It will have to do with your camshaft duration, more specifically when you close the intake valve. I know people who have run 13:1 on 93 octane pump gas. I have run 11.9:1 with iron heads and .160" quench on 91 octane.
And this so far is the best response. Static compression ratio is a useless number and really means nothing. Dynamic compression ratio is the only one relevant. This number takes into account the closing of the intake valve.
Ex. A 10.5:1 motor might have a 9:1 dynamic compression ratio and will detonate on pump gas. Change the camshaft and say now the intake valve closes later and its still a 10.5:1 motor but the dynamic compression ratio is now 8.5:1 and now it runs fine on 93 octane gas.
See the problem?
Also, keep in mind, aluminum heads keep a cooler combution chamber vs. Cast iron heads and for that reason they can run a higher dynamic compression ratio without detonating.
DISCLAIMER: The example demonstrating the difference between DCR and SCR is just that, an example. Those are numbers I pulled out my ***.
