Unleaded Plus vs. Unleaded Premium

[PSL-Boxster]

is there really a difference if i use unleaded plus gas instead of the suggested unleaded premium? my sister does it in her car (which isnt a porsche but also is suggested to run on premium) and has no noticeable difference in hp or gas mileage - is it ok for porsche engines to run on this type of gas for the sake of saving a couple bucks every time i fill up?

[6speed]

I would think the porsche would run on it. It has what porsche calls a Knock sensore. But you failed to mension exactly what car she has. You know porsches are fine tuned and i would think it would make a diffrence.

[Jeph]

Yes it will run on less. I wouldn't do it -but that's just me. Mine's not a daily driver. Boxsters have been known to throw check engine lights if less than optimum gas is used. So I don't put anything less than 93 octane in mine.

My cousin has an '08 Jetta that recommends premium. He only fills that up with premium about every 4th tank. Says it's doing fine. (Got it in January new and has already racked up 14,000 miles on it!)

[dmenn]

Last time I checked you owned a Porsche - suck up the couple bucks and put in the type of gas that it's supposed to take.

[Rob-00BoxsterS]

I also owned a BMW E46 323is that I daily drive. 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.

I know this test was not very scientific since there are variables I didn't account for (like driving style, conditions, weather, etc..) but I tried to keep everything as consistent as possible.

End results: Although I could not feel a difference in performance, I did notice my fuel economy went up 12% while the added cost went up only 8% (this is back when fuel was 3 dollars a gallon) so it was actually less expensive for me to run premium over standard (as fuel prices rise, the savings is even greater.) Couple this with the fact that the car was designed to run on premium (like the Boxster) and my decision became a no brainer.

Let's say:

-You drive 15,000 miles per year
- And you average 22 MPG
-That's about 681 gallons of fuel
- At a $0.12 price premium for the 93 octane (vs. plus)
- You pay an extra $82 for the "right" gas
- That's about equal to 38 cups of Vente Starbucks!
- So, use the right fuel, and cut back on 1 Starbucks per week, and you'll come out ahead.

[Brucelee]

Originally Posted by Rob-00BoxsterS

I also owned a BMW E46 323is that I daily drive. 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.

I know this test was not very scientific since there are variables I didn't account for (like driveling style, conditions, weather, etc..) but I tried to keep everything as consistent as possible.

End results: Although I could not feel a difference in performance, I did notice my fuel economy went up 12% while the added cost went up only 8% (this is back when fuel was 3 dollars a gallon) so it was actually less expensive for me to run premium over standard (as fuel prices rise, the savings is even greater.) Couple this with the fact that the car was designed to run on premium (like the Boxster) and my decision became a no brainer.

That is interesting work and I think it synchs up with the theory on octane and mielage. Thanks for telling us.

[blue2000s]

Originally Posted by Rob-00BoxsterS

I also owned a BMW E46 323is that I daily drive. 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.

I know this test was not very scientific since there are variables I didn't account for (like driving style, conditions, weather, etc..) but I tried to keep everything as consistent as possible.

End results: Although I could not feel a difference in performance, I did notice my fuel economy went up 12% while the added cost went up only 8% (this is back when fuel was 3 dollars a gallon) so it was actually less expensive for me to run premium over standard (as fuel prices rise, the savings is even greater.) Couple this with the fact that the car was designed to run on premium (like the Boxster) and my decision became a no brainer.

I know you mentioned that your test wasn't scientific. I appreciate your admission. Fuels have different formulations at different times of the year, and economy is heavily effected by the air temperature and humidity as well as the other factors you noted.

[Hops]

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.

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.