[johnunit]

I doubt hotter plugs will gain you anything. In general, the stock plugs are best for stock engines. Motorcraft (Ford) copper plugs are quite good.


Hotter plugs will also limit your ability to advance the ignition timing before encountering detonation. I'd experiment with more timing, for sure. Keep an ear out, but you MAY not even need to step up to premium fuel. You may be able to advance timing by enough to make a difference in fuel economy before encountering ping. However, if you don't have the equipment to tap into the factory detonation sensor, and the exhaust leak makes a tapping/knocking sound... You can probably see where I'm going here. My personal experience has been both a 4.0L OHV Explorer and a 5.8l Carbed Crown Vic saving gas (even in terms of dollars per mile) with advanced timing and premium fuel. Whether that translates to a Probe? No promises.