I think in threads like this before what we concluded was SCR's of 9.5 on iron heads and just about 1 point more or 10.5 for aluminum heads with a DCR of 8.5. is a save bet given the variances of pump gas.That there have been guys push the DCR to 9.0,but that gets towards the ragged edge.That the possibility of headers covering up any noise detection is very real.Interesting Techinspector 1 posted tight quench helps prevent dentition as well.