I see this statement, describing the 350's in 68's, here, my Motor's Manual and other places........."standard two-barrel 350 cu in version, four-barrel "Ultra High Compression" 310 hp". So, my query is, is there a physical / build difference in these engines between the 2 and 4bbl's? Correct me if I am wrong but merely putting a 4bbl on instead of a 2bbl does absolutely nothing for compression. Are the extra horses attributable solely to the 4bbl? Assuming what I am writing here is correct, why did they call it a "high compression" engine then? Just for advertising?

 

Compression is due to small dishes on the piston tops.

 

I realize that is part of what determines a compression ratio ...so did the 4 bbl engines come with different pistons?

 

Yes, in that era, generally speaking, shallow dish pistons were used with the 4-bbl carb. Along with a hotter camshaft, and typically dual exhaust. It's a package.

"low compression" was like 9:1
high compression 10:1, and Ultra High Compression like W31 or Toronado engines approached 10.5:1