As long as you are on the subject what the heck is the difference between a regular 318 and a poly 318?
Poly is short for Polyspherical head. Some say it was a forerunner to the Hemi (Hemispherical head). Basically it was based on a theory that you could get more combustion or volume by angling the intake and exhaust valves then if you had them parallel. From Hemmings Motor News book of Chrysler Performance Cars…”The ploy’s exhaust valve is located parallel to, but offset from, the cylinder axis.”
This increased the efficiency. The hemi was constructed to increase the efficiency with a more angled system. The shape that the valves make are semi-circular with the Poly being a little flatter than the Hemi. I did the below image in Paint.
Thanks for the question Steve!!! More to come on the 318.
Thanks for reading.