1.6 rockers with stock 318 cam??

Just to back up what others have said about the rocker arm ratio change. The stock 318 cam has .373"/.399" lift. Changing to 1.6 rockers will result in .397"/.425" lift. I just recently tested a 318 head with some bowl work that flowed 155.8 cfm at .300" lift, 168.7 cfm @ .400" lift and 169.9 cfm @ .500" lift. A switch from 1.5 to 1.6 rockers would result in no perceptable performance benefit with the stock cam and heads.

As to stock cam duration. The stock 340 camshaft is 268/276 advertised and 208/214 @ .050".

I don't have the duration at .050" for the 318 camshaft, but the advertised duration is 240/248. If the 318 cam used only 50 degrees rather than the 60 degrees the 340 cam uses to get the .050" duration, the 318 cam would still have only 190/198 degrees. Seems to be a fairly mild camshaft.

But......

Unless you have tested the duration of the aftermarket cam and the stock cam, at the same starting (advertised) and .050" lifts, you cannot accurately tell the difference between the aftermarket cam and the stock cam.