I've never found a difference in fuel economy using different brands or types of spark plugs. NGK or Nippon Denso were probably original equipment in your engine so you won't go wrong with those brands.
For my Saturn, I have read that the OEM NGK spark plugs have the best result. I think there are spark plugs that are ideal for each car, so I think you need to hunt around for a "Nissan Sentra Fans" forum.
If you are willing to spend the money, the very best seem to be your specific engines part number: DOR17LGS at BriskRacing.com!
OR the next one down would be a triple platinum splitfire plug!
And also, I've tested champion platinums, NGK v-powers+Iridiums and Bosch supers and single platinums and all ran VERY differently with VERY different fuel economy results. In fact, the NGK v-power on my 2.8L v6 did not even let me get past 35mph.
Then on the other hand on the same motor, the NGK tr55ix Iridium plug was second best only to the bosch super plug, which is the cheapest plug you can buy for any car and the one that has to be changed every 15k which is a pita...
What I really want to do is try those Brisk plugs. Or maybe after Bob has had them in there for 5000 miles, so I can see how the silver electrode held up...
"You have to know the truth, and seek the truth, and the truth will set you free."
I've never really noticed a difference in FE when trying different plugs either. I've experimented w/OE Volvo, Bosch copper, Bosch Plat.+4, NGK Iriduim, and NGK V-Power. The V-Power plugs seem to give the best driveabilty and they're cheap....so I've stuck with them.
__________________ 1993 Volvo 240 Wagon - 323k miles (awaiting recommissioning) 1999 Audi A6 Avant Quattro - 149k miles(the NEW daily driver)
copper is very conductive, the only metal that would be better in the core of a spark plug would be silver, and you can get silver spark plugs but they cost alot more, the draw back of haveing a silver or copper electrode is that the tip that has the spark jumping accrosse it in the combustion chamber will wear away over time, and platnum and irideum does not wear away nearly as fast, but it is a much higher resistant metal, sparks also like to jump from sharp points, not rounded edges, so NGK V-power plugs have a v-grove in the tip of the center electrode to give it two sharp points for the spark to jump, split fire plugs work with a simaler idea of spliting the other electrode while opening up the tip of the spark plug at the same time, platnum plugs can shrink the tip of the electrode down very very small so it's a sharp point.
Japanese made engines seem to work best with NGK plugs, Bosh seems to be designing plugs to work well in german made engines, and some exspensive plugs that help poorly designed amarican made engines.
because platnum tiped plugs are less prone to wear over time, but higher resistance when new they tend to give a much flatter performance curve, basicly they will never perform as good when new, but over time they will not get as bad, so if you are lazy and don't mind taking a small hit in mileage go with platnum, but if you don't mind changing your spark plugs ever 20,000-30,000 miles then go with the cheaper copper spark plugs.
Just ordered some NGK Iridium plugs for my xB - should arrive just before Christmas - all the way from Oregon . . . what a ship happy world we live in! Should be interesting to see if there is any mileage increase. BTW the resistance of various metals at that high a voltage should have very little effect on the spark - more important to have a small electrode to make it easier for the spark to jump the gap at high engine combustion pressures. Makes the E Field more concentrated by having a smaller electrode but along with the smaller electrode you need a tougher metal so that it doesn't wear and that is where the Iridium comes in. NGK also V the outer electrode to allow the spark to project into the combustion chamber better.