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Old 11-13-2012, 03:36 PM   #1 (permalink)
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4bt swap fuel economy

Hi guys, I've been doing a lot of research on 4bt swaps, and Ive got some questions. Basically I'm looking to build a small truck that would be great on fuel to drive to work everyday so I don't have to drive my 12v all the time.
My question is, what is some of the best mileage one could expect to see out of one of these engines. I've been doing some searching but there are so many different set ups it's tough to find one built specifically for economy. I'd like to know if its even worth me considering.
My thoughts were swapping one into an older Chevy 1/2 ton, late 70's through early and mid 80's preferably. I'd like to stick with a manual trans, maybe using an nv4500. 2wd would probably be my best bet, but I really don't know how much 4wd will eat up mileage. Does this sound like a decent base for a mileage truck? Any advice or input would be appreciated
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Old 11-13-2012, 03:44 PM   #2 (permalink)
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I can get 25mpg around town consistantly and 30mpg on high with my truck when i had the 46rh. just rebuilt a 47rh and on my first tank i got 25mpg but i also towed a little trailer around for a couple days loaded with wood. so i am expcecting 26 or 27 with my next tank.
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Old 11-13-2012, 04:01 PM   #3 (permalink)
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Thanks for the info, I'm actually a little surprised, I had hoped to see mileage closer to low to mid 30's on highway, 30 still isn't bad though don't take me the wrong way.
Saw a guy who had a Extd cab Chevy dually with a 4bt and a 4l80e trans lifted 6" with 22.5 rims and big tires claiming he got 28 mpg with it. I figured if it was true then a truck that was much smaller could get past that fairly easy. Again that's assuming if it was true, seems like a stretch to me but I won't call bs without seeing it.
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Old 11-13-2012, 04:16 PM   #4 (permalink)
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I havent done a highway run with the 47rh yet but a 65mph im only turning about 1500 rpms and a little more than 1600rpms at 70 with boost below 10psi. I think that it is possible for me to get closer to that mid thirties mark on the highway.

Most ppl with 4bts usually are at the high 20s on the highway, there arent that many above thirty. tough to do with a truck.
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Old 11-13-2012, 04:28 PM   #5 (permalink)
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If you leave it bone stock 25mpg and up, mess with it and it goes down from there plus life expectincy goes down by alot......
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Old 11-13-2012, 07:11 PM   #6 (permalink)
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Even advancing the timing up a bit and changing boost/fuel limit wouldn't give you a bump in economy? I know in my truck just timing helped tremendously an upping the boost with my fuel level helped. All together I've seen my best mileage @ 23 mpgs all highway. If I can't get much better than that with a 3.9 in a smaller truck I don't think it would be worth my while to go with that engine, ill just keep driving my 12v. As cool as it would be, maybe a smaller engine in a smaller vehicle would be better.
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Old 11-14-2012, 09:04 AM   #7 (permalink)
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Quote:
Originally Posted by jpavao1066 View Post
Even advancing the timing up a bit and changing boost/fuel limit wouldn't give you a bump in economy? I know in my truck just timing helped tremendously an upping the boost with my fuel level helped. All together I've seen my best mileage @ 23 mpgs all highway. If I can't get much better than that with a 3.9 in a smaller truck I don't think it would be worth my while to go with that engine, ill just keep driving my 12v. As cool as it would be, maybe a smaller engine in a smaller vehicle would be better.
Yea you can get better mpg by doing different things like you said. Porting the head is something else that you can do.

And there are plenty 4bts that arent stock still getting good mpgs is you drive them right. they will only use the fuel if you tell it to with your foot lol.
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Old 11-14-2012, 10:10 AM   #8 (permalink)
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Ok that's what I would guess, increasing the efficiency of any diesel should bring its fuel economy up some. I just didn't know really how much more you can go up from stock
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Old 11-14-2012, 01:44 PM   #9 (permalink)
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Guess all the cummins engineers are wrong when it comes to fuel effiency after all they designed a 4cyl engine that pulls a 1 ton bread truck around in the wind up + down hills loaded andget aprox 25mpg. It amazes me how many folk want to turn there engine up and have never even driven a 4bt.guess its me. These engines have inherent weak spots and its all they can do to live at factory hp rateings. I,ve bilt abot 50 of these engines, burnned pistons and cracked heads are very common in stock engines and trying to increase its output magnifies these problems. There at there limits stock in the hi-hp cpl's.
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Old 11-14-2012, 03:05 PM   #10 (permalink)
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Your absolutely right in saying the cummins engineers designed an engine to pull a loaded bread truck around and still get 25 mpg. But my concern is not with "turning the engine up" and pushing its limits. My question pertained to using this engine in a much different application, using it in a vehicle half the weight of a bread truck and using it strictly for mileage. I was only curious as to what some people who have done what I am asking have seen for best fuel economy. I personally have not had a lot of experience with these particular engines, so I'm not arguing anything, but remember cummins engineers designed and built these engines to perform well in a variety of conditions and applications, and on a budget as well. There are always improvements that can be made to a factory made machine, that's what I believe personally, it just depends how much money you want to spend. Don't take this the wrong way, I just don't want this thread to turn into a pi$$ing match about "turning up" these engines.
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Old 11-14-2012, 05:09 PM   #11 (permalink)
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That's just the way he seems to post, so don't take the pissy sounding responses/answers personally.

If you're all about mileage have you looked at all into the Isuzu 4 cyl. I think it's the 4bd - common in the Isuzu NPR trucks
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Old 11-15-2012, 12:41 AM   #12 (permalink)
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That's just the way he seems to post, so don't take the pissy sounding responses/answers personally
Take it with a grain of salt for sure... Looks like he was banned from 4btswaps

Just got my swap on the road tonight. I was running 50-55 in 3rd gear.
I'm guessing my 3.55s are too tall. Also running the 47rh and expecting 25 on the highway
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