You must be thinking about this picture.
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That picture was my desktop for over a year, and I thought long and hard about putting one in. I am actually a forum member at 4btswaps.com and was long before I joined here.
Problem is, the 4bt is rather low in the sophistication, and weighs in at over 700 lbs. Fitting it in the Explorer is entirely possible, but for the engine you get I'm not sure it's worth it. Stock, the 4bt is rated at 105 hp and 200+ ft lb torque. The max rpm is 2800, but with the governor spring swap it can do up to 3200 rpm.
The 4bta nets 130 hp and about 250 ft lbs torque. With a few mods you can get a bit higher, but then I came across this YJ with a 3.3 QSB
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The specs on this smaller engine is similar to the 4bt, but is 200 lbs lighter. Then I looked deeper and realized the power available in the smaller diesels is rather impressive. So while I'm not yet decided on which engine I want, I know I want one smaller than the 3.9.