Hi,
The Ignition System was designed to work at it's best with the Plugs the manufacturer calls for - Bosch FGR 7KQC or BERU 14 FGR 7KQC. This is because the Head was specifically tapped to properly index these Plugs, meaning that they seat to the proper depth in the Combustion Chamber, have the proper Temperature Range, and are aligned properly in terms of Flamefront propogation.
The specific functions of any Spark Plug are:
- To Ignite the Air/Fuel mixture
To EXTRACT the heat out of the combustion chamber - it is a Heat Exchanger
The temperature of the spark plug's firing end must be kept low enough to prevent Pre-Ignition or Detonation, but high enough to prevent fouling and lies between 450°-850° C.
The Heat Range is a measure of the Spark Plug's ability to remove heat from the Combustion Chamber. The Spark Plug removes about 22% (or more than 1/5th) of all the Heat from the Cylinder - 20% through the Porcelain Insulator and 2% through the Spark Plug Wire from the Connector at one end to the Electrode Tip at the other. This
Thermal Performance is determined by the Heat Range of the Spark Plug. A Spark Plug's Heat Range has
no relationship whatever to the actual voltage transferred through the Spark Plug.
Using Aftermarket Plugs does not guarantee that any of these things will be achieved. This is because there is no Industry Standard on Temperature Range - while the difference between between successive Heat Range numbers by the
same Plug from the
same Manufacturer is about 70°-100° C (in terms of operating Heat, this is the equivalent of advancing the timing 10°), this in no way assures that the same Heat Range Number of a different Plug from another Manufacturer will operate in the same range.
In fact, some Manufacturer's Heat Range increases as the numbers ascend, while for other Manufacturers the Heat Range increases as the numbers
descend.
Thermal Performance is the single most important factor of a Spark Plug.
So far as igniting the A/F mixture, there is no such thing as a
better Spark or a
greater Spark, there's just the Spark. The AF Mixture doesn't ignite any better with a Spark temperature of 80,000° K than it does at 60,000° K (59726.85°C). Nor does it combust the mix more completely, or quickly, that is determined by the Fuel Quality and Octane Rating, Compression Ratio, Combustion Chamber, and Piston Dome design, and Flamefront propogation.
Precious Metal Spark Plugs are mainly a gimmick to increase Market Share in a very competitive Market. There are no Performance advantages to be had from them at all. One supposed
advantage is that the Precious Metals slow down the rate of degradation of the Tip, allowing the Spark Plugs to remain in service longer. Another
advantage is that the plug's Center Wire can be made more thin - an advantage for the Manufacturer, not you.
There's a False advantage in making the Plugs last longer. While the Tip may not degrade as quickly, the Ceramic Insulator does not possess a corresponding longer life and can crack just as quickly as a non-Precious Metal Plug requiring it to be changed. And, Carbon Buildup around the Metal Case will impede Heat Transfer from the Combustion Chamber, again, reducing Engine Perfomance and Longevity.
Spark Plugs are
Cheap and easy to change and there is simply no advantage to increasing their Service Interval (especially on a Sports Car). These parts perform literally 400-500 Million cycles (or individual firings) during a 2yr./30k mi. Service Interval. It's simply prudent to inspect any piece of Machinery after this many Cycles, especially the one which makes the whole IC process possible.
And, since inspecting them (especially in the Boxster) equates in function to changing them, considering the minimal cost involved, it's advantageous to simply replace them at that time.
In order to increase Market Share, these Precious Metal Plugs are often marketed with much Fanfare and unsupported claims to lure you into buying them.
At one point, Firestone (a Tire Manufacturer) tried to enter this fierce market by offering a Spark Plug which had a
Polonium tip. Supposedly, this radioactive metal would
ionize the air in the gap, easing Spark formation. But, the high cost of these plugs - $25@ (in 1970's $), a questionable theory at best, the AEC's concern that radioactivity may be released into the environment, and people's aversion to using anything radioactive, resulted in so little demand for this product that it was quickly pulled from the Market and Firestone withdrew from the field.
So, don't be drawn in by all the Hype - stick with the OEM Spark Plug. You really cannot do any better...
Happy Motoring!... Jim'99
Turbo
Christine
Frank N Stein
Ghost
Linear Mode
