Originally Posted by bwc175

If I recall the details correctly, the engines on the MD90 are significantly heavier than the JT8s on the MD88. The weight difference necessitated stretching the MD90 forward of the wings to achieve proper balance for controllable flight, so the MD88, among other reasons, can't take the newer engines without being too tail-heavy.

There is however a product on the market to reduce fuel consumption, engine wear, and noise on the MD88-- see http://www.dugankinetics.com/

With Delta apparently intending to keep it's fleet of 100+ MD88s for several more years, it would seem this mod would be a good fit for them and pay off relatively quickly. Of course we don't know all the decision-making factors involved, but there must be some reason they have not jumped on it.