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Date: Tue, 25 Jan 2005 23:59:09 -0600
From: Jim Sower <canarder@frontiernet.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: FW: Re: Plugs and Mags
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Ed,
I've been hearing about SAG periodically.  I don't fell like I know all 
of the details or the finer points of the symptoms, but some brain farts 
have occurred to me: 

If as you say it's the plugs getting dirty enough around the porcelain 
shroud, to lay down a carbon arc (like in a damp distributor cap) I 
would doubt that trick electrodes would help. 

Since it's peculiar to rotary engines and is basically unheard of on 
recips, one might reasonably attribute it to the shrouded plug(s). 

I'm wondering if SAG can be replicated on the ground.  Like if you 
experienced it on takeoff, can you reasonably expect (after you've 
recovered) to make it happen on the ground, say during a prolonged high 
power turn up? 

I would intuit that since power SAGs but doesn't DISAPPEAR, that it only 
involves one plug.  I would furthermore intuit that it would be the 
shrouded plug. 

Is there an accessible primary wire that powers the leading or lagging 
plugs that might be switched for test purposes.  If you could replicate 
the symptoms by shutting down the leading or trailing plugs I think it 
would give you some insights into the problem.

If I'm not mistaken, a very "cold" plug in a recip engine will start 
missing after a relatively short period, and a very "hot" plug can get 
hot enough to lead to detonation. 
What if you were to replace your shrouded plugs with plugs about 3 or 4 
heat ranges hotter and see how long they last.  I can't imagine how a 
shrouded plug could make a hot spot prominent enough to cause detonation.

Or am I missing something REALLY important here ... Jim S.


Ed Anderson wrote:

>No, I'm not certain, Mark,
>
>However, the plugs are not necessary worn out at 20-30 or 100 hours on MoGas
>but they must have the center ceramic cleaned of lead or carbon deposits.
>Not arguing that platinum and Iridium plugs won't give longer life - just
>don't  believe they will go significant longer before the SAG hits.
>
>Those that have not yet hit the SAG have a thrill awaiting.  The first time
>your engine drops in rpm and the EGT drops 300F, you'll likely think some
>major problem has occurred.  For months I though I had erratic fuel
>injectors or some intermittent fuel problem.  Tracy is the one that put me
>onto the plugs - change to new plugs and the problem ends immediately.
>Generally  backing off on the power seems to help temporarily and the
>condition will go away on its on (after getting your heart beat accelerated)
>to recur at ever increasingly frequent intervals.  Seems to happened mostly
>at high power settings - like on takeoff where you really don't want
>anything unusual from the engine.  Even though I have had it happen a number
>of times, I still have not gotten use to it {:>)
>
>Ed A
>
>  
>
>