I also remember the J.C.Whitney Rapidfire sparkplugs (LOL). If memory serves, they were the same as the Splitfires of today. However, they are different in concept, and design, from the Bosch Plus 4. With the Plus 4, if an electrode wears, or erodes, there are three more for the spark to choose from. Therefore, there is always a place for the spark to be generated. If not one electrode, then another, and if not that, another, etc. I believe the Plus 4 concept makes them the longest-lived plug on the market. In my mind, the plugs work great...very smooth idle and very responsive. I have never seen conclusive proof that different manufacturers' plugs for an engine, assuming they're all of the proper heat range and gapping, make a measureable performance difference. I think they're all that close to each other.
Ed

 

As an employee of a very large plug manufacturer (won't mention the name) and having worked in a technical capacity I will tell you that multi ground electrode plugs have a place but not in your car. The spark can only jump to one electrode at a time and it's a hit & miss as to which one it will jump to each time. Splitfire is an absolutely fantastic example of what a great marketing campaign can do to promote false promises and create a demand for something that can't live up to its billing. They were brought to court for misleading advertising and were told to either provide factual evidence of the claims they were making or otherwise pull the plug on their ads.

Multi ground electrode plugs are found mostly in heavy duty industrial engines that are required to run continuously. The reason they are used is that they will last longer because it takes more time to wear out 4 ground electodes as opposed to one and don't have to be replaced as often. Downtime to replace these plugs in these engines usually means a loss of revenue while the plugs are being replaced. For this reason, multi grounds are used along with platinum center electrodes to extend the service interval.

The use of platinum and double platinum plugs where the ground electrode also has a platinum pad extend the service interval required over conventional copper cored plugs but do so at a premium price. Since the density of the platinum is greater it takes a lot more energy to erode the electrodes but this also means it takes more energy for spark to fire across the gap. Combined with larger plug gaps, modern ignition systems must have a much higher energy output that those in the past.

If you get a chance to go to any NHRA or IHRA event and go look around the pit areas take a close look the spark plugs that the different classes use. I'm pretty sure you won't see any multi ground plugs in any of their engines and these are guys that are trying to gain every advantage they can, even if its 1/100 of a second. With regards to spark plug indexing, it is done when there is a clearance issue between the ground electrode and the piston. It's been proven on a dyno that it doesn't make any more power!

Although precious metal spark plugs (platinum, iridium) can last longer, they won't if the fuel you're using isn't of good quality. This mostly has to do with the additives used for getting the proper octane rating. I've seen platinum plugs with less tha 10,000 kms (6,600 miles) ruined because deposits from these additives have masked the ceramic around the center electrode causing the plugs to misfire. If your car is not running well or has a rough idle, replacing the plugs with new ones of any type will instantly make the car run better and you'll think the new plugs are great. Next time this happens, replace with the originals and see if the outcome isn't the same.

My advice would be that you get the plugs changed more often, regardless of what the manufacturer's recommended service interval is. Engines are put on dynos to have their heat characteristics charted in order to determine the proper heat range for proper operation. As long as you replace your plugs with the appropriate heat range as the original (based on manufacturers catalog) you shouldn't have a problem. Also,have you ever experienced trying to remove a plug out of an aluminum head after it's been in there for 100,000 or less miles? The plug gets seized in the head and SNAP! Now you've really got a problem.

For my money, replacing the plugs sooner is still the least expensive method of getting the best performance, not to mention gas mileage, you can from your car. 'Nuff said.