A quick guess would be that the extended tip is acting as a hot- spot in the
head, and preigniting the fuel.
Toby
Second: The other day I came across a set
> of new NGK BPR7ES plugs I had bought (one step colder than the 6ES). I noted
> that they have an extended nose. I have always run BR6ES plugs, which are
>not
> extended. The NGK site calls for a BPR (extended nose) plug for the
>Roadster.
> To make a long story short, I installed the extended nose plugs and the car
> would knock at 3500 rpms under moderate to hard acceleration. Switching back
>to
> the BR plugs stopped the knock. Don't know why it makes a
> difference, but it does in my car.
> Greg Burrows
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