IG-101 is a brief discussion on the controversial topic of SPARK PLUGS. Among rampant opinions and market strategy, we can find common sense.

At 08:59 PM 11/8/02 -0500, Steve Demko wrote:

".... for my 1959 1500 MGA. The plug I was recommended to get ( from a fellow club member who is driving a 1958 1500 MGA) were NPR plugs # BPR6ES. When I checked at the local NAPA store their computer shows the proper plug for a 1500 MGA is NPR # BPR7ES. Who is right NAPA or my club buddy? The NAPA guy thinks that the BPR7ES plug fires hotter. Also is .025 the correct gap setting for my plugs."

If you're a concours nut they're both wrong. If you want your car to run right, they might both be right, and either one might work as well in your car, as well as a very large number of other part numbers from various manufacturers.

By 'the book', "MG Series MGA Workshop Manual" with several issue dates between 1955 and 1961, the correct plug is the Champion N5, which was the NA8 (some years earlier). But that's similar to saying that the correct engine oil is 30 weight non-detergent oil, just because that's what "the book" says. In fact, that's what was state of the art and/or generally available in the late 1950's, but no one in their right mind uses the stuff today. What we use today is multi-weight detergent oil, or even synthetic oil, because it's better stuff than the original spec. The same can be said for spark plugs. Original type would generally work as well today as 40 years ago, but sometimes the good old days weren't all that good by today's standards.

As for your noted part numbers, those are both resistor plugs (see the "R" in the part number), and the factory never recommended resistor plugs (or resistor wires). But that doesn't mean they can't be used. The resistor feature is used for suppression of radio frequency electrical noise that can cause static on electronic radios by disturbing (corrupting) the incoming radio signal. If the rest of your ignition system is in good condition they work okay. The resistor plugs are often recommended as a substitute part for no other reason than the originally specified type may no longer be manufactured. This is a matter of consolidation of the part numbers for similar types to reduce required inventory. There have been so many different types of plugs made by so many different companies that an unabridged cross reference manual would be larger than the workshop manual for this car. Cross references for part numbers that might be used in this particular engine could fill a whole page, and they include various heat ranges, resistor or non-resistor, single or multiple or ring electrodes, platinum contacts, silicon dielectric material, single or two piece top end contact. Any one of those might be the "correct" plug for your use, depending on operating conditions and local availability.

The real correct plug for your car may depend on the physical condition of your engine, what non-standard modifications it may have, large variety possible, and what motor fuel is available, also large variation and none available like the stuff that was around when the cars were built. Think about where you drive (urban or rural, altitude or hills), and how you drive (casual cruise, lead footed, trailer towing or racing). The point is that you should use whatever plug works best for your engine and your driving habits under your local ambient conditions. Additionally you might also have the choice to pay more for a plug that lasts longer, or less for one that's "good enough", in either case maybe the best economical proposition (think cheap).

The most important feature of a spark plug (and maybe the only important feature) is the heat range, and that may not be the same optimum for every engine, maybe even different for engines of the same spec and build parameters. In reality you may have to experiment a little to figure out what works best for you. If the plug runs cold it can misfire or collect oil and foul out. If the plug runs hot it can cause pre-ignition or detonation which can damage internal engine parts or even physical failure of the plug itself. When in doubt you should start with a cooler plug, and if it works well stop worrying. If it fouls out try a hotter plug. Black deposits on the plug usually mean it's too cool. Stark white color or a cracked insulator around the tip would mean it's too hot. Good operation makes the ceramic tip insulator an even light tan color, possibly just a little darker on one side because of variations in temperature in different parts of the combustion chamber. A badly worn engine might leave black deposits on the plugs regardless.

If they run cold in the winter, and you step up one heat range, they will likely run just fine in the summer without stepping back. If they run hot in the summer or during mild competition or trailer towing, and you step down one heat range, they will still likely run well in other conditions without having to change again. When you get the heat range "in the ballpark", there is very little else that matters in the selection of a spark plug. Chances are that you could step up or down one heat range with no noticeable difference, or you can drive in a lot of variable conditions without changing plugs. And whatever someone recommends for your car, it will likely work okay, as there is a wide range of tolerance for such variations.

The originally specified plug gap of .025 inch works well with the original equipment ignition system. And if you maintain the original type equipment in good working order all will be well with the world. There may be some advantage to raising the system output voltage for a hotter spark, like easier starting for instance. You do this by installing a high voltage ignition coil, like the Bosh Blue or the Lucas 40,000 volt Sport Coil for instance. The size of the plug gap that a spark "can" jump is proportional to the output voltage capability of the coil. In fact the actual voltage seen at the spark plug is determined by the size of the plug gap, as long as you don't exceed the voltage output capability of the coil. When the plug gap is small the voltage required to make the spark is minimal. When the gap is increased the voltage require to make a spark jump the gap increases. In this manner you get a hotter spark by increasing the gap. If you get the gap too large you get a misfire under high demand (marginal) operating conditions when the coil can't put out enough voltage.

Highest voltage requirement at the plugs will occur with the highest chamber compression, with the engine ingesting the largest possible amount of air around 2500 rpm with full throttle. This is the same condition where you might get spark knock if the compression ratio is too high or the fuel octane rating is too low or the spark timing is advanced too far. As plugs and points deteriorate with use the operating conditions get more marginal. To avoid having to readjust the plug gap too often, you should stay away from the maximum possible gap and settle for something considerably smaller. With the stock coil you might be able to use .035 gap, but should stick with .025 to minimize the bother with maintenance. With a high output coil it might run well initially with up to .060 gap, but you should still set it to only .035 in order to run a lot of miles without having to fiddle with it. When you have a spark that's hot enough to fire the fuel consistently, then anything hotter is unnecessary overkill that just causes faster wear on the spark plugs, distributor cap, rotor and contact points, and more frequent demand for maintenance stops.

For the record, I use a Lucas 40,000 volt Sport Coil and .035 plug gap, but neither is actually necessary if you keep up good maintenance. I just happen to like driving in any kind of adverse conditions, and sometimes lots of miles in short time with little maintenance, and the higher system voltage is more forgiving to neglect. So I have paid the one time fee for the high output coil, and I like it. I also use Autolite model 64 (non-resistor) spark plugs in my MGA, because they work, because they're available at any local NAPA store, and because they're cheap (which matches my personal requirements perfectly).

And I defy anyone to prove to me that it might run better or that I could save money by using any other plug. In fact I occasionally get free spark plugs on that dare. I have recently installed a set of Halo plugs given to me by a distributor who would like me to test them diligently and return a rosy report. But I know in advance how it will turn out. The end result will be no verifiable improvement in performance, power, fuel economy, running temperature, emissions, or anything else. The primary problem with verifying any improvement in fuel economy is that driving conditions vary so widely on a daily basis that the fuel economy can vary anywhere between 30 mpg and 18 mpg on any single tank of gas, and no two weeks or calendar months ever see the same operating conditions. For any verifiable improvement the change would have to be dramatic, like at least 15%, and by the inherent design of the engine the thermal efficiency cannot possibly be improved that much through improved spark.

If there is any improvement in operating life it still cannot justify the high cost of the special plugs unless they happen to last for more than 100,000 miles with no maintenance. This is not very likely, but as much as I drive the car it won't take too long to find out. Even if they did last forever it would be hard to justify the cost simply on the time value of money, meaning in simple terms that I wouldn't want to spend the extra money that many years in advance of when it might otherwise be required. The guy who gave me these plugs is in fact running the risk of a no-win proposition, and will most likely not like the end report. When you combine "good enough" with "cheap" and it works okay, that combination is nearly impossible to beat with anything that costs substantially more money.

I know all of this rambling isn't going to settle your initial argument, because neither one of your part numbers can be judged to be wrong without a lot of personal field testing, and only you can ever do that. If you were really intent on answering your own question you would have to run each model of plug in turn until it might wear out or otherwise fail, or until you discover some deficiency in operation with one but not the other. Those two part numbers are not a lot different in spec, so you might more likely find that they are both okay (most likely), or that they are both bad (not so likely). Of course if you drive it that much just to test the plugs it would be a very expensive test indeed, but you can certainly have a lot of fun doing it.

My recommendation is to start with any spark plugs that are cheap and keep the rest of your money in your pocket, and if it works quit worrying about it and drive the wheels off of the beast. These cars were designed to provide inexpensive fun, so it may actually be immoral to spend more money than required.

$.02,

Barney Gaylord
1958 MGA with an attitude (and $.99 spark plugs)