lots of good plug info from grassroots good for faq

What makes good ignition? Ignition occurs in a modern automobile when an arc is struck and current flows between the electrodes of a spark plug, or when current migrates across the conductive medium in a surface gap plug. While that may sound simple at first, the process becomes progressively more complicated as engineers try to optimize the type of spark plug with the ignition system generating the required voltage.

The amount of voltage necessary to arc the electrode gap is set by the following characteristics:

The size of the gap... arc-over voltage is roughly proportional to the gap size
The air/fuel ratio within the gap... the richer the air/fuel ratio (more gasoline vs. Air), the lower the required arc-over voltage
The compression at the moment arc-over is to occur... the higher the compression, the higher the required arc-over voltage
The composition of the electrode... certain metals for all the same conditions stated above will require less arc-over voltage than other metals. For example, platinum requires less arc-over voltage, all other things equal, than does steel
The shape of the electrode... the sharper and more jagged the shape, the easier it is for voltage to jump
The amount of fouling deposits trying to remove the electron flow from the arc... more fouling deposits and lower resistance to ground pulls more energy out of the spark gap.
While it may therefore seem desirable to lower the required arc-over voltage, since without arc-over there is a total misfire and no ignition, low arc-over voltage produces low spark power because spark power is directly proportional to arc-over voltage. That is, by doubling the required arc-over voltage, you double the instantaneous peak spark power, and the higher the spark power, the better the ignition.

All ignition is, therefore, a balance between the requirement to have sufficient arc-over voltage and increasing peak spark power for better, quicker ignition.

What benefits to specialty plugs bring to this mix? One popular specialty type is platinum plugs. The primary advantage of these plugs, especially when used in an OEM ignition system (especially an older system, which may not be producing as much voltage as when it was new), is that platinum will require less arc-over voltage and therefore, particularly in a weak ignition, allows the gap to be jumped a higher percentage of the time.

For example, if at factory gap and with steel electrode plugs, it requires as much as 18,000 volts five percent of the time to jump the arc... due to the changing engine environment and running conditions... and if the OEM only produces 17,000 volts, then it follows that five percent of the time there would be a misfire.

Now, if one installs platinum plugs, which may only require, say, 13,000 volts to arc, the five percent misfiring with steel plugs would be eliminated. Since the ignition output on OEM ignition rolls off as rpm increases, platinum plugs in this case would allow the motor to reliably turn to higher rpm, thereby giving and increase in performance and possibly gas mileage.

The disadvantage of this method of reducing misfires is that the higher arc-over voltage, the better the spark when it does fire. Therefore, platinum plugs will show a performance improvement with a weak ignition because the benefit from reducing the percentage of misfires more than outweighs the loss from reduced spark power.

SplitFire (split electrode ignition) plugs also reduce the amount of arc-over voltage, but they do this in a different way. Rather than using a different metal as the platinum plugs do, a V is cut into the ground electrode, thereby giving three more "pointy" areas for the spark to jump from. It is this increase in the number of pointy areas available to the electrons migrating from the center electrode to contact which lowers the arc-over voltage. All the advantages and disadvantages of platinum plugs described above are inherent with the installation of SplitFire plugs.

While certain geometry's of electrode design, such as splitting the ground electrode, have gained popularity, the principle behind all unusually shaped electrodes is virtually the same. That is, spark likes to jump from and to pointy objects. Therefore, the pointier the object, the lower the voltage to arc over. By the way, this explains why newer plugs often increase miles per gallon and performance: the center and side electrodes are cut sharp, but after being used for many miles, the constant flow of arcing electricity invariably tends to round them off.

Surface gap plugs, which provide a solid-state medium for the electrons to migrate across, generally allow electron flow, regardless of spark voltage. However, they do not allow the air/fuel ratio 360 degrees of contact area with the migrating electron as is accomplished with an open gap type spark plug, but rather only 180 degrees. This is because 50 percent of the electron path is shrouded in the solid state medium. Like platinum and split electrode ignition plugs, they mask a problem inherent in weak ignitions.

Jacobs Electronics, a manufacturer of aftermarket ignition systems, has performed extensive testing of assorted spark plugs. They have noticed an increase in horsepower, on some occasions, with weak ignitions using specialty plugs. However, Jacobs' testers measured no advantage to using specialty plugs with their Energy Team, Omni-Pak and especially their Ultra Team ignition systems, and in some cases noticed a slight loss when specialty plugs, at the same gap, were installed in the place of standard steel plugs. On the other hand, Jacobs has seen a gain with specialty plugs when their lowered arc-over voltage has allowed users to increase the plug gap above that possible with steel electrode plugs.

For example, Jacobs tested a 253-cubic-inch V6 engine, which was slightly modified; the factory called for a 45-thousandths (0.045") spark plug gap. With stock ignition, it produced 168 horsepower. Platinum plugs increased horsepower to 171. SplitFire plugs also brought horsepower to 171. Surface gap plugs increased horsepower to 172.5 - actually a little closer to 173. Jacobs then installed an Ultra Team and replaced the steel gapped spark plugs; horsepower increased to 180.5. Retaining the 0.045" gap, the testers installed the specialty plugs, and the horsepower fell to 178.5 for all three types. The reason for this power loss was because it required less arc-over voltage at the same gap; therefore, the peak spark power fell slightly.

Further experiments with the Ultra Team and different spark plug gaps produced some interesting numbers. With steel electrode plugs, the peak horsepower achieved was 186 at 0.063" gap. With SplitFire plugs, the peak horsepower achieved was 184, but it required a 0.067" gap, which caused some concern about the piston contacting the side electrode. The platinum plug achieved a peak horsepower off slightly under 185, with an optimum plug gap of 0.066".

So what can we conclude? While specialty plugs can mask the effect of an inadequate ignition, so can reducing the plug gap in many cases. One of the true tests of a good ignition is to install a specialty plug instead of a standard steel electrode plug; of the performance of the engine significantly increases at factory spark plug gap, then the ignition was either inadequate or operating in its marginal range.

While these numbers won't apply to all engines, the results are representative and explain the balance between lowered arc-over voltage to assure a spark actual jumped, and increased arc-over voltage to get higher peaked spark power. The general rule is, the better the ignition system, the less benefit that will be achieved from specialty plugs, unless the installer is willing to go with monstrously large gaps.

Naturally, in certain situations, like when you're campaigning an older car (assumably with a weaker ignition) in stock classes where other ignition modifications are not allowed, testing specialty plugs would make a lot of sense.