Actually I have taken to using 87 PON fuel in my 190E 2.6, but I only drive it from November to May nowadays when it is fairly cool in So Cal.

I remember when I drove it everyday I could sometimes hear it ping on hot days with the A/C on and that was on premium (91 PON) fuel.

My car has a five-speed, and I tend to shift at low revs in normal in-town driving. I've learned to feed the throttle in a little slow on upshifts to avoid the brief transient detonation than can occur if you feed the throttle in a little too fast at low revs.

Detonation damage can be cumulative, but brief transient detonation will rarely lead to a burned piston or valve, however, sustained moderate detonation can eventually cause a burned piston or valve.

Detonation causes shock waves in the combustion chamber. These shock waves "ring" the engine structure, which is what we hear as "knock" or "ping", and these shock waves increase the rate of heat transfer to the combustion chamber boundaries by up to an order of magnitude, so it is ultimately local overheating of the material that can burn a piston or valve since pistons and exhaust valves normally operate pretty close to the temperatures at which their strength properties rapidly degrade.

Carbon buildup can reduce combustion chamber volume, which can increase actual compression ratio, and carbon can also cause hot spots that can create preigntion, which can lead to detonation; 87 octane fuel will not usually lead to carbon buildup. Carbon buildup is usually the result of either high oil consumption for extended periods or an overly rich fuel mixture for extended periods.

The key is to keep you ear open for detonation. Driving the car with the radio off and a window open in the city will usually allow you to hear any detonation. If anything, as has been mentioned previously, you are more likely to hear detonation echoing off the surroundings through an open window as opposed to through the vehicle structure.

Duke