Indications were that sag was due to preignition caused by the spark plugs. The hottest part of the spark plug is the tip of the insulator. The temperature there is controlled by conduction of heat through the alumina insulator into the base
of the plug and by conduction through the center electrode also to the base of the plug. The vast majority of the heat is conducted through the center electrode which has a copper core to help do this because of copper’s high thermal conductivity.
As the spark plugs age, salts form between the corroding center electrode and the tip of the insulator. Those salts have a lower thermal conductivity than the alumina insulator so less heat is conducted to the center electrode.
As the plugs age, the center electrode copper core becomes porous due to the cyclic temperature change with each combustion event. This reduces the thermal conductivity of the copper core.
Those two processes result in the tip of the insulator becoming hot enough to cause preignition at high power conditions. Throttling back stops the preignition. Using leaded fuel appears to cause the preignition to occur with less spark plug
time in use.
For the rpm decrease (sag) to occur, preignition must occur at both the leading and trailing locations simultaneously. Preignition at only one location actually gives a slight power increase.
No amount of cleaning can reverse those changes.
Although the heat range of new stock spark plugs is initially appropriate, they essentially become hotter plugs as they age.
NGK plugs are colder with higher number. Using stock trailing plugs in both the leading and trailing locations will help. I’ve been using BR10EIX plugs in both locations which helps even more and they have been lasting well over 100 hours whereas
the stock plugs would often develop sag at less than 20.
Steve Boese
On May 25, 2023, at 2:51 PM, Charlie England ceengland7@gmail.com <flyrotary@lancaironline.net> wrote:
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Steve Boese did a scientific analysis years ago on what happens to the plugs as they start to break down. I *think* he did a presentation at one of our Roundups, but I can't find the document in any of my records at the moment.
IIRC, it had to do with the insulator around the center electrode starting to break down (fail) from the heat. Hopefully, Steve will see his name & respond.
Charlie
On 5/25/2023 3:20 PM, Finn Lassen
finn.lassen@verizon.net wrote:
Thanks Ed.
But I've not been running AvGas at all. Only 87 10% ethanol pump gas.
I bought the AR2592 plugs as recommended replacements.
Now having watched some youtube videos I may try cleaning the stock plugs in my ultrasound cleaner.
Found one site that claims the RX-8 gap is 1.2mm. So now I don't know why the AR2592 plugs with their 0.6mm gap are recommended.
Finn
On 5/25/2023 3:25 PM,
eanderson@carolina.rr.com wrote:
Finn,
That sounds a lot like the onset of rpm drop caused by lead fouled plugs. I used to get around 100 hrs and then would start getting Sparkplug SAG (engine rpm dropping off). It seem to get worst during high power (such as take off) engine regimes.
Regapping did not help and I never found a way of removing the lead suspected of providing a grounding path inside the plug.
I would recommend a new set of plugs.
Ed
------ Original Message ------
Date 5/25/2023 9:13:07 AM
Subject [FlyRotary] AR2592 gap
For those of you who use the AR2592 spark plug in the Renesis, do you use the 0.6mm gap the plug comes with? I bought 8 of them some years ago.
I've been using the stock plugs (RE7A and RE9B) for maybe 100 hours by now. Experienced a brief 200 RPM drop on last takeoff Wondering if it's time to change plugs. Or re-gap the stock plugs to what?
What can I learn by looking at plugs?
Best way to clean them?
Finn