Quote:
Originally posted by psfred
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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Detonation damage is due to local overheating caused by the dramatically increased rate of heat transfer caused by the detonation the shock waves. It doesn't have anything to do with spark plug location.
The center of the piston is its hottest point, and most aluminum alloys rapidly weaken above 400 degrees F., so in the absense of unfavorable piston crown geometry, detonation failure will usually be near the center.
Pistons for four valve engines or two inline valves (like a Chevy V8) that have machined valve clearance notches a prone to failure where the notch boundaries pass close to the edge of the piston leaving a ridge that has a high surface area to volume ratio. Even though this part of the piston is normally cooler than the center, the ridge can experience rapid overheating due to detonation and fail.
The best pistons have crown geometry "built in" to the forging or casting tooling with no sharp edges that are typical of machined piston crowns. Rather than individual valve clearance notches, a chamfer clear across the top of the crown will not leave high surface to volume ratio ridges near the edge of the piston, but reduce the potential compression ratio.
Duke