Clark,
Yep, she's broke!
It will be very informative to know why the unit failed so as to
permanently operate at the max advance.
Most of my high power operations, >2450 RPM, >24" MAP, <6000 Palt)
result in an advance between 21 to 25 DBTDC. Lower power, 2480 RPM, 19-20"
MAP, >8000 Palt, result in 28-29 DBTDC. I only see the 30's during idle
or taxi (very low power since I don't like "low" power).
If the RPM sensor failed so as to always report RPM less than
400, the timing would probably have been set to 0 DBTDC (engine start
conditions). If the MAP sensor sent near 0" indications, I could
imagine the timing going to max advance. This would be different
than selecting the "ignore the MAP" jumper. Also, it would have been
interesting to see what would have happened if you had selected the fixed 25
degree setting.
Note, removing the MAP "vacuum" tube cannot eliminate timing adjustments
based on the MAP sensor since it would see 29" (or so) on the ground and
less ambient atmospheric pressure as one climbed. Just like there was no
effect from ram air on the induction system running at WOT. There must be
a jumper or switch setting that would allow you to operate independent of MAP
sensing.
Consider this relationship (assuming 8:1 CR) - At takeoff power,
2700 RPM and 29" MAP, 25 DBTDC may allow the combustion event to deliver
max pressure at the optimum piston position (perhaps even better at some
timing less than 25). At highly reduced power, say 1200 RPM and 10"
MAP (fast taxi), at 25 DBTDC the peak pressure may occur long after the best
piston position, thus is inefficient(so what, efficiency at idle is not a
priority objective). If the timing is advanced, the peak pressure is
advanced to the better piston position or the fuel can be reduced to deliver the
same pressure at the same piston position. This is also true at high
altitudes - even at 2500 RPM, the MAP (NA engine) may only be 16-18" (low
power) and timing advance may deliver the same power (pressure) with less fuel
or more power with the same fuel as with the standard mistimed combustion
event (i.e. fixed at 25 DBTDC).
Of course, there are other benefits of electronic ignitions (see any
automobile).
Scott
PS: Knowledge is power (especially the timing info).
In a message dated 12/7/2006 3:12:26 P.M. Central Standard Time,
bakercdb@gmail.com writes:
Interesting about the observations on your LSE system.
Just to expand a little:
* I was getting considerably different readings between the two
units.
* The suspect unit was at ~39degrees at 1,000 to 1,500 rpm (I couldn't
take any more prop blast while holding the timing light), while the other unit
remained near 25 degrees.
* Maximum advance should have been limited to 34 degrees as I had the low
advance curve jumper installed (not the 39 shown on the suspect unit).
* Pulling the vacuum advance connection is supposed to remove any advance
due to manifold pressure. Pulling the tube made no difference to the
suspect unit (the other unit was already at minimum advance). However, I
am not familiar with the exact dynamics between RPM, MP, and timing.
Finally, as more substantive evidence, I manually retarded the suspect
unit by 15 degrees (ie, set the timing while at 15degrees ATDC, not
TDC as is usual procedure. A test flight showed the temps back to
normal. I have not yet received the manufacturer's findings.
Regards
Clark
On 12/7/06, Sky2high@aol.com <Sky2high@aol.com> wrote:
In a message dated 12/7/2006 1:06:47 P.M. Central Standard Time,
bakercdb@gmail.com
writes:
On the 2nd leg of a long cross country trip, I noticed that CHTs were
running noticeably higher than usual on climb out. After leveling
off at 6,500msl, CHTs were uniformly running approximately 40+ degrees
higher than normal. Upon landing, the problem was eventually traced
to one of my PMag ignitions that, was going to full advance at idle
(39degrees). This was a failure mode of electronic ignitions that I
hadn't considered before. Given the engine seemed to be running fine
otherwise, I didn't try switching off one ignition in flight, which likely
would have pinpointed the problem.
Clark,
Low MAP and low RPM are exactly the low power combination that
encourages electronic ignitions to fully advance the timing. Indeed,
the P-Mag may be broken (always delivering max advance), but a
significant advance at idle is not a good failure indication. I
regularly see 34-35 DBTDC at low RPM (idle or close to idle) and my base
timing is 20 DBTDC because of a CR greater than 8.7 (equivalent to your 39
DBTDC unless your jumpers also are set for a base timing of 20 DBTDC).
Perhaps the P-Mag goes to a large advance because the
MAP sensor is always indicating low pressure. What happens if you
set the jumpers to use RPM only for the timing? You shouldn't get much
advance for operation above 2500 RPM. I have to assume the RPM sensor
is working for the P-Mag to operate at all.
PS: You may have experienced the kind of condition that led
me to the conclusion that I must have a display of the in flight timing
for any ignition that changes the timing.
Scott Krueger AKA
Grayhawk
.