it's somewhat awkwardly put i think the distinction is between a main bearing six and a main bearing six in that the former gets crankshaft whip under control and gives you more potential for high end horsepower which you can exploit with an easily added overhead cam or two however the general gist is that the american manufacturers went off down a technological blind alley with the big pushrod v s and got sucker punched by the fuel crisis while bmw mercedes and the japanese managed to crank out as much power from more fuel efficient overhead cam straight sixes there's probably a better way to put it the point is probably moot now that v s have taken over the market but american manufacturers are still off down a blind alley with the big ohv gas guzzlers