head vs head vs.............

In my opinion a "true" Hemi head has a deep spherical chamber (no quench pads) and the valves directly in line at the north/south poles (like the classic Chrysler/Keith Black 426 hemi). The modern DCX Chrysler hemi head is similar to the classic head with the north/south valve placement, but with slightly recessed quench pads. The Ford Boss 429 hemi isn't technically 100% a true hemi (but damn close) since the valves are rotated a few degrees off of the dead center north/south poles, and some have quench pads, some don't. The new Jon kaase Boss 429 street hemi head chamber kinda looks somewhat his 426-460 P-51 head's chamber, but rotated more so the valve placement is similar to the normal boss 429 valve placement.   

 

The factory angled valve heads thought of as "twisted wedge" or "semi hemi" stuff (351-C, 429/460, and big block chebby) while not a "true" inline valve wedge chamber (inline east/west valve centerlines), are also not anywhere close to being a true hemi valve chamber layout either (as in north/south valve centerlines). I always just think of them as neither wedge nor hemi, but a completely separate class of their own. And while it's true the 351-C open chamber might have a somewhat spherical shape (no quench) it's not a true 1/2 sphere as the roof of the chamber is too flat/shallow. And the valve placement is nowhere near a north/south hemi layout.

 

The big block chebby head while available in what was called both open and closed chambers, was never really a true "open chamber" because it still has quench pads. The bbc "open chamber" is really just a real big fat "D" shaped chamber with quench pads. And the bbc closed chamber is more of super skinny "D" shaped chamber (really more of an oval with flat sides) with bigger quench pads.   

 

If you look at the Pro Mod class, all most 100% of the blown alcohol cars use a "true" classic open chambered hemi head/valve layout (aka KB 426 hemi). While some of the gas/nitrous P/M cars use either some form of an updated  twisted wedge/semi hemi layout (BBF, BBC) with an exaggerated "figure 8" chamber, or the updated Boss 429 ford "almost true hemi" layout. The new Sonny Lenard (sp) monster cubic inch "chebby hemi" head is making big power numbers with & w/o nitrous. But I haven't yet seen if they are a true hemi, or more like the Boss 429 hemi layout.  

 

As for the 351-C open vs closed chamber debate I think there is in fact some HP/torque difference to be had in a high compression race engine (w/wo a power adder) even if both are at the same static compression. The open chamber piston will be a little heavier with a bigger dome. And the quench properties of the closed chamber mixing air/fuel better & helping fight detonation is a plus, especially with nitrous use. But it might be less of a difference between the two on a turbo motor because of the increased air speed/velocity the turbo brings to the table to help mix the air fuel.

 

I know one guy who many years ago basically did his own 351-C open vs closed head "test" on his 67-68 nitrous Mustang. His thinking at the time was that the open chamber head & flat top piston had more room inside for more nitrous vs the closed chamber head & flat piston thus making the car faster. Only problem was the motor made less power NA with the open chambered head, so in the end it actually needed the extra added nitrous just to keep up with the closed chamber head and less nitrous. I think his best with the closed chamber heads & less nitrous was around 6.60's-6.50's  in the 1/8 mile. When he first switched to the open chamber heads it slowed down to 6.90's with the same nitrous and had detonation flecks starting to show up on the plugs. He then added a ton more nitrous and finally got it back down to the 6.60's range.