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Fine wire vs. massive: which is better? Although fine wire spark plugs cost three times as much as massives, they last longer and deliver more efficient combustion. We think they're worth the cost.

What kind of spark plug do you have in your aircraft engine? Do you even know or care? Or should you just leave that up to the shop and buy what's cheapest? While delegating this choice to your shop is the no-hassle option, we think having an informed opinion on spark plugs might save you a few bucks. Maybe a lot of bucks, actually.

But it's a case of spending more to save more. In aviation as in everything else, you're confronted with the dilemma of one product that does the same thing as another, but costs twice or three times as much. That's definitely the case with spark plugs. The popular massive electrode plug sells for a third less than its fine-wire electrode cousin. In this article, we'll make the pitch for spending the additional money, at least for owners who fly high-performance aircraft and who are interested in fuel economy and long-term durability.

SPARK PLUG ANATOMY

As shown in the illustrations here, a massive electrode plug looks just like the name implies: two (usually) huge outer electrodes surrounding a center electrode provide an air gap across which the spark travels to ignite the fuel/air mixture. Fine wire plugs have more in common with traditional automotive plugs, in that a single electrode--consisting of a small-gauge wire--forms the spark gap with a center electrode.

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There are several variations in electrode designs within the fine wire and massive categories. In the fine wire group, the single iridium wire is the most common. For massive plugs, the standard is the two-electrode configuration shown in the photos here.

The latest Champion guide lists nine different electrode configurations in its offerings. It's up to the user to decide if the cost advantage of the more common massive electrode design outweighs the more costly fine wire design advantages.

The cost difference is slightly more than three to one for Champion plugs--roughly $22 versus $70 each at list prices. Occasionally, other brands such as Autolite go on sale at $30 for fine wire plugs and $15 for the massive types, so watch for these. We saw one recently at Aircraft Spruce and Specialty (www.aircraftspruce.com). Even if you don't need the plugs right away, buying them on sale saves a pile of money.

[ILLUSTRATION OMITTED]

Because of the expense of fine wire plugs, we have some guidelines and testimonials to help you decide if they're right for you, This decision, if based on cost alone, mostly revolves around how much you fly the airplane and if a few percent better fuel economy is worth the extra cost of the more expensive plugs. But before we run some scenarios on which is the best spark plug for your engine, let's take a moment to describe what the differences are between massive and fine wire spark plugs.

DIFFERENCES

Typical massive plugs refer to the electrode design, not the actual physical size of the plug itself. Massive electrode types typically have two large nickel-alloy electrodes. Looking at this design, it's easy to see how constricted the plug well firing area becomes, thus it's not as efficient at delivering hot spark into the fuel/air mixture. The massive design definitely doesn't minimize the formation of lead and carbon deposits.

Then why make the electrodes so big? It's primarily about cost. If you want the electrode to last and don't use exotic materials, there's a trade off between performance and life versus cost. If you look at the firing end of a fine wire plug, you'll see one or two fine wire electrodes. Currently, the single iridium electrode is the pre dominant design. Older designs have included platinum electrodes, but these don't last as long as iridium.

With fine wire designs, the plug well firing area is exposed to a clean, hot spark and more effective scavenging action is thus possible. This plug design is also resistant to oil fouling and can last much longer in service, while offering small performance improvements over massives. That said, it's easy to damage the fine wire electrode because the wire is brittle. This also means you need the right tools and a careful pair of hands to get the gap adjustment just right.

PERFORMANCE

Some of the performance difference between massives and fine wires is anecdotal, but one company, RAM Aircraft Limited, has done some detailed and documented research on the difference between these two plug designs. For years, RAM has specialized in re-engining Cessna twins, Cessna 210s and other Continental 10-520, GTSTO-520 and IO-550-powered aircraft. The company has does instrumented test flight for its engine work and has tested the plugs head-to-head and cylinder-to-cylinder.

RAM is a strong proponent of fine wire plugs and has found the efficiency gain--that is, improved fuel specifics--to be as much as 2.2 per cent. That doesn't sound like much, but carry it out over the life of a large-displacement Continental engine and it could amount to more than $4000 in fuel savings. That more than pays for the additional cost of the fine wires over massives. Further, RAM's investigation revealed that fine wire plugs last much longer.

RAM says that for turbocharged piston twins flying at high altitude, fine wire plugs have proven more effective at igniting the fuel/air mixture and they induce less strain on magnetos. Shortly after Continental developed the liquid-cooled Voyager engine, RAM offered it as a conversion product in Cessna 414s. According to the company's Web site, "since advising operators ... to switch to fine wire plugs, our service managers do not recall any customer contacts regarding ignition performance at altitude."

RAM says there are two reasons why fine wire plugs are more effective than massive designs: First, the massive electrode's large size shields its own spark from some of the fuel/air mixture around it. This causes uneven ignition and a loss of efficiency. Fuel is burned less completely and wasted. Reason two: The iridium alloy used in the fine wire plug permits using a larger spark gap. The larger gap results in a hotter, more powerful spark that's more exposed to the fuel/air mixture, encouraging complete combustion.

One problem all spark plugs must contend with is erosion and melting caused by the spark itself. Although the massive plugs have more material, says RAM, the metal used in massive plugs is less durable and thus more susceptible to erosion. That means that massive plugs don't last as long. RAM reports that customers see as many as 1000 hours or more on a set of properly maintained fine wire plugs.

The manufacturers back up this observation in their product recommendations. Auburn Spark Plug Corporation estimates massive electrode plug life to be 400 hours, but they suggest 1200 hours for the fine wire products. You can readily see that if a plug lasts three times as long but costs twice as much, you still come out ahead, even if the fuel savings are a wash, which probably won't be the case. These durability estimates assume proper gapping and cleaning. Like anything else, an abused spark plug won't give long service.

To back up its fuel efficiency findings, RAM collected operating data from a TSIO-520-NB engine. At high cruise or climb power (232 HP at 2400 RPM; EGT at 100 degrees rich of peak at 10,50 feet) RAM found a brake specific fuel consumption of .498 for fine wire plugs and .509 for massive plugs. For this test, both plus had the same gap settings: 0.018 inch.

WHAT TO DO?

In our view, if you fly fewer than 100 hours a year and your engine doesn't oil foul the bottom plugs much, fine wires will be hard to justify on cost alone. Figuring the typical 300- to 400-hour life for massives and 1000-plus hours for fine wires, the cost difference is a tie on a plug life basis.

However, if you fly a real fuel hog, then adding in the 2 percent savings begins to tilt the investment away from the break-even point and into the black column. At 16 CPH and only 100 hours a year at $5 per gallon, that's $160 in fuel savings per 100 hours. In fuel savings alone, you'd pay for 12 fine wire plugs in 360 hours--that's a $48 difference for each plug at list. The numbers are better if you buy the plugs on sale.

In any case, a minimum of 100-hour servicing intervals is recommended, regardless of plug type. That said, most massive plug users find 50-hour intervals at minimum are required to keep deposits down and gaps at optimal specifications. Thus there is also a potential maintenance saving in time and money by investing in fine wire plugs.

If you fly a turbocharged aircraft at high altitudes, you know that magneto misfiring and other ignition issues can be a nuisance, so we think fine wires are the best choice in this situation, too. Our sister magazine, Light Plane Maintenance, reports that operators of high utilization aircraft--missionary or utility work--like the fine wires because they're just less troublesome to maintain. This anecdotal evidence also supports RAM's findings of better durability from fine wire plugs.

RELATED ARTICLE: FOR AN OIL BURNER, TRY MIX AND MATCH

I have several hundred hours of personal use with a mixture of fine wire and massive plugs in my Continental IO-520-powered Bonanza. Yes, I mixed the massives and fine wire plugs with no problems whatsoever.

I got a great deal on six platinum fine wires that were surplus to an FBO, as 1 had heard that fine wire plugs worked well firing in oily cylinder conditions. My engine was at 1350 hours at 14 years since new, and had begun to have oil-wet bottom plugs routinely, especially on my frequent shorter flights--not much fun to constantly have to clean six bottom plugs to go flying. Since compression was still acceptable in the low 60s, I thought I'd give the fine wire plugs a try since they were alleged to do so well in oil. (Occasionally slightly oily bottom plugs is not necessarily a sign of trouble, particularly with engines 50 percent of the way to TBO, but wet top plugs are another story.)

On local flights, I was burning a quart every three to four hours and on longer flights, about every six hours. Without question, the fine wire plugs helped me go five more years to pass the 1700-hour TBO with significantly reduced plug fouling from the oil leaking past the rings.

How much of an oil burner you have and the nature of the oil leaks limit the degree to which you can make this work. The fine wire plugs can't do miracles, but they definitely can reduce the plug-cleaning chore and slightly improve the combustion in an oil-burning engine.

It was money well spent for my application over a five-year period. With respect to plug rotation to optimize plug life, I didn't bother, but you may be able to do ft with a modification of the standard top to bottom rotation to better equalize electrode wear.

And if, like me, your engine is burning more oil than it used to and fouling the bottom plugs more often than you like, fine wires may be just the ticket There is no reason that you cannot just buy four or six and install them in only the bottom plug holes.

Even if only one or two cylinders are oil burners--usually affecting the bottom plugs--then try fine wires for only those cylinders, but be sure there's nothing wrong other than mild blow-by past the rings, or very modest valve guide leaking, and that the cylinder is otherwise sound. If the top plugs are constantly oil fouled as well, the cylinder needs work now.

A change to fine wires can help with reduced plug maintenance, but it will not change an oil burner from being an oil burner--you will still use as much oil. Oil leaking past the guides will tend to worsen and may ultimately stick a valve. This is by no means a solution, only a Band-Aid for temporarily reduced plug maintenance.

RAM also recommends gapping at the allowable maximum to get the best fuel economy and hottest spark. If you have your plugs serviced by a mechanic, then the reduced plug maintenance has to be factored into the decision costs as well.

- Kim Santerre

CHECK LIST

+ The fine wire electrode spark well is more open, allowing better access to fuel/air mixture.

+ Fine wires promote more efficient combustion, thus saving fuel.

- At two three times the cost of massive electrode products, fine wires aren't cost effective for all owners.

- The wire electrode and brittle and can be broken by ham-handed gap setting.

Kim Santerre is editor of Light Plane Maintenance, Aviation Consumer's sister publication.
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Title Annotation:SAVVY MAINTENANCE
Author:Santerre, Kim
Publication:The Aviation Consumer
Geographic Code:1USA
Date:Apr 1, 2008
Words:2131
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