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Doug wrote
"Of interest, within the limits of my setup (VM1000, all
altitude and engine parameters the same with the exception of a 15degF cooler
OAT) I did notice a 20-30degF cooler CHT with the new Iridium plugs over a broad
range of fuel flows, and a 2-3 knot improved speed at lean-of-peak operations
(no change at ROP operations)."
Walter wrote:
" I know of no scientific reason that the type of plug making the
spark could make such differences. It's more likely the OAT. More O2 molecules
for combustion and better air cooling."
Walter, I tend to believe you are correct, the
improvement was more air temp related, however, don't diminish the effects of a
good ignition system. Poor plugs, bad wires, worn out mag (even hotter
spark) all can make a significant affect for the same reason the RPM drops on
one mag vs both. If the flame is lit but the flame kernel is "poor" the cylinder
will make power but not as much as if the flame kernel is "good". The idea is to
have as fast of combustion as possible without it being an explosion (non
controlled combustion). With 2 flame fronts (two spark plugs per cylinder) the
combustion time is greatly reduced and therefore power is increased and BSFC is
reduced (fuel economy is improved). Particularly on our big bore aircraft
engines. Just changing your fuel from 100 octane to 130 octane will reduce power
and fuel economy for two reasons. 1. The 130 octane fuel has less BTU's per
gallon. 2. The 130 octane fuel burns slower. Some of this loss came be regained
by advancing the ignition timing when running higher octane fuel.
Craig Berland
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