I am still flying off hours and playing at low altitudes, but running way lean of peak at 12k ft I am burning 15.5 gph and seeing about 205 kt TAS.  These investigation hours are a mix of fun and frustration.  The EI readouts are all over the map and do not agree with the RPM and MAP displayed on the Grand Rapids EIS.  If the firing angle readout is correct we are at 40 degrees on take off, 32 degrees when cruising ROP but that drops off as we go LOP - and that seems strange.  Could anyone enlighten me on this?

Robert,

 

You are using Light Speed EI, right?  And a Lycoming engine, right?  If it is a Continental engine or not LSE, ignore most of this e-mail.

 

Otherwise, remember that the timing is tied to power as represented by RPM and MAP.  At idle (lo MAP, lo RPM), the timing would be quite advanced (35-38 DBTDC).  At takeoff power (hi MAP, hi RPM), timing advance is minimal, i.e. 25 DBTDC.  As power declines by an increase in altitude (lower MAP) and cruise RPM (2300-2500), the advance should increase towards 35 or so DBTDC.  If running LOP causes a reduction in RPM or MAP, the advance should increase a bit further.  Reduce all the timing numbers by 5 degrees if the compression ratio (CR) is greater than 8.7:1.

 

If mag hole timing sensors are being used along with a hi CR, the sensors are offset and the indicator readouts are 5 degrees higher than actual.  However, if a big engine (6-cyl, >200 HP) is being used, the very accurate crank sensor should be installed. Once again, if a hi CR, the sensor magnets imbedded in the flywheel are offset by 5 degrees and the timing indications are accurately reported. 

 

In my airplane, the MAP and RPM are low by .5" and 60, respectively as against those displayed by VM instrumentation.  There is no damping of the display so the numbers can jump around quite a bit.

 

Finally, if the above doesn't explain what you are seeing, re-check the actual timing with a timing light. If that is good, check the wiring and connections.  The display is a Simpson volt meter showing hundredths of volts.  The selector switch not only connects the appropriate source, but adjusts the display decimal point location.

 

Good Luck,