Optimize the spark curve for a SlantSix.
I'm tuning up the 225 in my 1973 Duster, and I feel that the distributors' centrifugal weight / advance curve could be better. I took the distributor apart last winter and fully cleaned and regreased it, but made no changes. It had the one light and one heavy spring.
That's how they were built.
I'm running around 3-4 degrees initial advance of timing, but even with 93 octane I think I hear it pinging at higher rpms under heavy load.
High-test gasoline is not necessary or beneficial for a stockish Slant-6 tuned properly, so finding and fixing the cause of this ping should be near the top of your list.
I have to wonder if it is getting too much advance from the weights and slots.
Maybe, but there are other possible causes, too. Restricted exhaust, improper or sub-optimal spark plug selection and/or plugs installed without removing their metal ring washers first, lean mixture, carbon buildup in combustion chambers, mud buildup in cooling passages, poor oil control (oil getting into combustion chamber) — even if it's not enough to cause much or any visible smoke, it'll take cause the effective antiknock performance of the gasoline to nosedive, and it'll carbon up the combustion chambers, aggravating the ping further.
I already reduced the base timing several degrees, which helped
These engines with those distributors tend to run optimally with base timing around 5° BTDC. The factory setting of TDC gives mushy acceleration and driveability and poor MPG.
It isn't leaking oil but it is using a quart every 600-700 miles, and I don't think it was ever apart, so I'm sure the valve seals and ring seal is not optimal.
There y'go. That right there will easily cause your pinging, or at least contribute to it.
For distributor dial-in, see Doug Dutra's article
here. For better ignition, do the
HEI upgrade . Spark plugs: as described in
this post.