[kenikh]

13:1? Wow, that is super duper high, even on race gas and GE80 cams, especially for big 100mm pistons. You will definitely need to use C12, C14, e85 or e100 and likely an EMS w/ knock control.

Your dragster buddy may be right for dragsters, but totally wrong for an air cooled 911 motor.

As far as your octane to compression question, I wish I had the math to do it, but a refernce point is that @15:1 CR, you'd need 137 octane. All of the 2 liter racers use C16 for their 13:1 cars, so you mgiht be able to interpolate a calculation from those numbers. Remember, your pistons are 1.5 cm larger in diameter than the 2 liter racers, so you will have a higher propensity to knock due to increased flame travel distances.