Timing issues on '71 318

The stock spec for a '71 318 is + or - 2.5 degrees.

But, that's not anywhere near where I run mine. Most do pretty good with 10-12 degrees advanced. For some reason, the 318 in my Challenger runs best around 18 degrees advanced. Don't know why, engine was rebuilt by the previous owner so I don't have all the specs, could be cam timing etc. But I run it 18* advanced, with all in at 36*. Vacuum advance is disabled, mechanical advance only because starting at 18* and adding vacuum and mechanical advance gave me way too much timing. I even had to tweak the mechanical advance to end up there. I also run an edelbrock streetmaster, 600 cfm Carter and headers.