".........For some reason, I assumed that the Chrysler Hemi/Poly had the same factory compression rating as the Dodge......."

The mid 50s thru the 60s were an interesting time for engine development. As performance increased so did the quality of the fuel to support it. The guy who could afford the top of the line car didn't have a problem with the few extra cents for the better grade of gas. From 55-58 Chryslers gradually went from 8:1 to 10:1 advertised (optimistic) compression.

The guy who went in and bought a Plymouth or Dodge with a grocery getter 6 or 2 bbl Poly was more likely to appreciate only having to pay for regular grade gas. Historically trucks and industrial got whatever cheap crap gas was available resulting in a requirement to keep compression low.

"........This discussion raises a question... probably could end up hijacking this otherwise useful thread... what do we call these engines? The group that includes:
DeSoto Hemi
Dodge Hemi
Dodge Poly
Chrysler Hemi
Chrysler Poly ........"

MOPAR ?


Adding the Polys and Hemis to the mix sure made keeping track of parts interesting.......likely a big part of the reason Chrysler became a leader in standardizing engines across corporate lines in the late 50s.

As far as Plymouth having its own V8 engine that occurred in 57 with the 301/318. Just speculating, but with the 318 being a "new" design I suspect that its development had an eye on corporate sharing.