I'm not looking to gain HP, I'm just talking about the torque curves between the different TDI engines. In general most diesels will have much more low end torque vs the 4.0 and more often HP numbers come from high in the rpm band which makes higher hp often meaningless, especially because it's all about torque in the rpm range one drives in.Just my perspective from dealing with previous modified diesels, I would not swap a TDI with the intention of gaining "much" power over the 4.2/4.0 engine. You will likely spend a ton of money on parts and chase down the next weak link of engine and drivetrain components and fixing forever. These engines are great for mileage, simplicity and long term reliability if you leave them relatively stock. People will argue with me about this, but you have to consider long term when going through the effort of swapping engines. How long is something going to last running at 100% load and demand. Just something to think about.
I use the 10% rule on increasing hp and will only consider a bump if it occurs in the usable rpm window.
Typically for turbo diesels, I put WAY more stock in displacement for all around functionality, but I'm interested in the tdi, due to availability rather than doing another R2.8 or something. Plus this will be a slow build rather than a fast build for me. If it takes a year or 2 then that's ok, especially if my money investment is fairly small. I will probably buy an adapter but I'm gonna copy and fabricate most stuff unless the price point is right. I do plan on doing some engine mods as deemed prudent, just due to a long service history that is known for typical issues.