what is the deference

What do you mean by the small motor & the big motor? All the isx's are 15 liter engines.They make a ism which is a 12 or 13 liter.

 

The "small" motor is the ISX 12. It is a 12 liter engine. It is narrower at the top. Rear mounted fuel pump. The valve cover will be about 6 inches wide on the top.

The "big" engine is the ISX 15 which is a 15 liter engine. The highest rating provided on the 12 liter is 425 or 450 depending on what you put it in. The 15 liter starts at 400 hp and goes to 600 hp. The fuel pump is mounted on the front gear train of the engine. The valve cover on the 15 liter will be about 16 inches wide.

Anything smaller than that is a midrange engine and no one cares about them.

 

It all depends on who you talk to. Generally the egr valves and coolers go. The VG turbos do cause a few problems also.