Is taking off on single mag bad for engine

flyer wrote:
: O-320-E2D. The engine only developed 1900 rpm (2200 is normal for us
: on 2 mags with fixed pitch prop) and climbed poorly and ran rough.
Sounds about right, but...

: would like but the engine runs strong. My partner said that the
: single mag takeoff could have caused "detonation" or "cylinder
: glazing" or "a bad flame front in the cylinders" and ruined the
: compression.
Running at only 1900 RPM instead of the normal 2200 RPM on
climbout means that there's more absolute time from the spark (at 25
degrees BTDC) until TDC. For that matter, there's more time from the
beginning of the compression stroke to the spark. That extra time could
cause either preignition or detonation. Not likely, but possible


: I don't think that at that low power setting in a 7:1 compression
: engine anything bad could happen.
7:1 is pretty low compression, granted. BUT, it's rated for a
certain octane gasoline at certain min/max RPM settings.


I am not aware of anything bad that
: could happen in general from single mag operation at any power
: setting, other than rough performance.
Aside from roughness and the potential for lower-than-normal RPM,
I'd agree


This would be the same thing
: as having a fouled plug in a cylinder, and nothing bad happens then.
: Does anyone have any other feelings about this phenomenon.
... except that with one fouled plug, the other three jugs still
run on 2 plugs, so the RPM wouldn't decrease nearly as much.


Just for a bit more clarification... this on avgas (100LL), or cargas?
I'll give you one more data point that might be somewhat related. I've
noticed on my O-360 180 hp 8.5:1 Lycoming with the 91 octane cargas STC
that with a hot (recently flown) engine, it will actually diesel when I
cut the mags off. I was at idle, and rather than cut the mixture, I cut
the mags. It slowly shook to close to a stop, but at 100 rpm or so, it
continued to sputter and run enough to keep shaking and running. I was
very surprised at this, but when I started thinking about it, I realized
that at this (very) low RPM, there was lots of time to get the mixture
burning in the jugs, and also the idle throttle position allowed the MP to
build up to 20" or so (again very small airflow). It's almost like
lugging the engine at full throttle at 100 RPM... pretty tough.

But still... in your situation that 1900 instead of 2200
*shouldn't* have done it.... but it doesn't mean that it didn't. Try a
static RPM runup (and lean if DA is an issue). Make sure the tach is
calibrated, and the runup is within spec as per TCDS (Type certificate...
www.faa.gov)

-Cory


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----------snip--------

Just for a bit more clarification... this on avgas (100LL), or cargas?
I'll give you one more data point that might be somewhat related. I've
noticed on my O-360 180 hp 8.5:1 Lycoming with the 91 octane cargas STC
that with a hot (recently flown) engine, it will actually diesel when I
cut the mags off. I was at idle, and rather than cut the mixture, I cut
the mags. It slowly shook to close to a stop, but at 100 rpm or so, it
continued to sputter and run enough to keep shaking and running. I was
very surprised at this, but when I started thinking about it, I realized
that at this (very) low RPM, there was lots of time to get the mixture
burning in the jugs, and also the idle throttle position allowed the MP to
build up to 20" or so (again very small airflow). It's almost like
lugging the engine at full throttle at 100 RPM... pretty tough.

-----------snip---------

I was present when one of the mechanics, who I respect a lot, told a friend at
FXE that mogas and avgas have significantly different viscosities; and that an
engine will run much richer on one than the other. Me recollection fails me,
but your account indicates that magas has the lower viscosity and will run
rich. A rich mixture at idle, and carbon deposits from rich operation, used to
be the classic causes of "dieseling" in cars--sometimes people had to kill the
engines with the clutch! :-(

My best recollection of the mechanic's advice was to only use mogas if you could
use it all of the time. It might be possible to eliminate the problem by
leaning under all permissible modes of operation; and I plan to explore that
avenue when I have a plane avaible and start flying again.

Peter