I'm wondering what the real 'knock' protection difference there is between 90 and 91 oc. A new gas station just opened up near work and they sell a 90 oc gas. Never seen this before. They stock 87, 89, 90 & 91. I gather they are blending 87 and 91 to get the two other grades.
I'm currently running the Stage 91 ACN Cobb tune. Tops out at 14.5 or 15psi (forget which).
Keep in mind I'm in Reno (4500 feet), and some gas stations don't sell an 87 they have 85 instead.
1. Is this something to do with our altitude?
2. What is the real difference in 'knock' protect between each octane rating? IE is it a 4 fold improvement from 87 to 91?
I understand the basics on knock protection, higher oc the better for higher compression, however the specifics on the difference have always escaped me.
PS - this is not at money saving post, so avoid the stupidness of 'buy a different car' comments please.