The NGK CR9EK or NGK CR10EK are the only ones I've ever seen make a power difference. They cost about $7.00 - $9.00 each. The gain is from 1 to 2 horsepower increase, depending on the condition of the plugs you're replacing. That's a huge money to power ratio!

The "9" or "10" number refers to the heat range, which also doesn't matter. (it really doesn't)

The "EK" refers to the fact that they have dual electrodes. There isn't any advantage to having 2 electrodes. Think about it: You can't beat physics and get 2 sparks at one time. (That's the "Splitfire Theory")

Why Do They Work Better ??
One reason the CR9EK's or CR10EK's make more power is because they take up more space in the combustion chamber, which increases the compression ratio. The second reason is that the spark is .040" / 1mm deeper into the combustion chamber. I think that increases the burning efficiency at high speeds.

They work so well that Yamaha uses installs them in the R6 from the factory.

One word of warning: they have to be replaced more often since the last thread of the plug extends into the combustion chamber. This means that as the thread on the plug gets loaded with carbon over time, it can damage the threads in the head on the way back out. If you don't want to change your plugs once a season, these aren't for you.

This spark plug came out of one of the many 1999 #1 Plate
Championship Winning 

   
Supersport race bikes built by:

This spark plug was installed new June 1999 ...

100 Roadrace sprint races later...look at it now !!

Notice the hourglass shape of the worn center electrode... this bike was losing lots of power !!  

4 worn sparkplugs in a Yamaha R6 causing a small misfire of 1-2 % per cylinder won't make a misfire "sound"....so this problem went unnoticed. This bike will probably gain 2-3 h.p. just by installing the same type of new plugs. 

For maximum Whoop ~ Ass, I recommend that you replace plugs once every 3 race weekends! Don't forget to use small drop of oil on the threads!  

Leaving these plugs in too long almost wrecked the cylinder head... See the shiny thread ?? That's actually aluminum from the head!! The hard carbon build up at the start of the spark plug's threads tore up the aluminum cylinder head's threads as it was removed!!    

 REMEMBER BOYZ:

"Use Fresh Sparkplugs !"

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