Yeah, you normally don't hear detonation at speeds, but it's there -- usually a noticable degredation in performance if it's bad.

My friend Hans had an Alfa Romeo come into the shop in Germany once with a "died on the highway and won't start" complaint.

No compression AT ALL, and when they pulled the heads, there was a nice, neat, smooth sided hole in each piston directly below the spark plug. Turns out if you cheat on the octane rating or turn the timing up too far, the flame from the resulting detonation comes straight out of the spark plug, and on the Alfa, it was pointed directly at the piston crown at a right angle -- burned right through.

Got the exhaust valves too, I think.

On an MB, the plugs are usually canted, so you don't have to worry about buring a hole right through, but you can still melt the piston crown and splash aluminum around -- or flame cut down through the rings (I've seen this on american v8s where some idiot "set the timing by ear"). Will usually roast the exhaust valves too. The M110 has vertical spark plugs, though....

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!