Spark Plugs

[Bob Axsom]

I've always taken these little buggers for granted but since I'm doing my own Condition Inspection I had to deal with them this week. Several things surprized me.

(1) The manual that came with my direct from Lycoming O-360-A1A does not specify what spark plug to use.
(2) The Aircraft Spruce catalog says the the REM40E is correct but my engine was delivered with REM38E (colder) plugs.
(3) The correct spark gap is not specified in (1) or (2).
(4) I found the acceptable gap range on a Champion web site as 0.016" to 0.021".
(5) All of my plug gaps were oversize by several thousanths.

I have a Lycoming factory [improperly] installed LASAR ignition system so that may require the colder plug. After I vibrator cleaned and abrasive blast cleaned and regapped the plugs to ~0.016" the engine ran smoother. OK the question - are spark plugs really worth researching to try and eak out some performance gains.

Bob Axsom

[tacchi88]

Spark plugs are well worth researching, although, with the dinosaurs we use for power will not make as much difference as a more sophiticated auto or lawnmower engine. Still it is a source of performance, as well as longevity for the beast.
From racing days, incorrect heat range was enough to make shrapnells out of a perfectly good running fire breather. One heat range mistake and poof. The construction of the plug itself, was also a factor. As a result, I do not use certain brands of plugs, Champions not withstanding.
Again from racing, the construction of our Champion race plug failed, raising the heat range. The result was a ventilated crankcase. After lenghty investigation, the culprit was the plug. The engine required a gap of .016 to .018. Two of the 3 cylinders survived, and the remaining plugs were headed for the same fate. The center electrode began to"dislodge" and increased the heat range. (Yes, heat range can be increased by simply increasing plug gap). Gap increased from .018 to nearly .050 and the second was already starting at .030, as I recall.
There were other findings with various brands and conditions with the help of a dynomometer.
Heat range is most important. Too cold, and it can easily foul, too hot and it will become a glow plug and ripe for detonation. It has an important role in maintaining proper operating temperature in the combustion chamber, which affects, fuel economy, not to mention engine life, and such mundane features as oil consumption. Yep, they can affect oil consumption.
As to my own personal preferences, I find Auto Lites to be far superior to the other available AC plug, the ubuquitous Champ. Based on dyno experience, the Champ plugs exibited the same anomalies, a mysterious, occasional "skip". Same engine same performance, gas, etc, and Auto Lite, the mysterious "skip" was gone.
With LASAR, the Champoin was a better performer, but over shadowed by the Auto Lite. My recently sold Arrow to pay for the RV10, had nearly 900 hrs, with little signs of the eminent "football" shaped center electrode, which is the begining of plug wear identification.
BTW, a friend who built a small amphib using a Rotax 100hp 4 cycle engine, began having hard starts and "rumblings" after barely 30 hrs. The plugs used were NGKs. The plugs were cleaned and gaps set, but shortly failed with the same problem. The were replace with Auto Lites of the same heat range, and to date have more than 300 hrs, and still installed. The NGK's outer ceramic (center electrode) was saturated with carbon from unburnt fuel, and couild not be completely removed.
These were our our experiences, and yes, plugs maintainance, condition, and heat range. play an important part.
T88
RV10 w/Auto Lites and LASAR

[Bob Axsom]

OK, you have my attention. You noticed that I have a colder plug (38 vs. 40) than was specified for the O-360-A1A in the only source I had available (Aircraft Spruce catalog) but I personally know that they have a few substitution problems. There is some question of which Autolite to purchase, UREM38E or UREM40E. I will research it. Thank you very much for the information.

Bob Axsom

[Mel]

Bob,
I'm guessing here, so don't hold me to anything. I suspect that the "40" heat range you are refering to is for straight mags. Your Lasar ignition should have a hotter spark, thereby requiring a "colder" plug.
Just a $.02 guess. (Sorry, I just noticed that you had already stated this.)
Mel...DAR

[Bob Axsom]

I value that input and it influences me to stay with the "38" I will try a set of the Autolite UREM38Es next.

Bob Axsom

[tacchi88]

Mr. Bob,
In all cases the engine manufacturer specifies heat range. The first specification that affects heat range is compression ratio. Engine speeds, cams, breathing are also related, but first is compression ratio.
Under extreme conditions, just about any engine can run with a colder than specified plug, but under normal useage, it is not a good idea, since the plug has to accomodate a wide spectrum of performance. Under normal condition, too cold would lower combustion temperature, which may sound like a good idea (more cooling) but a poor idea since it will lower operating temperature, and prevent the plug from staying clean, and completing combustion.
In the case of LASAR or any "mamaged" timing, despite the more powerful spark, and the ability to adapt timing based on use, the heat range factor remains the same, as far as the plug itself is concerned.
Electronic ignitions have the ability to provide a stronger spark, which will "help" to minimize build ups in the plug, but better still is managing the spark timing for proper load further completing combustion. In all cases, the plug it self must transfer heat to the cylinder head for cooling, hence the heat range compatibility.
BTW, the heat range number for Champs is the same for Auto Lite. As rumors go, Champion was not to happy when Unison did this.. Oh well.
T88
Torello Tacchi

[az_gila]

Or you could just go to the Lycoming web site and download their approved spark plug list.....

http://www.lycoming.textron.com/supp...ns/SI1042X.pdf

gil in Tucson

[Bob Axsom]

Thanks Tachi88 and az_gila. It is interesting that Lycoming "neither recommends nor approves" REM40E for a Lycoming IO-360-A (not my engine I know) but does approve the Unison (Autolite) UREM40E for that engine. That implies that they are not directly interchangable even though both are approved for my O-360-A as are the "38" heat range plugs from both companys. From what Tachi88 and Mel said earlier and the fact that REM38E plugs were delivered with my LASAR equiped engine, I will go with UREM38E for my next spark plug purchase.

Bob Axsom

[mrreddick]

I ordered a full set of Unison plugs from Van's. I have an O-360-A1A with a plenum. I installed the lower plugs first. When I went to install the uppers I found that there's a big difference between Champion and Unison. The wrenching flats on a Unison are at the very bottom of the plug, close to the threaded portion. The Champion's flats are over an inch higher on the plug.
With the Unison's you have to use a socket, there's no way I could get a crow's foot or box end wrench that deep without damaging the pushrod shrouds (that I just recently replaced!). I tried 3 different brands of aviation sparkplug sockets and none of them will fit between the plenum wall and the top of the plug.
I'm forced to stick with Champions!

[tacchi88]

Mr. Mike,
I have two different (3/8" and 1/2") drive sockets that work on both Champs and AL plugs. Both purchased from NAPA.
T88