As for the detonation it is rare like you say and cars these days have anti-knock sensors to prevent this from happening. If detonation does happen you will know it's going on...by your fast yelling from under the hood!DrJones said:Low octane gas in a high compression of forced induction motor can created a condition of early ignition called detontation. When the fuel-air mixture ignites before it's supposed to, the downforce placed on the piston before it completes it's upstroke can cause serious damage to the motor. While this condition is rare, it is a possibility. The motor in the GTI is designed to run best on 91 octane gas.