If by mechanical you mean no computer control necessary, I say 603. Good durable engine will last many miles with minimal maintenance, quieter, more efficient, and smoother than the old iron-head engines, cross-flow head, hydraulic lifters, more power, ... if you consider the 606 to be mechanical, I'd take the 606 turbo over the 603 in a heartbeat, more fuel efficient and more power.

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Gone to the dark side

- Jeff