Plymouth offered a 318 poly power pack engine option from 1958 through 1962. It included a 4 bbl carb, dual exhaust and a up graded dist vacuum advance and produced 260 HP. The better choice seems like it would be the 350/361 golden commando at 305 HP. For that reason the 318 power pack is a rare find. I have heard rumor's that there was a special camshaft in the power pack engine but could never fine any reference to that.
Any information about that?...
318 poly power pack
