AOPA Pilot Magazine - February 2015 - 84

P&E

FRUGAL FLIER
AOPA Frugal Flier coverage sponsored by AIRCRAFT SPRUCE

How it 'auto' be
Automotive spark plugs can be an Experimental option
B Y D AV E H I R S C H M A N

I RECENTLY HAD THE PLEASURE of changing the
spark plugs in the Lycoming O-320 that powers my
Experimental Van's Aircraft RV-4, and, for a change,
it really was a pleasure.
In fact, since it had only been about 150 flight
hours since I had installed an E-Mag dual electronic
ignition system (with auto plugs and harnesses),
I really didn't need to change them at all. The bottom plugs showed a hint of lead fouling, however, so
I decided to replace (instead of clean) them.
At a mere $2 per plug, why not?
The top plugs looked just fine, but while the cowling was off, I replaced them, too. The whole set of
eight auto spark plugs cost half as much as a single
new aviation spark plug, and the kicker is that the auto
plugs perform better. In fact, E-Mag strongly recommends auto plugs for its customers, and Experimental
engine manufacturers are making cylinders drilled for
metric auto plugs, so adaptors aren't necessary.
Lightspeed Engineering, maker of Plasma electronic ignition systems, recommends auto spark
plugs. ElectroAir requires FAA-approved spark plugs
for Standard category engines, but gives owners the
option in Experimentals.
Pilots often ask how long automotive plugs last in
aircraft engines, and the unsatisfying answer is that
no one seems to know. Mike Stewart, leader of Team
Aerodynamix-a formation aerobatic team that flies
RVs-operated a set of auto plugs more than 600 flight
hours, and they were doing just fine when he replaced
them with an identical set.
The availability and low cost of auto plugs makes
them practically disposable.
Kevin Eldridge, an air racer and head of business
development for Titan Aircraft Engines, said the vast
majority of the engines the San Antonio, Texas, firm
manufactures are delivered with auto plugs.
"I've flown and raced with auto plugs for years,
and the real attraction at first was the lower cost,"
Eldridge said. "But the reason the vast majority of our
customers are using them now is that they work better, are more reliable, and are more resistant to lead
fouling."

84 | AOPA PILOT February 2015

Not all spark plugs are
equal, of course, and gaps
and temperature specifications must be appropriate for
each engine installation. Highperformance auto plugs (usually
about $6 each) can cover an exceptionally broad range.
A fellow pilot with an identical ignition system and a similar Lycoming engine recommended
NGK plugs, and Klaus Savier, founder of Lightspeed
Engineering, endorses that manufacturer (as long as
the part number begins with the letter B).
The reason auto plugs are preferable is that,
when combined with an electronic ignition system
with variable timing, they provide a bigger, hotter, longer-duration spark that burns the fuel/air
mixture in each cylinder more thoroughly and efficiently. That means easier starting, more power at
altitude, and better fuel economy. In contrast, magnetos with fixed timing produce less energy, and
require more costly FAA-certified aviation spark
plugs that have smaller gaps and a shorter-duration
spark.
I felt a bit ungrateful tossing out a set of perfectly
performing spark plugs. After all, each of them had
provided about 100,000 faithful sparks per flight
hour while being subjected to a hellish mix of flame,
heat, pressure, cold, and vibration. During that time,
they had received virtually no maintenance, attention, or appreciation at all. That's a pretty thankless
existence.
If an aircraft owner wants to clean and gap
automotive spark plugs, that's certainly doable.
Lightspeed Engineering recommends 0.032 to 0.040
inches for low compression engines timed to fire at
25 degrees before top dead center, and 0.026 to 0.035
inches for engines with higher compression.
But I'll simply replace the spark plugs at every
second or third oil change. I bought a dozen the last
time I was at the auto parts store-so I'm ready. AOPA
EMAIL dave.hirschman@aopa.org

AOPA Pilot Magazine - February 2015

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