I am considering adjusting my mag timing in order to lower my chts. My engine is an io360 exp with a name plate stamped at 25 degrees. My eci flywheel is only marked at TC and 25 degrees but I figure each tooth is worth 2.5 degrees. The slick mags are 4373 and 4370 both marked at 25 degrees lag
There was a lycoming service instruction back in the mid 70s si1325a that advocated backing timing off to 20 degrees to improve CHTs.
I have three questions for the group.
1. Anybody running 20 degrees advance after changing from 25 degrees? Worth the effort on CHT?
2. Anybody running 20 degrees on this engine type and chts running in the 380 max range?
3. If I move my timing down to 22 or so and leave the magneto lag on the impulse magneto alone would there be any adverse effects other than lower power and maybe a little more difficulty starting?
The si stated that the lag should be adjusted to 15 degrees for better starts
Thanks in advance
Tom
Lnc2 285 hrs
Service Instruction No. 1325A
(Supersedes Service Instruction No. 1325)
Engineering Aspects are
FAA (DEER) Approved
DATE: May 14, 1976
SUBJECT:
MODELS AFFECTED:
TIME OF COMPLIANCE:
Timing Change for IO-360 Series Engines
IO-360-A series (not -A1B6D), -C, -D series, HIO-360-C1A, -C1B, AEIO-
360-A1B6 with serial numbers prior to 14435-51; IO-360-C1D6 with
serial numbers prior to 14445-51, LIO-360-C1E6 with serial numbers
prior to 1063-67 and AIO-360-A1A, -A1B, -B1B with serial numbers prior
to 219-63; TIO-360-A1B with serial numbers prior to 145-64. See
Service Bulletin No. 380 for IO-360-C1C and -C1F models.
At overhaul or at owner's discretion.
Certain advantages in engine operation have
been found by changing engine timing from 25°
BTC to 20 BTC and reducing the lag angle to
15° on impulse coupling magnetos and 15
retard angle on retard breaker magnetos whi-chever
is applicable. Significant improve-ments
in engine operating characteristics are
achieved in above listed aircraft engine
installations particularly in the areas of
normal operation and cold weather starting.
During cruise operation, cooler cylinder head
temperatures can be expected resulting in
improved service life of intake and exhaust
valves. During full throttle operation the
new spark setting will reduce possibility of
detonation during extreme cold weather opera-tion.
Consequently this modification has been
incorporated in the production of new engines
and may be incorporated in engines in service
as follows:
1. Remove the left magneto and determine if
it has an impulse coupling or if it is a
retard breaker magneto; if it is a retard
breaker magneto it cannot be converted to
the 15 retard angle. However, a Bendix
blue magneto which incorporates a 15 re-tard
angle is available through Avco Ly-coming
distributors. See "Parts Data" be-low.
Install the replacement magneto as
described in step 3.
2. If the left magneto has an impulse
coupling, replace the coupling with a new
one, LW-391429 on all models except reverse
rotation engines; on these use impulse
coupling LW-391427.
3. Install the magneto on the engine and
retime both magnetos using the 20 timing
mark on the starter ring gear instead of
the 25 mark. See Avco Lycoming Operator's
Manual 60297-12 for ignition timing proce-dure.
4. Replace the engine nameplate with a new
one, Avco Lycoming P/N 61548 which indi-cates
new timing angle. Also restamp the
magneto nameplate to indicate the magneto
is -9, -10 or -55 instead of -3, -6 or -45.
PARTS DATA:
15 Lag Angle
Magneto
LW-349365-9
LW-349365-10
LW-51360-55
Replaces
Magneto
LW-349365-3
Magneto
Model Engine Application
S4LN-1227 IO-360-A1B,-A1B6,-C1C,-C1F,-C1D 6, -C1E6, AIO-360-A1A,
-A1B, -B1B, AEIO-360-A1B6
LW-349365-6 S4RN-1227 LIO-360-C1E6
LW-51360-45 S4LN-21 IO-360-A1D
PARTS DATA (CONT.):
15° Retard Angle
Magneto
LW-349285-7
LW-349285-8
LW-163005-11
LW-163005-12
Replaces
Magneto
LW-349285-1
LW-163005-2
Magneto
Model
S4LN-1208
S4RN-1208
S4LN-200
S4RN-200
Engine Application
IO-360-A1C, -C1C, -D1A; HIO-360-C1B; AIO-360-
A1A; TIO-360-A1B
LHIO-360 C1B
IO-360-AlA, -A2A, -C1A; HIO-360-ClA; AEIO-360-
A1A
LHIO-360-C1A
LW-391429
LW-391427
61548
18484, 18485 -
Impulse coupling, 15° lag angle (replaces LW-349359)
Impulse coupling, 15 lag angle (replaces LW-349358 for magnetos on reverse ro-tation
engines)
Engine nameplate (replaces nameplate with 25° BTC timing specified). See Ser-vice
Instruction No. 1304 for procurement procedure.
These numbers for Avco Lycoming reference only.