4bt Jeep Conversion-Facts, Figures, Cautionary Notes
Below is a post by ArizonaPerfect on the dieselpowermag.com forum. He did the 4bt-Jeep conversion and offers some interesting facts, figures and a few cautionary notes:
http://forums.dieselpowermag.com/70...swaps/39l-cummins-in-jeep-wrangler/index.html
"I did the 3.9 Cummins in a 1995 Wrangler. It has a AX15 five speed transmission, 33 inch ProComp tires, and 3.07 gears (front and rear).
First off be prepared for a lot of attention. No matter where I go people stop and ask about the vehicle. It is very rare that I can get fuel without having to open the hood and show people. Pull into Home Depot and you become an instant "show stopper". At least two or three people come over to check it out and the ones that don't come over have a look of disbelief on their face. It really is a lot of fun!
Fuel economy in the winter is around 25 mpg. In the summer it goes up to around 27 - 29 mpg.
Be prepared to do a lot of changes. The stock rear axle has to go. A Dana 35 is not going to live very long. I upgraded to a Ford 9 inch. Find a set of 3.07 gears (eBay) to match the front. Since you are limited on engine rpm's, going with 4.10s or larger will limit your top speed.
My Wrangler has 3.07 gears and 33 inch tall tires. It turns about 1800 rpm's at 70 and the engine seems to like that alot. I had 3.55 gears, but the engine seemed to rev excessively. With 3.55 gears on a normal take off I'd be in fourth gear before I got through a typical intersection. A 4BT has more than enough power and torque.....it does not need big gears.
Finding a 4BT is as easy as going onto eBay. There are always 3 or 4 engines for sale. Plan on spending $2,500 to $3,500 for a good engine. Make sure they include the trans adapter. Even if it isn't the one you need, they all have value.
Your Jeep will also require a suspension lift kit of at least 4 inches. 4BT engines are very tall. The front axle gets to close to the oil pan on a non-lifted Jeep.
I installed all the cold start aids from a 1989 Dodge truck. 1989 - 1991.5 had a seperate module to run the intake grid heaters. The module also controlled the "Wait to Start" light and the "Water in Fuel" light. I used the original "Check Engine" light in the instrument cluster as a "Wait to Start" light and used the ABS light as the "Water in Fuel" light. I also installed the block heater from the same truck.
I live in Michigan and so far we've had an extremely cold winter. There have been several days where the temperature never got above 10 degrees and nights would go below zero. So far it has started every time. When the block heater is plugged in, the "Wait to Start" light does not come on. The block heater keeps the engine warm enough to where the module determines intake heat is not required.
Speaking of heat......the heater in the passenger compartment has no problem roasting you out of the vehicle. When the block heater is plugged in over night, heat is almost instant. The temp gauge goes to about 110 degree on start up, so I have to assume the six cylinder block heater is keeping the engine at about 110 degrees.
4BT's do vibrate. It is the nature of the beast. It does not bother me. The best way I can describe it is to find a recliner that has a vibrating massage feature. Put the chair setting on low and that is what it feels like.
Any four cylinder engine over about 2.0 liters will vibrate. The only way to fix it is to add balance shafts. Good luck find a set. They are hard to find. I've been looking for years and have yet to find a set. I know they exist because the Cummins service manual shows how to install and time them. They bolt to the bottom of the crankshaft main bearing caps. Does anyone know where to find a set of balance shafts?
Here is a question for everyone out there.....since I work for Chrysler, does that make my 4BT Wrangler the only one built by the factory? Hmmmmmmmm?..."
