As one who has usually driven marginally powered vehicles, BMW motorcycles, 4cyl autos, I've always looked for ways to give an easy boost to engine power. Spark plugs such as Splitfire, AC/Delco Rapidfire, Bosch +2 or +4 promise improved performance but I have always found they don't when applied to an otherwise stock motor, fuel-injected or carburated. Results have always been consistant. There may be a slightly detectable improvement in starting and idling. There is always a very noticable decrease in mid-range or roll-on response. Like any other modification one change requires another to get optimum performance. So while these plugs may work they seem to upset the balance on otherwise stock fuel settings. My latest experiment was to try Bosch +4 in my Chrysler 3.5 V-6. In this case roll-on response was typically soft, the engine not wanting to respond when pulling out to pass for example. Worse I also suffered a 2-4 mpg loss in fuel use. I used the plug for over a month before returning to single electrode Bosch Platinums which immediately restored gas mileage and crisp throttle response. The worst I ever experienced was the AC Rapidfires which I don't think are sold any more. Good idle and starting but the engine simply wouldn't respond when I wanted to accelerate to pass at typical highway speeds.
On spark plugs e.g. splitfire, rapidfire, bosch +4
- Thread starter ripdaddy
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Most likely wrong plug for the motor. Anything other than the oe plug for an engine is often "close enough". Heat range, projection, etc are determined by the oe manufacturer, and a plug is made or selected for that application. Another manufacturers plug may be a close aproximate, but is usually just slightly off. This shows more on some motors than others. The fancy plugs have a very limited range of part numbers, and are known more in the industry for their creative marketing than for their performance. Any performance increase noted is usually because the old plugs were junk, and just about anything would work better than what's being replaced. If the car came with AC's, Champs, NGK's or Motorcrafts, thats usually the plug thats gonna work best in that app, and the one to go with. For what it's worth, I've had my best luck with Autolites or NGK's in place of oe on the app's I've seen. 
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Penforhire
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It depends. Some work.
My race club found that Bosch +4 plugs gave a definite slight improvement to C5 Z06 corvette dyno output. We did it in two vehicles with known-good OEM plugs (good gap and visual condition) and the results were repeatable but barely above the dyno's noise-threshold (about 3 HP added to 350 average at the wheels, taken away when OEM plugs were swapped back in). Large club dyno days are fun because the cost-per-pull is low enough ($10 or less) to try these things.
Interestingly enough, that was much more of a difference than going from crappy California premium gas 91 octane) to 104 octane race gas in these same vehicles. This second test might have been foiled by the ECU's learned fuel trims but we also tried it on a reset ECU.
My race club found that Bosch +4 plugs gave a definite slight improvement to C5 Z06 corvette dyno output. We did it in two vehicles with known-good OEM plugs (good gap and visual condition) and the results were repeatable but barely above the dyno's noise-threshold (about 3 HP added to 350 average at the wheels, taken away when OEM plugs were swapped back in). Large club dyno days are fun because the cost-per-pull is low enough ($10 or less) to try these things.
Interestingly enough, that was much more of a difference than going from crappy California premium gas 91 octane) to 104 octane race gas in these same vehicles. This second test might have been foiled by the ECU's learned fuel trims but we also tried it on a reset ECU.
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