Which Spark Plug?

Fine wires cost three times more than massives. But theyre more efficient and appear to last longer.

-by Coy Jacob

While the world of piston-engine R&D focuses on electronic ignition and diesel engines, one aspect of internal combustion for airplanes hasnt changed much: the lowly spark plug. As electronic ignition gains more service history, that may change. But for now, owners have to solve their plug problems with essentially two choices: massive electrode or fine wire plug designs.

Given how badly an engine can run without clean spark plugs of the right type, youd think performance and economy gains are there for taking with improved spark plug technology. And while that has happened in the automotive world, as usual, light aircraft GA engines putt along in a technological backwater, awaiting further developments.

Why? The usual suspects. GA volume is low, the profitability is slim and product liability remains an ever-present worry. In other words, if it aint broke, don’t fix it.

Thats not to say there’s anything wrong with aviation spark plugs, just that there’s nothing really new out there, either. That said, confusion abounds about which type of plug is best, which delivers the most bang for the buck and how long you can run a set of plugs before theyre junk.

Plug Players
At the moment, there are two sources of spark plugs for GA engines: Champion and Unison/Autolite, with Liberty, North Carolina-based Champion having the lions share of the market. The Auburn plug is no longer in production; the rights to the Auburn plug were sold to Champion. That said, there are still plenty of Auburn plugs out there.

Rockford, Illinois-based Unison Industries manufactures and sells the Autolite aviation line of spark plugs. As of early this year, it markets only massive electrode versions. They dodged our questions about whether or when they plan to introduce a line of fine wire plugs.

Our guess is that they have more up their sleeve than just another me-too version of whats already on the market, especially given the companys experience with the LASAR electronically advancing magneto. Keep your eye on Unison, for theyre promising something new soon.

Meanwhile, the choices in plugs remain simple if not crystal clear. Will it be massive electrode plugs or fine wire plugs? There’s ongoing debate about which is better for which application and there’s precious little data to support the arguments for either side, other than direct anecdotal experience.

Both massive and fine wire plugs operate on the same principle, that being that the spark arcs across a small air gap between two electrodes. Massive electrode plugs have a larger center electrode made of a less expensive nickel-based material than that used in fine wire plugs.

Fine wires were developed to improve starting and anti-fouling characteristics in two-stroke engines and they have migrated into other applications.

Because aircraft engines burn leaded fuel, plug fouling is a worry. Some engines are more susceptible to this than others but all plugs foul to some degree. Fine wire electrodes reduce the required firing voltage and thus more efficiently convert the energy available into spark. This reduces fouling.

For turbocharged piston twins flying at high altitude, fine wire spark plugs have been thought to be more effective at igniting a fuel air mixture than massive designs using less exotic metals for their electrodes.

Whether these performance gains are worth the additional cost is debatable, however. And the cost difference is not trivial. With its high conductivity, high melting point, high temperature stability and corrosion resistance, a metal called ruthenium is ideal for fine wire firing tips. Iridium is also favored. Cost wise, a fine wire is two to three times more than an equivalent massive electrode model. That adds up.

Lets say youve got a O-540 series Lycoming that requires a Champion RHM38E massive plug. That plug will retail for about $16 at discount. The equivalent fine wire version-the Champion REM38S-sells for about $42. Run the numbers and installing a new set of massives will cost $192 while the fine wires will cost $504.

Is it worth the cost delta? Probably. The carbureted Lyc is a poster child for plug fouling and if you grow weary of taxiing back in after a bad run-up, the additional cost will seem trivial. On the other hand, in a four-cylinder engine that doesnt tend toward fouling and is flown infrequently, the additional cost wont be worth it.

There’s some indication that the more efficient spark fine wire plugs produce may also improve overall engine efficiency. Data collected by RAM, a well-known mod shop and overhauler, indicated that fine wire plugs were 2.2 percent more efficient than massive electrode spark plugs.

RAM used a TSIO-520-NB engine at high cruise or climb power (232 HP at 2400 RPM; EGT 100 degrees rich of peak at 10,500 feet), yielding a brake specific fuel consumption of .498 for fine wire plugs and .509 for massive plugs.

Obviously, thats not a huge difference and could easily be buried by other aircraft and operating variables. On the other hand, if youre miserly with mixture and like to run lean, fine wires are probably the better choice. (For more, see RAMs website at www.ramaircraft.com.)

They Last Longer
Fine wires do seem to last longer than massive electrode designs, based on field experience across a range of engine types. Further, they seem to improve starting performance in some balky starting fuel-injected engines, especially Lycomings.

How much longer they last is a bit of a crap shoot. For example, following proper maintenance procedures, when in production, Auburn Spark Plugs claimed plug life to be 400 hours for massive plugs and up to 1200 hours for fine wire plugs.

However, without periodic re-gapping-typically done at 100- hour inspections-and proper rotation, spark plug life may be reduced to half those recommended numbers.

According to RAM, which published the results of their own operational study on fine wires versus massive electrode plugs, there are two reasons why fine wires are more effective than massive plugs.

First, the massive electrodes shear size somewhat shields its own spark from some of the fuel/air mixture around it. The result is a less-than-even ignition, which is less efficient. Second, the iridium alloy used in the fine wire plug allows the use of a larger spark gap. The larger gap results in a more uniform combustion event, which affects both perceived smoothness and economy. This second point is worth some elaboration. General Aviation Modifications, Inc., has done extensive research on cylinder-to-cylinder variation in fuel/air ratio and power output. Indeed, smoothing out these variations is the basis of the companys flagship product, GAMIjectors.

GAMIs George Braly says that in addition to cylinder-to-cylinder variation in fuel/air ratios, cycle-to-cycle variability between combustion events can be improved by the ignition system and, in particular, by improving the uniformity of the initial combustion flame front. Fine wire plugs reduce the randomness between combustion events, improving smoothness.

Further, the gap of any electrode is vulnerable to erosion over time. The massive plug is especially susceptible due to the medium melting point alloy used in manufacturing it. The alloy erodes more quickly than does a fine wire electrode. So even though you pay twice as much or more for a fine wire, you get better performance and longer life. Does that argue for fine wire plugs in every engine? Not necessarily. Champions William Austin and high-performance engine guru Terry Capehart tell us that massive plugs make sense for a low-altitude, low performance engine, say a Cub or Cessna 150. But high-performance engines should clearly get fine wire plugs.

Heat Range
The heat rating of a spark plug is the measure of its ability to transfer heat received from the firing end of the plug into the cylinder head and cooling fins.

A cold plug transfers heat rapidly away from its firing end into the cooling system and is used to avoid excessive plug core nose temperatures where combustion chamber pressures and temperatures are relatively high, as in a turbocharged engine.

A higher heat range spark plug has a slower rate of heat transfer and is used to avoid fouling where combustion chamber or cylinder head temperatures are relatively low and lead or oil deposits are likely to be a problem.

Operating temperature of the spark plug insulator core nose is one factor that governs formation of troublesome combustion deposits. Spark plugs are susceptible to carbon deposits when the operating temperature of the core nose insulator is at or below 800 degrees F. But an increase of just 100 degrees F is sufficient to eliminate formation of these deposits. Also, lead deposits form because the bromide scavenger contained in tetraethyl lead is in-active at low temperatures.

At 900 degrees F, the bromide scavenger is activated, disposing of lead deposits with combustion gases during the exhaust cycle. To eliminate or to keep this problem at a minimum, avoid prolonged idling at low RPM, avoid power-off let downs and after flooded starts, run engine at medium RPM before taxiing. And always lean aggressively on the ground.

Deposits formed between 1000 and 1300 degrees F are low in volume and electrical conductivity and are least apt to cause spark plug fouling. This is the reason for selecting a plug that will operate within the aforementioned temperature range at all power settings.

Normally, a hot plug is used in a cold engine-low horsepower-and a cold plug in a hot engine – high horsepower. To avoid spark plug overheating where combustion chamber or cylinder head temperatures are relatively high, a cold plug is recommended, such as in a high compression engine.

The primary means of adjusting heat range is by varying the length of the core nose and the alloy material used in the electrodes. Hot plugs have a relatively long insulator nose with a long heat transfer path.

Cold plugs have a much shorter insulator nose and thus transfer heat more rapidly.

The correct heat rating for the engine design ensures that the plug operates cool enough to prevent pre-ignition but warm enough to resist the accumulation of conductive plug-fouling lead deposits.

With Champion plugs, the higher the number the hotter the plug. For example, a REM38E is a colder plug than a REM40E.

All things considered, isn’t the coolest plug the best plug? Yes, probably. GAMIs George Braly suggests that most all practical heat range testing was done back in the 1950s and 1960s when fuel had higher lead content.

There are now better instrumentation methods and he believes that it might be worthwhile for the OEM plug makers to revisit this subject.

In the mean time, Braly suggests it may be a good idea to use the coldest heat range plug thats authorized for your particular engine, provided you don’t have lead build-up or oil fouling problems.

One reason for this is because its easy for mechanics to damage spark plugs in handling and cleaning and this can affect the subsequent operating temperature of the plug, reducing the assumed heat range margin. There’s also some manufacturing variability in the spark plugs themselves. Using the lowest heat range available lets the pilot take advantage of the largest possible pre-ignition protection margin.

Size and Reach
For the most part, engines can use either 3/4-inch-also known as all weather variety-or 5/8-inch barrel spark plugs. Generally, higher performance and most all turbocharged engines use 3/4-inch and smaller engines tend to use 5/8-inch barrels. The 3/4-inch variety offers better moisture resistant seals and less opportunity for arcing.

Before you order spark plugs, you must know if the ignition harness lead nut is 3/4 or 5/8 inch. Be sure to double check before screwing the plugs into the engine. A 5/8-24 plug takes a 3/4-inch wrench on the lead nut while a 3/4-20 plug requires a 7/8-inch wrench.

Generally, if youre replacing the plugs and harness, the 3/4-inch all weather is a better set-up because the lead end of the spark plug is completely sealed from moisture. REM designates a 5/8-inch barrel plug while RHM designates a 3/4-inch plug.

Spark plugs can be either short reach or long reach. This refers to the length of the threaded portion of the spark plug that screws into the cylinder.

Larger engines generally use long reach plugs while smaller engines use short reach plugs. Be sure to check, as some engines, such as the Continental IO-470, require short or long reach depending upon model.

Summing Up
No rock solid data exists proving fine wire plugs work better than massive but its generally accepted that they are less prone to lead fouling and that they do provide a more energetic spark.

While they cost substantially more upfront, the sting of the higher price is offset by the fact that they last longer, perhaps up to three or four times longer than equivalent massive plugs.

Overall, properly maintained and rotated massives may be cheaper in the long haul but if cleaner firing fine wires save you just one run-up hassle a year, the cost difference will seem trivial.

We think substantial cost savings can be had by proper maintenance, something most plugs may never see. This includes proper rotation and using care in both cleaning and handling.

Its entirely possible that rough handling can crack or damage the fragile insulator to the point that internal arcing occurs. A three-inch drop on a metal workbench may be enough to trash a plug.

And while youre considering replacement plugs, buy a couple of spares, wrap them carefully, and carry them in your baggage compartment toolkit. They could come in handy some Sunday afternoon when the FBO is closed and one of your plugs is fouled and misfiring.

Also With This Article
Click here to view “Checklist.”
Click here to view “Plug Rotation: Not Just Bottom to Top.”
Click here to view “Spark Plug 101.”

-Coy Jacob operates the Mooney Mart in Venice, Florida. Hes an Aviation Consumer contributing editor.