The ISX Cummins is the 14-liter Cummins engine, while the ISM is the 11-liter smaller block Cummins. There was an L-10(liter) engine about 10 years ago but was replaced by the M11, which in turn was replaced by the ISM. If absolute max power(500+HP) is needed, use the ISX, otherwise find an M11, as it will give you much better fuel economy. Hope this helps you.
Gary
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