4BT swaps I recommend visiting the 4BT Swaps Forum at:

USMCredneck 03-25-2008 10:23 PM

That is one killer jeep. I have been planning a 4bt swap into my ranger Ive got sitting at the house for a while now. Just have to wait until the jeep is done.

limegreenCJ7 03-25-2008 10:58 PM

Nice, real nice
bahaha I can imagin the looks you get in that thing
people turning thier heads expecting to see a big truck

John Strenk 03-26-2008 06:07 AM

603 Attachment(s)
I should be so lucky to find an engine that!

BigTorqueCJ 03-26-2008 03:15 PM

The entire swap took a passive year to complete. I took my time finding parts used on eBay and from other sources. I wanted to keep it as simple as possible so I stayed with all Jeep, Dodge Ram, and Cummins parts.

BigTorqueCJ 03-27-2008 10:40 AM

The question came up in a PM to me regarding who did all the work for the engine installation...

To answer, I did basically all the work:
-Acquiring the engine off e-bay and driving to pick it up
-Installing a new rear main seal
-Buying a Flywheel housing from Dodge
-Acquiring the bellhousing and clutch through a friend
-Acquiring a new input shaft for my NV4500 from a friend, then installing it
-Attaching the transmission/t-case to the engine
-Dry fitting the engine and transmission into place about 20 times
-Measuring a few times
-Designing and welding up engine motor mounts
-Designing and welding up transmission mount
-Cutting more transmission tunnel out of my Jeep body to make for a raised transmission
-then making new covers for that area
-Drain gas tank, blow out fuel lines, and fill tank with Diesel
-And Everything In-between!

What I had a shop(s) do:
-Custom bend my exhaust
-Shorten the height of the charge air tube; so I could close the hood without hitting the engine.
-Inspect my Jeep
-Attach a new speedometer cable and calibrate my odometer
-Rebuild my alternator that, in the shop owners words, "looks like it was in a flood"

I have always taken pride in doing all my own work and fabricating all the custom parts for my Jeep with a few exceptions. I think everyone knows where their limits are. Usually the only time my Jeep has to go to see a mechanic is when it needs inspected.

BigTorqueCJ 03-27-2008 11:30 AM

Why did I decide to pick this engine over all the other options out there?

The Background:
First off, my CJ originally had the AMC 304 in it. As I found out they typically blow out the rings when they are older and before all was said and done I was putting all kinds of oil in the engine because it was all just passing by the rings.

I had just started college and needed a new engine. I acquired a used 1991 AMC 360 and once home I had it swapped and was driving again in 9 hours. I was able to enjoy that engine through my 4 years of college. (Of coarse, it had it's quirks like all AMC V8's do.) I do truly love AMC V8 gas engines, but when I was faced with another compression problem with the 360 I had a lot of thinking to do. I had seen the Cummins put into another CJ-7 a year earlier and thought to look into a diesel conversion vs. another gasser.

Other engines I considered before deciding on the Cummins include: AMC 360, Dodge Magnum 360, Dodge Magnum V10, Jeep 4.0 I6, Nissan/Chrysler SD33(T), and probably a few more. I did know that I will never have another engine with a carburetor on it, it's fuel injection or nothing.

The Debate:
Emission Inspection:
I was already faced with emission inspections and the limitations they put on you in Lancaster, PA. I knew if I wanted to continue with gas engines I would have to either re-apply all the emission crap or just drive my Jeep like never so that I don't go above the allowed miles for emission exemption.
Diesels, however are emission exempt. Except now, I believe, all 2007's and up have emission requirements.

Fuel Mileage
Diesels get better fuel mileage, period. That's why all big rigs run diesel. I have been getting twice the fuel mileage with the 4BT than what I got from the 360.
Diesel fuel works much different than gas in that you get much better torque, here's why:
In a gasser, when the piston comes up on compression stroke it makes the air hot and then when it gets to the top the fuel enters the compression chamber and is ignited by the spark plug. This causes an explosion and the piston travels down and turns the crankshaft.
In the Diesel, the compression is nearly twice that of a gas engine and the air is so hot when compressed; the diesel fuel is injected and the heat in the cylinder ignites the fuel and that fuel continues to burn as the piston travels down. The continuation of fuel burning is what gives diesel engines the extra torque, because they continue to push the piston down during the power-stroke.
What does that mean? Well, you get more torque = engine doesn't have to work as hard = better fuel mileage.

My conclusion:
-The 4BT is readily available and very very very easy to source part for. It is completely adaptable to all of the (heavy duty) Dodge Ram 6BT parts and my current transmission.
-I used to be a diesel mechanic, so I have my roots in diesel that way.
-I won't have to ever deal with emission inspection, as I know it today.
-I'll get much better torque and fuel mileage than compared to my V8 gas days.
-I hope to have much more torque off idle (and I do) for off-roading and rock-crawling.
-Diesels sound cool, are cool, and always turn heads. (I guess part of me is vein and likes attention, but I think most Jeep guys are like that. I think for me it's just that I like to feel that someone can appreciate all the time I put into my projects.)

NHwheelin 03-27-2008 12:29 PM

have you considered that you can also run veggie oil in the diesel and you can get that for free from most resturaunts. Thats what I am thinking about doing after I canvert to diesel. There are planty of fast food and sit downs around here. Will only have to pay for a couple gallons of fuel just to heat the veggie oil. Supposed to get a little better mileage and an awsome smell of french fries.

BigTorqueCJ 03-27-2008 12:48 PM

I'll be honest, I'm not the guy to go to about veggie oil questions. I would say that yes I did consider it for about 20 seconds. :o
I know several guys around here that run veggie oil in their diesels, but for me I think it's too much of a hassle for my Jeep that is not a daily driver. I get fuel mileage in the high 20's, so for me that is good enough. I know that what I get from the pump is not going to hurt my engine, so that makes it less of a headache as well. Currently I don't have to fill my 20 gallon tank too often, no big deal.

Anyway, from research on well done Veggie Oil Diesels, here is basically what you have to do:
1. Source the oil
2. Process/Blend/Filter the oil
3. Store the oil
4. Install a second fuel tank in your vehicle
5. Start your diesel on pump diesel
6. When engine is warmed up, then switch to the veggie tank
7. Switch back to pump diesel when a few miles from your destination/home.

In my case the step(s) are as follows:
1. Fill tank with pump diesel whenever necessary
2. Enjoy the drive!

Disclaimer: There are a ton of guys that swear by veggie oil in their diesels, and for them I am happy. I would not discourage you from exploring this option. You will find with diesel conversions (especially the 4BT) there is so much you can do to a diesel that you can't do to a gasser. Additionally, there are so many small performance mods for diesels with huge results and improved fuel mileage; compared to gassers that give you very small power gains and less fuel mileage.

jerry can 03-27-2008 01:56 PM

I live in northern california and have thought about doing a diesel swap does anyone know the rules for this. I have a 83 cj7?
Did you have to reinforce your frame for the weight of the diesel motor

BigTorqueCJ 03-28-2008 10:39 AM

I personally did not feel the need to reinforce the frame to handle this engine. What some people do (and recommend) is to sleeve the bolts that are going through the frame. I'm keeping an eye on the bolts for now.
I don't know what California restrictions are, but it was plain and simple where I live in Pennsylvania: If your vehicle has a diesel and it is 2006 or older, it is emission exempt. Nobody seems to care what engine was originally in my Jeep, just that it is now a diesel.

White92 04-01-2008 09:30 PM

I've got a fresh 4.2 but if it ever goes I'll be doing a diesel swap. What kind of highway cruising can you do with the engine? I do a lot of driving in my CJ and it burns some gas in overdrive.

BigTorqueCJ 04-02-2008 02:16 PM

I've had a 4.2 (you are talking about the AMC 258 right?), they are a nice, strong engine and I would think a fresh one would last you a long time! The only crapper is that they are carburetor run. Even though I'm 26 y/o I would say most of my driving is pokey style and I am comfortable cruising at 65 down the highway. I'll hold the engine up to 70 but for me that is about it. I have the NV4500, 4:10 gears, and 33in tires. I like my set-up, but I do think that with the diesel, overdrive is a must!
Happy Trails

White92 04-02-2008 08:25 PM

Yes it's an AMC. I am guessing this engine should get me at least another 100k miles. I drive it every day to school and work. And it's got fuel injection. So I have nothing to complain about and I have no business thinking about a diesel swap anytime soon. But in the back of my mind I'm always looking for 4TB engine.

BigTorqueCJ 04-04-2008 08:58 PM

Well, I've been convinced that the 4BT can be taken right up to the governor without any ill side-effects. So, I decided to take the 'ol Jeep out and see just what it can do.
Here is what I found out: the top rpm from stock is around 2500
3rd gear top speed: 39mph
4th gear top speed: 60mph
5th gear top speed: 79mph

What a rush, winding the diesel up to it's factory spring limit!
-don't feel a need for the 366 spring yet.

The time now is 08:45 PM.

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