Subaru refuses to call their heads, Hemi heads. They call them boxer heads. Dont they look like Hemi heads?
Well, there might be an educational tangent or two? "Pentroof" is an approximation: it's usually closer to a round-hipped thatch roof. The point is that the circles of the two intake valve heads are in the same plane, and likewise those of the exhaust valve heads, suggesting the two slopes of a pitched roof. Therefore none of the valve stem axes p*** through the cylinder axis. Therefore the circles don't describe a common part-sphere. Abarth called their take on the hemi head the Testa Radiale, i.e. both valve stems were on radii of the part-sphere which formed the combustion chamber. If I remember correctly they did develop a 4-valve version of this, with all the valve stem axes radial and none parallel as in a by-now-cl***ic pentroof, also based on the same Fiat pushrod architecture. The Subaru head has pairs of valves with their stem axes parallel and their heads in the same plane. There is no way to develop a single part-sphere from that, so not strictly a hemispherical combustion chamber. While we're doing tangents, a lot of guff gets spouted about the thermal efficiency of a hemispherical combustion chamber. A full-spherical combustion chamber would have the smallest surface area for its volume, which is desirable: but on an engine of square dimensions that would imply a compression ratio of precisely 1.5:1. That is hardly practical. Making the piston crown flat rather than hemispherically concave does improve that a bit, at the cost of a good deal of thermal efficiency. To get to anything like a decent compression ratio would involve domed piston crowns, leading to a combustion chamber shaped like half an orange peel, which is really extremely thermally inefficient, worse than any humdrum bathtub or wedge shape. It does afford the opportunity to get very big valves in, though, big enough to make it worthwhile if you're after power rather than efficiency as such.
The Jalopy Journal
