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I had a '99 9-3 SE with auto tranny (& red DIC). That MY w/auto was tuned to 185hp, but the 5-spd was tuned higher. Now I've got an '02 9-3 SE w/auto & it's tuned to 205hp. IIRC in '00 they dropped in the Viggen single-pass IC as std eqpt & tweaked the ECU (& changed the emissions evap sys). While a higher tune might warrant a cooler plug, a lot is going to depend on how you drive.
To help with Anders' explanation, the NGK rating system is "lower numbers = hotter plugs". Since our tubocharged engines generate more soot in the combustion chambers, it's easier for the plugs to get fouled than on a n/a engine. Also, since the ECU uses feedback directly from each individual plug, it's important to keep the plugs as free of debris as possible. (Each plug is individually monitored & has its own individual coil, & spark can be retarded or advanced independent of any other plugs.)
This is where heat range comes into play. Anders said, "7 are for hard drivers that runs their cars hotter and 6 are for most drivers that drives gently.
7's can be bad for very careful drivers in cold weather since the plug can get fouled up by the gentle driving style."
What he means is, if you're normally doing a lot of high speed (highway) driving, then run a cooler (7) plug because the constantly high engine revs will "self-clean" the plugs.
But if you're doing a lot of stop-&-go and/or short trip driving (city) driving, then run a hotter (6) plug to help "burn off" any accumulated deposits. This is especially problematic for cars in colder (snowy) regions, & is why a hotter (6) plug is routinely recommended for "winter driving."
I'd say run the 6 heat range for your driving style.
posted by 72.16.135...
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