I've been following the threads regarding the discussion of high altitude
magneto misfire and I thought that I would share some thoughts with the
group.  I wrote an article on pressurized magnetos that is relevant to this
discussion.  The link to this article can be found at:
http://www.amtonline.com/searchscripts/search.asp?SearchSiteURL=\articles\20
00\amt_07-00\amt_7-00-03.html

In a nutshell, electrons will always follow the path of least resistance.
When an engine is turbocharged, the voltage required to jump the spark plug
gap increases.  If the magneto is unpressurized, the voltage generated by
the magneto may follow other easier paths of discharge within the magneto
and an arc may not occur at the spark plug gap to ignite the fuel air
mixture.  Subsequently, the engine misses and runs rough.  In this regard,
Bendix and Slick magnetos are subject to the same laws of physics and will
function similarly.

The primary reason that engines are turbocharged is to improve performance
at altitude where there are fewer air molecules to mix with the fuel charge
to provide a combustible mixture of the proper chemically balanced ratio.
So, at high altitudes, the turbocharger provides an artificial environment
of increased air density like the lower altitudes where the engine runs more
efficiently.

The problem is, with some turbo applications, the power section of the
engine is optimized to run at sea level pressures while the magneto ignition
system is not.  To draw an analogy, it's like a runner with great lungs and
a stressed heart running a marathon- obviously the chances for a problem are
much greater than if both the runners lungs and heart are working in an
equal, balanced state of fitness.

The key to optimum performance of any turbocharged application is to
pressurize the magnetos.  Non-pressurized magnetos are often used with no
service issues, but occasional performance problems are more likely to occur
due to the lack of pressurization.  Once again, the demand voltage at the
spark plug gap and internal condition of the magneto will affect ignition
system operation.  Too wide of a plug gap increases voltage demand, worn or
out of spec internal components may reduce magneto output, internal
contaminants can result in arc paths, and so on.

Pressurized magnetos definitely enhance high altitude performance, but the
pressurized magneto system can also present other problems if it is not
designed properly.  The key to success is a large, relatively slow moving,
regulated air mass that is free of oil or moisture contaminants.  The
pressurized magnetos and ignition harnesses also feature special seals and
vents to ensure proper operation, so just blowing air through a standard
magneto will provide inadequate results.  The bottom line is that any
ignition system used on a turbocharged application will require more care
and more frequent maintenance than a normally aspirated application.  

As a point of interest, it should be noted that nearly all of the time to
climb and altitude records established by Bruce Bohannon in the Exxon Fly'n'
Tiger have been accomplished on non-pressurized magnetos!  Bruce has been to
almost 40,000 feet with a turbo'd engine, and routinely cruises at 12-16,000
feet with standard, off the shelf Slick 6300 series magnetos.  We decided to
pressurize the magnetos for the Oshkosh '02 attempt, but Bruce set up the
turbo and magneto bleed air system wrong which foiled the attempt.  We fixed
that and now Bruce reports that the engine runs better than ever at some
very, very high altitudes.  Watch for some exciting news during the AOPA
Convention at Palm Springs in October.

Speaking of which, I will be exhibiting at AOPA, so if you are attending,
feel free to visit with me at the Unison exhibit.  I'm a homebuilder and
engine builder, so always glad to talk about these topics.  Otherwise, feel
free to e-mail me at pistonhelp@unisonindustries.com.

Harry Fenton
Senior Field Service Engineer
Unison Industries, Piston Products