273 315 closed chamber heads

920..they're closed. The diff between the 315 and 920/234/302 is...the 315 is a true heart shape chamber with point.
How that might swirl effect or increase power in itself is theoretical at best..I've not proved anything myself. The 315 usually are about 66 cc. There is only a few cc diff between the open chamber 675 and the closed versions which ALL differ from one another.
302-64cc
315,920,234-66-67cc
675-68cc
Your results may vary.