3.9 Cummins 4BT

[shagrat]

Anyone here done this swap, or considered it? I'm considering different engines for my 68 W200. I was thinking the 4 BT might make a good replacement for the old tired 318. It wouldn't be a speed demon, but should get good mileage and plenty of power in low range.
Any thoughts????

[Kaegi]

have not done it. thought about it but I would go with a 6bt because it fits and cost about the same to convert. just have to move hte radiator to the front of the support and then plumb an intercooler.

[gww]

morn n, my .02 worth. as with any diesel swap. gearing should be at the top of the list (do to lost rpms.) the 3.9 is a good little engine but unless re balanced and mounted right could rattle your dash or teeth pretty good at a idle. as far as power, depends on your check book. 90-400hp. but moneys involed past 150.. guess its your end use that would dictate what will/won t work. if you go forward be sure to educate yourself on any diesel you choose as just with the 4b-4btta and high presure electronic, the fuel systems (pumps an injectors) , piston bowl designs and turbos are big money items that dictate hp.. before you jump in there get the engine history, a cpl or build number and do some question asking on a known diesel form.. may save some grief in the end. luck to ya. gw

[mopardwh]

There is unlimited info on cummins swaps. Here is the end result!

http://www.youtube.com/watch?v=8n32J9vq ... ata_player

[shagrat]

Very cool truck in that video. There is also a 71 D100 with a 4BT on youtube. He claims 38 MPG. I'm sold! I think the 6 cyl Cummins is overkill. I won't be pulling a semi-trailer, anyway.

[ecfuerst]

morn n, my .02 worth. as with any diesel swap. gearing should be at the top of the list (do to lost rpms.) the 3.9 is a good little engine but unless re balanced and mounted right could rattle your dash or teeth pretty good at a idle. as far as power, depends on your check book. 90-400hp. but moneys involed past 150.. guess its your end use that would dictate what will/won t work. if you go forward be sure to educate yourself on any diesel you choose as just with the 4b-4btta and high presure electronic, the fuel systems (pumps an injectors) , piston bowl designs and turbos are big money items that dictate hp.. before you jump in there get the engine history, a cpl or build number and do some question asking on a known diesel form.. may save some grief in the end. luck to ya. gw

I agree on the gearing annnnnd since you brought it up what are thoughts ? I am also considering a 4bt but i have never been able to figure out what gears I need.
can you give some possible suggestions on gearing for both tranny and axle?

[digdoug]

You want to be cruising at 1600-2000 rpm. With stock height tires you would want @ 3 to 1 rear end gearing with a direct tranny,or @ 3.5 to 1 with overdrive.

[gyman98]

You want to be cruising at 1600-2000 rpm. With stock height tires you would want @ 3 to 1 rear end gearing with a direct tranny,or @ 3.5 to 1 with overdrive.

Very good advice

According to Cummins the "sweet spot" for the highway cruising with the 6bt is 1600 rpm. I would imagine the 4bt would be similar

[ecfuerst]

You want to be cruising at 1600-2000 rpm. With stock height tires you would want @ 3 to 1 rear end gearing with a direct tranny,or @ 3.5 to 1 with overdrive.

Thanks
What if I were to have about a 33" tire?

[digdoug]

Same. I was thinking of a 31.5" tire as "stock height". If you were going with larger than 36" or towing heavy,than you might think about lower (numerically higher) gears.

[ecfuerst]

Awesome! thanks