Years ago there were a few companies like Fueling and Sputhe who made 4vlv EVO heads. Id like to buy a set if theres any around.
Harley guys had poor use with those heads because they used them on stroker motors. The 4vlv per cyl head flows better at higher flow rates found in shorter stroke motors. The rocker arms in most of the 4vlv heads were pretty heavy so heavier springs were required which set up harmonics which broke springs.
My head guy, Radar, has the molds..not selling heads yet. Busy Dude. I may have heard 40hp???Don't quote me. This guy did my heads back in the 90's on a couple of my HD's One of the best head guys in the mid-west.
M8 engines are the 4 valve heads. They can make good power but when you get into cam changes and bigger springs you have to stud the rockers or the rocker mounts will crack and break off.