Differences between automotive & airplane engines

On Wed, 15 Feb 2006 02:14:33 GMT, Ernest Christley
wrote:

Those caveats are the killer, aren't they? You CAN'T 'properly lean'
the average aircraft engine, because they have horribly designed intake
manifolds. The mixture isn't distributed evenly. So to get two of them
properly leaned, you have to send the other two down into detonation
territory. Not knowing who's on first, the best bet is to **** your
money away in wasted fuel out the exhaust pipe.

It gets worse. According to John Deakin who wrote a series of very
very interesting articles on AVWeb (still available by the way) about
how to properly lean an aircraft engine, it's not just useless but may
be harmful to the engine to attempt to lean below peak if you do not
have an EGR guage that reads all cylinders.

He had a chart that showed that "properly" leaned, that is leaned
according to the POH using rpm drop, you could very easily have one of
the cylinders reaching a redline cylinderhead temperature, while
others were safe.

That is, if you are above about 65% power. At or below 65% power, it
doesn't matter where you set the mixture, you won't be able to
overheat the engine.

He also advocated leaning even to the point of roughness, if you could
stand it and were at 65% power, saying that the roughness wouldn't
hurt anything and was just the result of relatively unbalanced
fuel/air charges in the cylinder combustion chamber. This is the kind
of thing you feel because the cylinders of the four cylinder 0-360's
are so big. Any unbalanced fuel charge results in substantially
different pressures inside the combustion chambers from one cylinder
to the other, which can result in a perceptably rough running engine.

Smaller displacement engines with more cylinders would be less
susceptible to this syndrome.

Adjusting the injectors such that they produce aproximately equal
fuel/air distribution within the combustion chambers allows the pilot
to lean to the point where the engine quits, without any roughness to
that point.

Corky Scott