Distributor advance springs

For example, here's the range of acceptable timing advance for a '71 distributor for manual transmission, compared with a '67.

1971 initial is TDC at 750 rpm.
1967 initial is 5* BTDC around 650 rpm.

With good fuel and such, may be able to set the initial a little higher and pick up a little fuel economy. The factory in most cases allowed plus minus 2.5* to account for variations in fiuel, elevation and advance curves at lower or higher end of the spec.

But setting that '71 distributor to have 5* or more at 650 or 750 rpm is likely to result in ping begining around 1600 rpm under any load situation.

Red line illustrates the resultant timing of a '71 distributor installed with 5* initial