360 Tune Up

Rock Able said
I usually use manifold vacuum on SBC's. Why would ported be recommended on SB Mopars? My engine is idling fine.

As far as I can tell it most has to do with the mechanical advance curve. Generally you'll find the factories prefered using ported. Doing so keeps the two advances related to their jobs. The mechanical advance main job is mostly about time in fractions of a second. Its job is to provide more lead time as increasing rpm brings the piston up quicker and quicker. It's not a 1:1 relation because the combustion efficiencies change as rpm increases. The efficiencies are different for every engine.

The vacuum advance's main job is to provide more time as the combustion is slowed due to lower fuel density. So leaner conditions require more time for combustion pressure to be maximized through the crank angles of maximum leverage. In terms of power efficiency engines run best at relatively rich idle mixtures. So ignoring emmissions considerations, timing at idle should be around 10-14* BTC at 550 - 650 rpm for best efficiency on these engines (stock). You can see that looking in the 67 and earlier service manuals.

This is somewhat related to the fact that most of us at one time or another came to believe that ported timing was an emissions thing and manifold was the way to hot rod things. But its not true.

One way we (seems like I'm not the only one) would fool ourselves was when we noticed that the engine at idle seemed to like more timing when we turn the distributor. The engine will run faster and faster. But there's no load on the engine when we're doing this, so it can do this and run lean. In gear - which is easy to check with an automatic but of course not with a standard - its different story.