688's lack the HP blow system that 637s have. We used to talk about how we'd line-up the ventiltion system to blow MBTs with the diesel, but we had no idea if it would actually work.
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So the choice is an LP blow or EMBT blow? I never knew that. Strange that is a system that would be removed from the mix, although I guess I could count on one hand the number of times any 637-class I was on surfaced using a HP blow.