3.9L Cummins/Isuzu diesel swap in 89 Suburban

[87Skier]

Originally Posted by 79c25

4bt, nv4500, np205 passenger drop. Should put you around 1800 rpm in 5th going 60. I know in my old 12 valve I got about 20 mpg going 65 everywhere, I think that the 4bt will do a little better. But, that's if you wanted to go with a manual. As far as an auto the 4L80E should be right about the same. But I'm interested in this swap as I'm looking to do the same in my 84 K5. As a side note the 12 valve does fit in the engine bay but I wouldn't use a 4L80E behind a 12 valve personally. That's my $0.02.

Why not use the 4L80? Personal preference? I was thinking of using it because of the cost of finding a manual transmission and having to convert my auto to manual. I really like driving manual transmissions though.