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Originally Posted by Rob-00BoxsterS
Last year I did a test where I filled up 10 tanks with 87 and 10 tanks with 93. I started the test with the tanks completely filled and ended them filled. I then averaged all 10 tanks so the differences in volume should be very minimal.
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Did you run 10 tanks of one octane followed by 10 of the other, or did you switch back and forth in some manner?
The extreme of 10 of one then 10 of the other is the least valid because the variation in temperature, season, mechanical factors (tire pressure among others), and gasoline blend would be difficult to control.
At the other end of the spectrum, alternating 20 tanks isn't a great idea because you are blending the two octanes to some extent with each fill (so the actual octane is somewhere in between). Also, I don't know for a fact how long it takes the car to completely adjust to the new octane... is it instantaneous (my assumption), or does it take 5 miles or 50 miles? (Insight on that question is welcome if anyone knows.)
If anyone plans to run this type of test, I might suggest running three tanks one and then three tanks of the other and repeat this for three or four cycles. Make some note of your balance of highway versus stop-and-go driving for each tank, check tire pressure with each fill, and note general patterns of high temperatures (for instance, 50s / 70s / 90s). We could draw some conclusions out of that sort of data.
I won't personally be doing this... I'm in CA with the highest octane of only 91 (aka craptane). I'd be happy to test 91 versus 93 or 94 if that were possible but have no interest in testing 89, which saves only ~12 cents a gallon and is below Porsche's minimum recommendation.
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