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Old 11-18-2017, 11:44 PM   #1
WyattTX
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4BT Cummins in a full size pickup, a good idea or bad one?

As we all know 6BT/12 valve swaps are common and seem to have good results in performance. But I have thought about building a truck for fuel economy, rather than raw power. I hear of people getting 25 or 30 miles to the gallon with a 4BT Cummins. Is this really possible in a full size pickup such as a 67-72, or 73-87? If it is possible, how much would you be sacrificing to get that kind of mileage? I would hate to end up with something super sluggish or weak. I know little about the 4BT, so I am interested to hear some real world results. Thanks.
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Old 11-19-2017, 03:12 AM   #2
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Re: 4BT Cummins in a full size pickup, a good idea or bad one?

I'm not sure you'd get that high of mileage in a full size 67-72 or 73-87. I have worked on 2 different gmt400 trucks that had 4bt's in them. After we bumped the timing up and put different injectors in them and tweaked the injection pump, the owners said the trucks were getting 20mpg hwy. These were 3/4 ton 4x4 extended cad long beds. They ran ok but not what I'd want after I did a swap. In my opinion I'd rather start with a 5.9 and tune it to get optimal mileage and not sacrifice the power. The main reason is because a p pump 4bt is usually more money than a 6bt. By the time you start making decent power with a 4bt, you start sacrificing economy in a truck that weighs 4-4500 on up.

Sure, it has been done. It would be nice if someone like 744btcrew would chime in about his mileage. I know of a member here that had a 4bt in his 68 but later removed it in favor of a duramax and is much happier with his truck. Do you have a truck in mind to start with?
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Old 11-19-2017, 07:31 PM   #3
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Re: 4BT Cummins in a full size pickup, a good idea or bad one?

I have a 1963 chevy K20 4X4 with a Dana 60 front and a custom extended cab it weights 6300lbs and I have had a 4bt in it since 2000 it gets 24mpg average combined over years of fill ups.
It is similar to a good 350 power wise empty and closer to a 454 loaded pulling a grade, it was a cheaper option back when I built it and I would not change it, but if I were to do it again I would use a 6bt or an LS.
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Old 11-19-2017, 09:00 PM   #4
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Re: 4BT Cummins in a full size pickup, a good idea or bad one?

Thanks guys. I notice everyone says 4BTs are a little expensive, but I dont see how they are anymore expensive than the 6BT (in fact they appear to be cheaper.). How much worse would the 6BT be on mielage than the 4BT? I know someone who had a bone stock 89 Ram 1 ton , with the 6BT, and the Ram pickups the era, are not what I call a lightweight vehicle by any means. He had claimed to get 24 to a gallon, so I assumed having two less cylinders would equal higher mielage, but the way you guys are talking, it does not seem to be the case. I have thought about newer generation engines, (both gas, and diesel) but I prefer to stay away from electronics if I can, so that is why I look these older Cummins offerings.
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Old 11-19-2017, 09:11 PM   #5
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Re: 4BT Cummins in a full size pickup, a good idea or bad one?

I also have a 1992 W350 and it is much smoother runs at lower egt's and gets 20ish for mpg's.
To me it makes no since to convert when you can buy a first gen factory built, unless time and money is no matter.
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Old 11-19-2017, 09:20 PM   #6
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Re: 4BT Cummins in a full size pickup, a good idea or bad one?

A 4bt works really well in a light vehicle such as a Bronco which I also built it was an 1983 with a zf5 from a small block and used a Power stroke tail pipe it is as close to bolt in as you can get and it did really well on fuel. It would average 28 mpg and had plenty of power for that size of vehicle.
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Old 11-19-2017, 10:39 PM   #7
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Re: 4BT Cummins in a full size pickup, a good idea or bad one?

Quote:
Originally Posted by dieseldude4bt View Post
I also have a 1992 W350 and it is much smoother runs at lower egt's and gets 20ish for mpg's.
To me it makes no since to convert when you can buy a first gen factory built, unless time and money is no matter.
I have thought about that. But 1st gens (and 12 valve 2nds.) that are not horribly abused, molested or super expensive, are getting rare. The people that are fortunate to own clean examples of these trucks are very reluctant to sell them for a discounted prices. I cant say I blame them.
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Old 11-20-2017, 01:01 AM   #8
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Re: 4BT Cummins in a full size pickup, a good idea or bad one?

But you can't build (or at least I can't ) for discount prices and that doesn't include any thing for your time.
I will admit that it is often easier to spend a little money at a time as you have it but it always takes twice as long and costs three times as much as planed.
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Old 11-20-2017, 10:43 AM   #9
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Re: 4BT Cummins in a full size pickup, a good idea or bad one?

Quote:
Originally Posted by dieseldude4bt View Post
But you can't build (or at least I can't ) for discount prices and that doesn't include any thing for your time.
I will admit that it is often easier to spend a little money at a time as you have it but it always takes twice as long and costs three times as much as planed.
You are probaly right. But for a majority of people, this a labor of love. You get to undertake the challenge of putting a massive engine in small area. For some people this can be a nightmare, but for others it is like playing with puzzle.
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Old 11-20-2017, 11:14 AM   #10
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Re: 4BT Cummins in a full size pickup, a good idea or bad one?

Quote:
Originally Posted by WyattTX View Post
You are probaly right. But for a majority of people, this a labor of love. You get to undertake the challenge of putting a massive engine in small area. For some people this can be a nightmare, but for others it is like playing with puzzle.
That is what I have done and enjoyed, (more when I was younger than now) and in that case I would recommend a 6bt for fullsize 4x4's and 3/4 ton and up 2x4's and a 4bt for light or smaller vehicles. The 6bt will be smoother and more versatile while the 4bt has more vibrations and needs the right combination of gears to stay in its limited power band to work well, that is easier in a lighter vehicle.

PS. what are you wanting to build?
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Old 11-20-2017, 11:59 AM   #11
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Re: 4BT Cummins in a full size pickup, a good idea or bad one?

i have not really decided for sure, what I want to build.But I do know someone with some complete, but non running m1008 CUCVs. I have thought about basing a Cummins builld off one of those or a K20, or K30 LWB. Would a TH400 mate well to a cummins, or would it be smarter to find another transmission? Thanks again, you guys have answered a lot of questions.
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Old 11-20-2017, 01:03 PM   #12
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Re: 4BT Cummins in a full size pickup, a good idea or bad one?

In my opinion you need overdrive and I would just use a dodge auto to make live easier.
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Old 11-20-2017, 01:50 PM   #13
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Re: 4BT Cummins in a full size pickup, a good idea or bad one?

Quote:
Originally Posted by WyattTX View Post
i have not really decided for sure, what I want to build.But I do know someone with some complete, but non running m1008 CUCVs. I have thought about basing a Cummins builld off one of those or a K20, or K30 LWB. Would a TH400 mate well to a cummins, or would it be smarter to find another transmission? Thanks again, you guys have answered a lot of questions.
TH400 would be fine if you never planned on any freeway driving. Diesels like low RPMs. You definitely need an overdrive. The 700R4 with a 3.31 gear and a 28" tire is an awesome combo in my Jeep.
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Old 11-20-2017, 05:07 PM   #14
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Re: 4BT Cummins in a full size pickup, a good idea or bad one?

.."The 700R4 with a 3.31 gear and a 28" tire "..

I would agree with this also.

Ive seen the prices for 12v 6BT's slowly climbing over the past 5-6 years. Not too long ago you could find them consistently in the $1k-$1500 range, now theyre typically $2k+. If your looking at a 6k lbs + truck and/or 4wd, I would choose the 6bt for a few reasons;
-More shops are familiar with them and how to diagnose/repair/tune them.
-Theyre still cheaper then 4BT's, overall.
-MPG will be dictated by engine rpm. With a 6bt, you can run a 3:08 or 3:23 etc. and the 6bt will have the low end grunt to pull it without lugging, if you tune it for lower rpm. Texas is a big state with 70-75mph limits all day, youre gonna want the ability to run those speeds, even if only a few days out of every a month.

If you end up with a lighter vehicle, then certainly the 4bt should be on your short list. You could also use compound turbos on it if you wanted best of both worlds, and you have the mechanical aptitude/confidence to tackle and tune it.
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Old 01-24-2018, 09:24 AM   #15
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Re: 4BT Cummins in a full size pickup, a good idea or bad one?

Good info guys.
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Old 03-25-2018, 12:02 AM   #16
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Re: 4BT Cummins in a full size pickup, a good idea or bad one?

Hi, live in western Colorado, not what you call flat land here. Have a 90 chevy 1500 4x4 with intercooled 4BT, 4500 trans and 3.42 gears. This truck has been in the family maybe 12 years, gets around 25 MPG on the average. We took it to Newhampshire to pickup another 4bt and pull back a 1500 lb trailer, at one point it got 34 mpg, I told the wife to go back through her figures again and it still came out 34, did it a third time no difference. Got home and it went back to 25, go figure, different blends of fuel, so many variables, who knows. Anyway I even put an RV fifth wheel in the back and pulled our smaller 2 axle camp trailer about 60 miles away and back, only had to down shift to 4th gear on couple of the big hills. We take it to Denver sometimes pulling Vail and Loveland pass and never drop below 4th gear but you gotta watch the temperature. This has been a great conversion. The 3.42 gears are just right as it will run 70 mph no problem, this truck is fun to drive and gets lots of thumbs up because it don't sound like no 350 chevy.
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Old 03-26-2018, 09:14 AM   #17
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Re: 4BT Cummins in a full size pickup, a good idea or bad one?

Paenutbutter Sounds like a fun truck 25mpg is good for a full size.

As for gearing in any diesel you want to have the rpm in the sweet spot. My peak torque rpm is where I cruse on the highway. My motor is quite happy.
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Old 03-27-2018, 01:14 AM   #18
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Re: 4BT Cummins in a full size pickup, a good idea or bad one?

This is the truck I was telling you about, We've had it from the Oregon coast to New Hampshire, this pic is at the Bonneville salt flats. Not exactly full size but when we're cross country its always half loaded.
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Old 03-27-2018, 08:41 PM   #19
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Re: 4BT Cummins in a full size pickup, a good idea or bad one?

Nice truck peanut butter!
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