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Chris,
Besides the two plugs being backup, the flame front is more effective with
2 ignition sources. Some dual EI's that communicate with each other
change the timing slightly if one of them has ceased operating. As
can be seen from my experience, 25 DBTDC (both mag and single EI)
cover a broad range of cruise flight conditions and the EI is producing a better
and more reliable spark. This is probably why there is a more modest
cruise performance improvement from only one EI. At high altitudes or
low power, the mag is lagging the advanced EI spark and the flame fronts
are started differently or the mag is already firing into an already started
combustion event. This still improves the power delivered from a better
timed fire.
To experience your question, turn off one of your mags in cruise. You
know what happens at the pre flight mag check - RPM drops and RPM is a
component of power. The loss of power is related to the entire combustion
event timing not matching the twin flame front condition.
Scott
In a message dated 11/28/2011 9:53:59 A.M. Central Standard Time,
chris_zavatson@yahoo.com writes:
Gary, Scott,
A little off-thread, but...
Do our Lycoming
cylinders require that each spark plug fires to properly propagate the combustion event throughout the
entire cylinder? On those planes that have one mag and one EI (and while the EI has advanced its spark), does the mag contribute or
can it even still fire?
Chris Zavatson
N91CZ
360std
Gary and Chris,
Here is detailed info from my most recent long trip (ARR-EDC-PLK-ARR). Perhaps you can deduce more as the RPM was
in the 2480 to 2500 range for all legs.
ARR->EDC 820 NM 4.7 hrs block
to block. Flown at 6500 MSL to
avoid higher headwinds at higher altitudes and LOP (economy) to
eliminate a fuel stop. Conditions were constant for the entire trip
and all four data points are very similar. OAT 19C, Dalt 7500, KIAS 168, KTAS 187, 24.8" MAP, 2480 RPM, 7.4 GPH and 24.5 DBTDC. Actual flight time was 4.3 hours and I put
32.1 gallons at EDC having been
left with a reserve of 9.9 gals.
EDC->PLK 440 NM 2.4 hours block to block. Flown at 9500
MSL to use the winds and ROP (best power) to go fast. OAT
16C, Dalt 11000, KIAS 163,
KTAS 190 (Ahhh, over PGO GS 208 Kts), 22.1" MAP, 2490 RPM, 8.5 GPH and 27 DBTDC.
PLK-> ARR 380 NM 2.2 hours
block to block. Flown at 7500 MSL and ROP.
OAT 18C, Dalt 8700, KIAS 171, KTAS 194, 24.1" MAP, 2500 RPM, 9.8 GPH and 25 DBTDC.
Scott Krueger
IO 320, 9:1 CR, unfiltered ram air, dual Plasma III EI, Hartzell CS prop, uh, a few drag
reducers....
Scott,
Thanks for the information. I took your numbers and did a crude
reqression analysis and came up
with the following: The advance increases by 2.2 degrees per
thousand rpm above 1700 and 0.85 degrees per inche of manifold pressure below 28. Just gives a
rough idea of how it works.
Gary
From Scott:
Restricting my comments to Lycoming engines (320, 360), one would note that 25
DBTDC is the fixed timing delivered
by standard magnetos and that timing is adequate for a wide range of
engine operations. My injected 320 engine has a 9:1 CR and uses a base
timing of 20 DBTDC. Normal ROP cruise settings in the 4000 to 9000
MSL range and 2500 RPM (say 21 to
25 MAP), the timing is between 24 and 26 DBTDC. At takeoff and low altitude race power settings
(2760 -2600 RPM, WOT, 28-30 MAP), the timing is about 22-23 DBTDC. At leaned idle/taxi power (<10"
MAP, <1100 RPM) the timing is about 34-35 DBTDC. At altitudes 10000- 14000 MSL, 2500 RPM, WOT and LOP the timings are generally
29-31 DBTDC.
Scott Krueger
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