I have a vague recollection that this used to be around 50 rpm in 2002.
Nowadays it is over 100rpm – on both halves of the mag equally.
The Lyco limit is 175.
The magneto is the D3000 single shaft dual type, so nothing to adjust there except the overall timing.
And the plane performs exactly to spec, within 1kt.
What could be the reason?
The 50 rpm difference between magnetos is probably the more relevant limit. 100 rpm drop on both magnetos sounds ok.
I never had less than 100 RPM on any plane I flew. I agree with Robert, it’s the difference which is more significant. 50 RPM sounds good.
Could it be the mixture setting, i.e. for same mixture lever setting that the actual mixture has changed?
I presume you would have tried different mixture lever positions during the magneto checks?
Are you able to look at EGT by cylinder from now vs before?
Peter wrote:
Nowadays it is over 100rpm – on both halves of the mag equally.
If I understand correctly there is no difference between the two sides, it’s just the increase from 50 to 100 with time.
I always do the mag checks on FULL mixture as per POH, 2000rpm.
Yes, the two sides are identical (within the width of the needle) which I think means the magneto itself is internally correctly adjusted. I always get the mag done at QAA in Oklahoma, and have a spare on the shelf ($2.5k) to avoid any downtime.
I can’t compare the EGTs because I don’t have the data going back. Well, I do have the EDM700 logs all the way back to 2002 and it may be in there somewhere, near the start of each flight…
Amount of rpm drop on grounding one mag is linked to ignition timing. Retarding the timing increases the rpm drop.
Does anyone have a rule of thumb for this? i.e. something like an additional 50rpm drop is about x degrees of timing shift?
Peter wrote:
I always do the mag checks on FULL mixture as per POH, 2000rpm.
When I looked at minimizing mag drop in detail a while ago, I remember reading a Lycoming procedure stating that if mag drop were excessive, then lean and try again. The procedure provides more detail. I’ve noticed this does reduce mag drop on my engine.
Alan_South wrote:
Amount of rpm drop on grounding one mag is linked to ignition timing. Retarding the timing increases the rpm drop.
Very interesting, makes sense.
I wonder why that Lyco leaning procedure would improve the mag drop?
Alan’s post makes sense; on half a mag there will be more unburnt mixture around, so a more advanced timing would help getting it burnt. Same with lean ops; you want an earlier spark (which is incidentally achieved by using a lower rpm).
Peter wrote:
I wonder why that Lyco leaning procedure would improve the mag drop?
The flame front moves slowly with over rich mixture, as with over lean mixture. With non-optimum mixture, twin plugs is equivalent to advancing ignition timing (the flame front has to travel half the distance) so single plug operation is like retarded spark. With better mixture the combustion does not benefit so much from twin plugs because the flame front speed is high enough to match the bore size and ignition timing.
