9's versus 10's plugs recommendation

[studdog]

Another new Yamaha guy stupid question. Why are lots of people running the 10's instead of recommended 9 Plugs? Which of the 10's are recommended for a stock machine? Does this engine run better on a colder plug? studdog

 

[grader]

these engines will produce abt. 2 hp more with cr10ek plugs over the stock cr9e plugs. freddie at ecp has done enough dyno runs to verify an increase, and he makes his living making horsepower, and others have reported the same.