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4BT or 6BT engine swap in a 1948 B-1-PW

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  • 4BT or 6BT engine swap in a 1948 B-1-PW

    Good day ladies and gents,

    not new to the forum, but first post on here. I acquired a year ago 2x FFPW, a '50 and a '48. the '50 is the donor truck (p.m. me if interested, I got a bunch of parts to get rid of).

    anyhow, I'm at the point of purchasing a ''new'' engine. The 230 still runs good, but will not stay since I don't want a gasser in my truck.

    I'm debating on the 3.9L Cummins (4BT) or the 5.9L Cummins (6BT), both with a P7100 pump for the engine swap.

    Thing is, plenty of people did the 4BT conversion, so I got the info on that, but not many with the 6BT, so I am looking for information, mostly on fitment, and what needs to be modified to get the engine in. I want specifics (i.e. new engine angle, mount locations, dimensions of the dog house in the cab, etc.). My biggest concern is weight and how much the stock frame will have to be reinforced and for sure the amount of space it'll go in the cab. In comparison, a 2019 Ram 1500 has 18'' of width in the foot well, I'd like to keep it somewhat similar.

    I could go with the 4BT, but I don't want to ever run in a situation where I wished I had the 6BT because I'm towing something. This being said, the truck will be my DD once complete and will see some trails, so I'm also looking for a good balance between power and fuel consumption.

    Either way, the engine will be backed by a NV4500, a NP203/205 doubler with a Dana 60 front and Corporate 14 bolt at the rear. Tire size will be 37 or 40'', not sure yet how big I want to go.

    Thoughts and opinions are appreciated.

  • #2
    I can't help with specifics, but you likely want the 6BT based on goals you have set. The 4BT is capable, but it is not as smooth as the 6 and you will absolutely be pushing it in a 7000+ lb truck at 70 mph and trying to tow anything with 37" or larger tires and the crazy low RPM limit of the stock engines.

    That said, the 6BT is a ton more work, the 4BT is a tight fit in the engine compartment without modifications length wise. Weight is definitely a big issue, my spring pack was a stock number of leaves with an additional 2" lift and a modified spring rate, and I likely need a leaf or two as I am still having interference issues with the front axle hitting the pan. A 6 will only make that worse.

    Honestly I still think the best option for a 6BT swap like you are talking about is to use the entire donor frame and running gear and fit the PW body over that. The work load is probably the same, but you get an entire chassis designed for doing modern driving and towing as well as the kind of power a 6BT puts out. It's not easier but it's both safer and easier to work on down the road as you are not trying to get parts for a Frankenstein of a vehicle. Plus in my opinion, once you toss more modern axles under these trucks and all that go with it it's usually too late to worry about how the modern suspension will look under the body, that ship has already sailed.

    Good luck! This is a massive undertaking with little in the way of a cookbook to make it "bolt on". without knowing what your background in automotive work is, my biggest recommendation having been in your shoes MANY years ago now is to take a careful look at this project and make sure you are being honest with yourself. I started mine thinking it would take 3-4 years. It's been triple that, and while the truck runs and drives, it's still not finished, and my time and excess money to continue is not what it once was, not to mention the mistakes I made that need to be fixed or re-done.

    I don't mean to talk anyone out of their dreams, I don't regret the project for all I learned and what I did accomplish, but too many people start with a dream in their head and a wrench in their hand and end up trying to sell a pile of parts at a loss.


    • #3

      thanks for your input. Putting the cab/body on a newer chassis is not my intention, it will look funny and its not a battle I wish to pick, I prefer upgrading the old to the new, and my frame is in really good shape, so I'm not too worried with safety per se.

      I was figuring too that the 4BT may be a stretch, but I did read that the application for the 4 BT can go up to 16,000 lbs GRVW, meaning that I could tow about 9000k ,which should be plenty for me. Note that I if I get a 4BT, I'm going to build it to get about 200 hp, 400 ft-lbs, which I hope should be enough. I also understand the suspension problem. Not planning on keeping that stock either, and you're right, a 2'' lift seems to be in order in the front, maybe even a body lift may be required to make everything fit without compromising the space in the cab.

      In term of my background, I'm a mechanical engineer, and I have a decent knowledge of automotive work. Whatever I don't know, I go ask my dad (heavy Diesel mechanic) or my coworkers (welders, mechanics, bodyworkers, etc.). So I am not afraid of taking big jobs. Also, I told my wife that it will take me 10 years and $100,000. So, I'm covered from the boss by that clause, I got time :)

      to everyone else, still looking for info, and thanks in advance!


      • #4
        I prefer upgrading the old to the new, and my frame is in really good shape, so I'm not too worried with safety per se.
        I'm not talking about crash safey. I'm talking about trying to put modern power through a chassis, suspension and steering designed for a truck that did 55. These trucks will always be death traps comparatively, it's about making sure the whole system is designed to work together, and you are essentially already replacing everything but the frame, which to me already looks funny with modern rims and axles in place of the old BUDD rims.

        As for the 4BT, what it's rated for and what it will comfortably do are different animals. Remember the NV4500 was rated to about 16k lbs and Dodge put it behind a 6BT to get the performance they needed. You will struggle to tow 9000K at anything approaching highway speeds and maintain it without pushing that engine hard. The 4BT is strong, but the small RPM window for diesel engines means gearing becomes super important, that's why dodge ended up going to a 6 speed with the diesels, they had the same 1st and OD, but added an extra gear in the middle as the 2-3 shift was a struggle for a diesel at GRVW in the NV4500. If you want to stay smaller you are better off finding the later common rail motors, but they get into even bigger money plus the electronics and such. They are much nicer motors from an NVH standpoint though.

        Stock governor is 2300 RPM, you can go to about 3K but you are giving up some longevity for that extra power. Best cruising RPM is about 1800 so play carefully with a gearing calculator before picking ring gears and tire sizes.


        • #5
          well, to whoever this interest, I have a 4BT on the way because I couldn't pass the deal, but I'm still searching....I did find a guy not too far from me that is putting a 6BT in his PW, and it looks like it will work very well, so I might change my mind along the way, either way, I don't think the 4BT will lose value in the next 1-2 years.. this is the timeline I got from niner domestic (i.e. wife) to get my sh*t together, and since we might be moving this summer or next summer, this timeline is even more relevant!


          • Matthew Welcher PWA
            Matthew Welcher PWA commented
            Editing a comment
            For years I have been intrigued at the power, mileage and torque that comes from the simple 4BT.
            I Have yet to do a diesel conversion into one. Someday I may. Until then I will live vicariously through you all.
            Please keep us posted and your time-line sounds very doable.

        • #6
          so what 4bt did you get? It does make a difference. i am running a 105 hp with arp head bolts, dynamic advance and compound turbos on stock injectors in what may be a 4 ton truck. I would not want to tow with it. The truck can cruise all day long on the highway at 60 - 65 miles an hour at 7 to 10 lbs boost, but it sure goes up hills slow. Basically I end up in third gear at 2000 rpm and about 30 mph. Steep grades and I have ended up matching highway trucks for speed and get passed by empty rigs. But boy is the milage great.


          • #7
            I'm going to go pick up the 4BT on 19 Feb, I traded it with another PW enthousiast for parts and stuff. I'm thinking that I'll keep it, but I won't put it in the PW, not this one anyways, I'll wait a bit and find a good deal on a first Gen Cummins truck and use it as a donor for the PW.


            • #8
              I know its been a while, but I decided to go with the 6BT route. We'll see how it goes.