I dont live in the USA and funny you mention SHELL as a culpret. Over here in South Africa SHELL is def the best feul, we have 95 over here. I experience knokking/ pinging or pre- ignition alot at high loads at low revs below 2500rpm. However in my feulcap it says min octane 98. When I feul her up with 98 racing feul knokking is absolutely gone and plenty more performance and responce. Try this and see if it works. VP racing feuls are brilliant!! USA product sure there are others!!