Although 4 valves work just as well as Quantime (member here), proved on his flowbench, he also proved that a 4 valve head was just as efficient too.
Never -the-less yamaha persevered with it for years, their top brass were reluctant to drop it I heard, even though manufacturing / parts / costs and servicing were much cheaper and shorter.
Genesis heads made it into microlight engines, snowmobiles and outboards too. I`m sure the snowmobile engine had a reversed head, exhausts to the rear.
Such a shame toffee inlet valves that `stretched` caused problems for a few years as many here have found.
It looks like Yamaha haven`t dropped the Genesis engine completely, they are still making these for American market snowmobiles I think, that Yamaha page is dated 2011:
http://www.yamaha-motor.com/sport/sno/tech_eng.aspx
130 horse triples, niiice!
