Just an update on my Impreza pinging problem.
Until now I ignored the advice that I should try higher octane gas because I just coudn't justify it.
Then I happened to read a couple of articles saying pinging can cause poor gas mileage.
So I filled up the last two tanks with 89.
TaDa.
95% of pinging is gone.
Even the remaining occasional 5% pinging is very short (like a half second).
And gas mileage was up almost 10% for the last two tanks.
I used to get 11L/100KM (Winter and Summer with A/C on), 10L/100KM (Sping/Fall).
The last two tanks were 10L/100KM even though it's Winter.
So I pay $2.5 more for 89, and the pingings almost disappeared and I saved $5 per tank due to the better mileage.
Awesome.
