[tubman]

That is my picture, and that is an Edmunds head, not an Evans. I have no experience with Evans heads, but as far as I know, Edmunds heads are the only ones with the spark plug moved far enough so it doesn't interfere with the exhaust valve. It allows the use if an extended tip plug, specifically a Champion L82YC. In Champion nomenclature, "L" means 14mm, 1/2" reach. while "82" is the heat range. "Y" means .060-.090" projected tip and "C" means a copper ground electrode. Please note that with this plug, the spark is well out in the combustion chamber, not up in a hole someplace.

This is not the place to expound upon combustion chamber theory, and anyone really interested should reference the works of Sir Harry Ricardo, who had this figured out 100 years ago. In an "L" head engine, combustion efficiency is mainly a result of efficient flame front travel. It will get a more robust manifestation when it has a mixture with good swirl (produced by proper quench) and a spark introduced in the midst of it. A set of properly massaged Edmunds heads does a far better job of this anything else out there (short of exotic custom built units.) I have also added a picture of an Edmunds head taken right down the spark plug hole, and there is nary a glimpse of a valve head in it. I have also added a picture of an L82YC plug in an Edmunds head along with the existing picture of a standard plug in the same head.

Barney Navarro did some wonderful stuff with flatheads (pop-up pistons and deep domed heads, for one) but it seems H&H picked the wrong heads to reproduce for street use. I will again reiterate that 100% of the ports and valves in a flathead Ford are in the block, so designing a head to be "the best flowing" seems to me to be a fools errand. Concentrating on compression and combustion efficiency would have been a much better idea. "OL' Ron" on this very forum was an advocate of the relocated plug, and often wondered why it hadn't been done.