“Rimfire” Spark Plug Prototypes by Paul Knapp, AZ, 1993
About Rimfire Spark Plugs...
Paul Knapp designed a new 1/4-32 miniature spark plug in 1993 for the model airplane industry. The “Rimfire” name was derived from his first design; a surface-gap racing plug that performed exceptionally well at high speed but was too small to retain enough heat at low speed to prevent fouling. After several electrode designs, public demand led to the typical single ground electrode so the builder could change the gap for different applications. In 1994, hand made samples were sent to many model racing enthusiasts in exchange for their reports on performance and/or failures. This valuable information was used to improve performance and reliability as Paul developed his new spark plug. By 1995, Rimfire spark plugs were used worldwide, and many were undergoing rigorous testing by several Aerospace companies such as XCOR Aerospace, Austrian Aerospace, German Aerospace, and NASA. The popularity among the model engine builders, the model airplane industry, and the need for different applications led to a variety of sizes and styles.
Bill Carpenter of CH Electronics in Wyoming produced the most reliable miniature CDI (capacitor discharge ignition) system used by modelers worldwide. In 1993 Mr. Knapp bet Mr. Carpenter that he could produce the world’s best, and most reliable, miniature spark plug to accompany the CH ignition system. Today’s line of “Rimfire” miniature spark plugs is the result of that bet.
Rimfires and Vipers are the world's first high-tech miniature spark plugs and they are specifically engineered to increase compression, run hotter and withstand the extreme temperatures and pressures produced by today's high-performance model engines.
The Rimfires were designed to outperform and replace the older generations of miniature spark plugs from the 1930s through the 1950s. Their physical sizes, designations, threads, and reach are nearly identical to the plugs that were most commonly used, but with added technological features to increase an engines performance. The Rimfire V plug for example, replaces the old Champion V plugs used in the early days, the VR2, replaces the old VR2, V3 replaces the old V3, etc. Rimfires have been extensively tested and successfully used in commercial and hand made model engines, experimental model airplanes, boats, cars, small jets, and miniature and full-size rocket engines.
The Vipers are the world's smallest commercially available spark plugs and are, by no means, any less rugged than their big brothers- the Rimfires. They were designed to accommodate the demand for smaller and smaller engines that require ever-increasing performance. The world of producing high horsepower from smaller packages is only in its infancy. The advent of the Viper spark plugs will set the pace and meet the demand as this industry develops and matures.
Rimfire and Viper spark plugs utilize only CERAMIC insulators. They are manufactured from the highest grades of alumina available and surpass that of most full-size automobile spark plugs. The intent: Produce the highest quality, strongest, and most reliable miniature spark plug the world has ever seen. We believe we have fulfilled that goal.
Generally speaking, the spark plugs with the longest ceramic inside the chamber are the hottest running plugs. The VR2L should run hotter than the others simply due to its long ceramic inside the engine.
The heat range of a plug is determined by the ability to conduct the heat away from the tip back into the engine head. Theoretically the longer the ceramic the longer distance the heat needs to travel to get back to the head, therefore, it should run hotter. Conversely, the shorter the ceramic the less time it takes conduct the heat back into the head, so the cooler it will run.
However, other factors can affect the heat range. Recently we learned that the longer ceramic spark plug may actually run cooler than the shorter ones due to the intake flow of a particular engine. For example, since the longer VR2L ceramic is exposed at the tip it may actually be cooled off much faster by the incoming air on intake. Therefore it can run cooler than a VR2 which may be tucked up and out of the jetstream from intake.
Rimfires were designed to run hotter and increase compression over the old Champion or NGK plugs, however they are also designed to help eliminate pre-ignition. Pre-ignition is not necessarily due to the heat range of the plug although a cooler plug can help.