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Old 05-09-2000, 05:11 PM   #11
Scooby Newbie
Member#: 143
Join Date: Aug 1999
Location: Kanata, ON, Canada
1999 Impreza 2.5RS
Supercharged - and STOLEN


Tim Prudence,

Since I installed the Rimmer s/c, I've tried three heat ranges with the NGKs.

1) At the time of my #3 mishap, I chucked the Champions for NGKs that were 2 steps colder. I also installed the RR FPR.
2) At the time I had the accident (hit a pole) I had the opportunity to check the plugs they were totally black and sooty. Carbon-like crystals were lodged between the ceramic insulator and the plug's body. My RR FPR could be at least part of the reason (running richer). I replaced them and went down one step hotter (or 1 step colder than stock).
3) At the time I rebuilt the motor (with the new short block, etc.) I saw the plugs again. They were still sooty but had some tan patches. I decided to go back to the stock heat range of 5 (that's NGK 5, not Champion 5 Champion's a different numbering system).

I now realize I had installed the colder spark plugs on the basis of some misguided and false presumptions.

The basis for selecting the heat range of a spark plug should not be on how you want the temperatures affected in the combustion chambers. Neither should it be based on how to compromise non-idyllic conditions that the chamber sees because of poor fuel management or whatever.

The sole and pure reason for selecting the proper heat range is to ensure the spark plug reaches its self-cleaning operating temperature while ensuring its max operating temperature is not exceeded in whatever environment it had been subjected to. The role of the spark plug is not based on what can it do to affect that environment but on how it can accommodate it.

The onus is on the person to make sure the engine's ignition and fuel management is tuned properly and that the conditions in the combustion chamber are right. In a properly tuned engine, the spark plug's heat range accommodates the imposed conditions.

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