Theyre specifically talking about the older 4d55 diesel from the 80s D50s. On mine I plan to add an intercooler, 3" exhaust and replace both F and R diffs with 3.55s and keep the stock 15" tires. It should have no problem with MPGs or reliability then. I'd like to add a 4d56 roller rocker head and lower the compression then up the boost with a good name brand turbo. Could I swap the 4d56 internals into the 4d55 block? Then I could afford to lower the compression to keep the engine cooler but should maintain the same feel as they've got a 5mm longer stroke making up for the torque. I'd like to make something to shut em up. When this truck is done I'll probably sell my 2nd gen and buy a mind 90s 12v cummins ram for a real drag truck since parts are way more available than performance parts for this engine. Plus it'll handle alot more abuse but I'd like to give this one something to brag about.