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Walter,
<<We have watched many detonation events--some
even heavy detonation events--some for extended periods-without any
ill effects on the piston, cylinder or engine in general. The danger
is these turning into pre-ignition, usually from a damaged spark plug
from the detonation. That can destroy the engine in 30 seconds and
while our observations are that detonation doesn't hole a piston,
pre-ignition will do it in a heartbeat.>>
Now I'm confused.
A few year back my plane was topped off with 20 gallons of fuel that was about 95% avgas and 5% Jet A. I didn't notice anything unusual while operating the engine. 6 flight hours later, I and 200 other unlucky aircraft owners were informed of the fuel contamination. Upon removal of the cylinders and connecting rods, we found that the rear pistons had been almost completely cleaned of all carbon deposits and a new uniform deposit layer had started forming. The front pistons had not been so completely cleaned and so the much of the original carbon deposit layer was still present. It had a coarsely pitted appearance. The exhaust valve seats had turned blue. Also, the babbitt in the connecting rods bearings had been smeared against the crank.
The diagnosis at the time was detonation. Do you agree? The scary part is that I couldn't tell anything was amiss. All indications in terms of sound, feel and temperatures were within typical operating ranges, yet it caused the engine to be scrapped. I imagine the plugs were not damaged as they continued to fly for six hours without any sign of trouble. I probably had 3 hours on a 2.5% blend of Jet A and 3 more on something much more dilute. The engine was a completely stock Lyc. 0-360A1A with 300 hours since new. I was left with the impression that a little (not noticeable) detonation over a long period of time (a few hours) will destroy an engine.
In the affected group, turbo charged engines, modified engines (hi-comp pistons etc.) and those receiving the highest concentrations of Jet-A did almost come apart at the seams leading to a number of emergency landings.
Chris Zavatson
N91CZ + engine #2
360 std
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