Re: 351C cylinder heads - squish and other issues

Thanks for another excellent post, George. Well written as usual.

Yes I would agree that the open chamber cleveland head is similar to a hemi head. I also would say that a hemi head is a poor choice for a pump gas motor that needs to be detonation resistant on modern fuel. A hemisphere is a good shape for a combustion chamber, but typically in order to get a proper compression ratio you need a domed piston, so the chamber ends up shaped more like a new moon (crescent) than like a sphere. That isn't a good shape for a combustion chamber, because the flame front has farther to travel.

Now, Ford designed their semi-hemi Boss 429 heads in a better way, with quench pads on the sides to reduce chamber volume while keeping the chamber more compact. The hemi's strength is flow, not street performance. I would argue that's why most modern heads have small, compact (and wedge shaped) combustion chambers and typically have quench areas. I'm not going to say the wedge is the best shape for a chamber, because it isn't - however I believe a more compact shape is generally better. The hemi chamber shape isn't the best for a street build, but it is great for a 10/10ths racing build with a blower running high boost and the chamber on the edge of hydro-locking.

Did Ford recommend the same ignition timing for open and closed chamber heads? I think ignition timing is a good indicator of how good a chamber design is. I understand that even the Cleveland quench chambers require a lot of spark advance - the better chamber design is part of why CHIs and other modern heads produce more power (and require less ignition advance).