In spite of the comments posted here by some very sincere people, there is no huge inherent engineering advantage to the inline six VS v-8 cylinder arrangement. However, there are some worth mentioning.
1. It fits nicely between the frame rails and provides room beside the engine for accessories and work space.
2. Large bore/stroke measurements means the engine breathes well at low speeds - better torque / cubic inch.
3. Reduced complexity. There are fewer parts, fewer gaskets, etc. This is only a minor consideration, to be honest. Major engine part failure is very, very rare for the Cummins and IH engines.
Many of the over-the-road truck engines are inline sixes especially for reasons 1 and 3. They require less machining and fewer complex operations to create.
As far as my gaze into the tea leaves of the future...
1. More Power! I don't think the horsepower war is over, so expect more power. Just remember that the more power you get, the more you can break stuff.
2. More Quiet! This forum has had spy reports that say that the next generation Cummins B engine will be much quieter, due to some very creative engineering of the block and heads.
Well, hope that catches your interest...
Attention: TDR Forum Junkies 
