Question from Reader Dodge/Plymouth 318 Poly

Steve asked. 

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. 

Poly and Hemi


Thanks for the question Steve!!!  More to come on the 318. 

Thanks for reading. 


Tagged , , , , , , , , . Bookmark the permalink.

2 Responses to Question from Reader Dodge/Plymouth 318 Poly

  1. Steve Sears says:

    Did the 318 poly have spark plugs centered between the valves like the Hemi?

    Thanks for your answer.

Let me hear from you!!! Drop a note!!