The 4l80-e was based on a turbo 400, 4l60-e replaced the turbo 350, 2004r replaced the metric 200. All gm's way of making gas mileage. I have built all the od's for street and all out racing with great results. Art carr has the best name in the industry for the 2004r but is very expensive. 4l80-e's were only in trucks and are very heavy.