You have to remember that the race plugs may be running a different gap than the nonadjustable stock gap. That could be affecting things.

I for one noticed nothing but improvements from switching back to stock plugs. The car idled far better.

 

something to consider;

the stock engines run 7L and 9T. the trailing is colder.

when you move to 9L should you not keep the colder trailings?

 

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