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Hello I am in the process or researching building a JEEP cj7 or 8 when my car payments are up in 6 months. The 4BT is proving hard to find in Ontario Canada and I am wondering how much more fuel will be consumed if I used a 6CYL out of a Dodge Pickup? I can not seem to find the Cummins fuel curves. I know the engine will be much heavier and harder to install.
Thanks Todd
 

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I didn't want to swap a 6BT into an F-250 because of the weight.....I would never consider swapping a 6BT into a CJ-7!!!! I know someone swapped a 6BT into a Scout, but I would never put that much weight up front. Maybe the Isuzu 3.9 is easier to find in Ontario, or perhaps a small Nissan 6 cyl diesel?
 

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Yes I know the weight is a major concern the jeep would need a boxed or new frame (most are junk by now anyway) and the front suspension beefed up. That is the down side to the 6 cyl plus the power will be exceccive so the axels will have to be upgraded to Dana 44 minimum probably 60. Also the firewall will need modification because of the size of the engine. The up side is that if I find the right truck the engine, transmition and transfer case all come together. This will be my daily driver ans i am looking for 30MPG on the highway. That is the reason for my question what will be the differnce in fuel comsumtion at the same power and RPM between the 4 and 6
Todd
 

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Yes I know the weight is a major concern the jeep would need a boxed or new frame (most are junk by now anyway) and the front suspension beefed up. That is the down side to the 6 cyl plus the power will be exceccive so the axels will have to be upgraded to Dana 44 minimum probably 60. Also the firewall will need modification because of the size of the engine. ...
Todd, you covered some of the mechanical problems with fitting a 6BT in a Jeep. There's also the handling on and off road that is adversely affected by that extra weight (engine and the mods to deal with it). After all is done, said jeep will stop worse, corner worse, sink sooner and faster in soft ground, be more prone to sliding on sidehills and on fast loose surface, etc.

Weight in a 4x4 is not as critical as in a sports car or aircraft but it still matters.

But, to each his own...

Ken
 

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There's also the handling on and off road that is adversely affected by that extra weight
Correct, the front/rear weight biases would be approaching 80%/20% ot 70/30 - - it would be dangerous to drive if the conditions weren't perfect (do roads ever ice over in Ontario? ;) )
 

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I must agree with the others: not the best idea. With the power available in a tweaked 4BT and the relative size/weight of your rig, plus some huge fabrication headaches for a 6BT, it's a no-brainer: 4BT!

If you haven't driven a 4BT powered rig it's hard to imagine the available torque, right off of idle. I'm very impressed with the excellent performance of a bone stock 4BT in my 6,500 lbs Dodge. I have about the same power as a Dodge 318 2bbl, and the torque of a 360 2bbl, but the torque is right there, all the time! After twisting screws and swapping a few parts it should be really great.

I don't think you'll see very good mileage with a 6BT, maybe very low 20's if that. You're dealing with weight, plus 4WD gear/drivetrain and tire resistance, and probably a fairly high number final drive gearing.

Buy a 6BT truck, strip the parts you want, sell the engine and leftovers to pay for a 4BT engine!
 

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Hi Todd;

To answer your original question, the 6BT will get approximately 2/3 the fuel economy of the 4BT with the same gearing. As others have said, the 6BT is not practical in a CJ.
 

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No 6BT in a stock CJ frame, period. Even with mods. I'd go aftermarket on the frame dept if you're destined to run a 6BT. The 4BT is pushing it, being nearly 200lbs over the stock engine. I've had to beef my CJ8 frame somewhat as the factory welds were done by cheap non-union labor.
 

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Hi Todd;

To answer your original question, the 6BT will get approximately 2/3 the fuel economy of the 4BT with the same gearing. As others have said, the 6BT is not practical in a CJ.
At Cummins factory fuel calibration settings for a 105 HP 4BT and a 160 HP 6BT this would be relatively correct using the same gearing and same chassis. This generalization however "all goes out the window" when you start re-adjusting the fuel rate delivery screws.
 

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Discussion Starter #10
Thanks

Thanks to all who responded. I expected the answer to be worse fuel economy but didnt expect that much very good information.
I knew there were significant obsticals to installing the 6 but there is always the hot-rod appeal of putting a big honkin engine in a small vehical. It is no suprise the result would be a lot of fun for a short period of time and overall not to practical but it never hurts to ask. Thanks again
Todd
 
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