Unleaded Plus vs. Unleaded Premium
 

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?

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.

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!)

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. :)

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. :)

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

So far as Calorific, BTU or Joules are concerned, generally speaking, lower octane fuel contains more energy than higher octane fuel.

This is because the additives used to boost octane contain less calories/volume than normal gasoline - they make the fuel more difficult to combust so that it can be better compressed and burn more thoroughly releasing more useful energy. Since these additives displace a % of normal gasoline when mixed for octane purposes, this gallon of octane boosted gas will contain less energy, somewhere on the order of 5-10%. This is also a problem with Ethanol - it contains 30% less energy/volume than regular gasoline.

Now, the issue becomes potential energy.

Some engines, due to better computer control, higher compression, etc. can release more useful energy from octane boosted fuels than normal gasoline, or perform better - create more HP or run more smoothly.

That is not to say they perform badly on regular gasoline, in fact, due to the wide variation in fuel quality around the world, most cars marketed internationally are setup to run just fine on a wide variance in octane. The Boxster is one of these.

Since there is no tetra-ethyl lead in any of these fuels, none of them will hurt the engine or emissions gear.

If mine were a daily driver, I'd use 90 Octane and only go to Super-premium for track or weekend spirited runs.

But, you hardly need to eek out that extra 0.1 sec. 0-60 when commuting.

Now, that said, there is often some variation between what is stated on the pump and the actual octane rating of any particular batch of fuel, in fact, it's usually slightly lower than stated due primarily to the volatility of the octane additives, which evaporate from the fuel as it sits in the Tankers or underground storage tanks. But, if you fill-up at the busiest name brand station you can find, this isn't usually a problem.

Try several grades of fuel, and use the one which gives you the best value for your money. Because of driving style, conditions, source of fuel, etc., there is no one answer to this question - you need to experiment on your own. You won't hurt the car at all. You may run a little rougher, or see reduced range and if so, just switch back. The savings may be small, but in this day of ever-rising fuel costs and uncertain immediate economic future, it pays to save wherever you can.

thanks guys, i never thought of the octane<>gasmileage feud. i was mainly wondering what long term affects could come other then the occasional check engine light

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.

The key here is "generally speaking" because there are various mixtures used to boost fuel octane and they don't all decrease the energy content of the fuel.

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.

Quote:

Originally Posted by Hops 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.)...

I didn't switch back and forth but I did run two tanks of regular (and discarded the results) before starting the test on the regular. The super was what I normally used so no data was dropped.

I realized I was not taking into account many factors but I did try to be as consistent as I could. Tire pressure was checked every weekend (habit I have, not just for the tests). There was a few times I need to drive a substantial distance on the interstate so I took a different car so as not to throw off my results. All traveling was done in town (but even that could be considered inconsistent) and the 20 tanks took me a good 5 to 6 months to complete so temperature would have been a big factor it didn't account for (although it is always hot in Florida :) .)

Quote:

Originally Posted by blue2000s 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.

That is one of the things I really like about this forum. People here are usually so respectful when they post a reply (I mean that sincerely, I'm not being sarcastic. :cheers: ) You are right and I didn't even think about different formulations or humidity.

My apologies for not being more thorough but when I did this, I was not expecting to pass my finding off as fact. I did it more out of curiosity because my wife was getting such bad fuel economy in her Land Rover and she was too thrifty to put in 93 octane.

I agree with you. I've only seen a few people bicker on here, and I don't understand the point of taking time out of your day to go online and bicker with someone. This is by and large a very friendly forum.

Thanks for providing a little more detail on your method. I am glad you ran your tests. None of the rest of us have gotten off our arses and done it. If I come across as an analyst or something, well, I am. ;) It would be cool to get enough data with enough variables trapped to really get a read on how big of a factor octane rating is in the overall fuel economy of our cars.

Unfortunately the number one factor is our right feet. I've done as high as 28mpg and as low as 16mpg in my '05 987S. As I think about this a little more, the best measure may be mpg over a controlled course using the onboard computer. So, driving multiple runs on a defined section of highway with the cruise set at 65 (or whatever) and resetting the onboard mpg at the beginning of each run might be the best apples to apples comparison... and it wouldn't take months to complete. Of course, this doesn't necessarily reflect the mpg difference in normal driving patterns, but it would be a start.

Exactly... There are fuels which contain more energy and higher octane, but these are usually special concoctions costing much more and not generally available at the pump.
:cheers:

the higher octane is primarily to run more timing. lower your timing down, and run all the 87 octane you want. the car can automatically retard timing, but only so much.

but why would you want to retard timing when you own a porsche, to save a couple of dollars/fill-up?

on the flip side, if you run higher octane, like 100, etc. then bump the timing, and get more power : )

Thats what porsches " knock sensor (TM) " does !

Unfortunately the number one factor is our right feet.

Boy, you got that one right! :D

double true -- i have the tiptronic option which helps with mileage more then i thought it would initially

Sorry to go OT but I found this interesting:

I also own an old Nissan 300ZX turbo and frequent a few Z31 sites. A thread was started almost identical to this one on one of those sites and I pretty much did a cut-n-paste from my above written posts.

Instead of respectfully pointing out the flaws in my data collection or seeing how the data could be beneficial to them (like what was done on this site) they turned it into a pissing match on what octane ratings mean. I was also called stupid for trying to say I know the car better "then the people who made the car from ground up and tested it's flaws.” (<= direct quote from that thread.) The data was ignored while the members there competed in a game of "Who has the bigger e-penis."

This forum and the Land Rover forums are the only two I have been on that show respect to their fellow members and talks to them like they would if they were face to face. It's very refreshing. :cheers:

**Note** This post is not meant to bash on other sites, just to compliment this one. Unfortunately, this site provides no information on my other vehicles no matter how hard I search so I do need to continue to frequent them.