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Old 06-03-2007, 07:25 PM   #1
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Low octane fuel for a 2,000 mile road trip to Phoenix

Hi folks,

I'm loading up my 2007 S tomorrow and moving out to Phoenix -- a good 2,000+ mile trip. In the interest of saving a bit of cash during the trip, I was thinking I could just put in some lower octane fuel throughout as I won't be doing any high revving with the car for the trip. Once I get to Phoenix I'll run the car super low on the low octane fuel, then go back to using 92+, so I should be rid of most of the bad stuff when I'll want to properly enjoy the car's performance again. Anyone have any compelling reasons to share to NOT do this ?


P.S. If anyone's in Phoenix, please drop me a line, would be nice to meet up, as I only know 2 other people out there at the moment!


Cheers,

Emmanuel

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Old 06-03-2007, 08:15 PM   #2
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Quote:
Originally Posted by ejmc11
Hi folks,

Once I get to Phoenix I'll run the car super low on the low octane fuel, so I should be rid of most of the bad stuff when I'll want to properly enjoy the car's performance again. Anyone have any compelling reasons to share to NOT do this ?
you already said yourself that it's a bad stuff, so why still put it

*my own experience*
It happen to my Suburban, use different brand (long interstate drive from CA. to Seattle) and low octane....I saved $0.20 per gallon but I end up changing my fuel filter after I got back
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Old 06-03-2007, 08:48 PM   #3
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Very poor idea, IMO. Just because you won't be doing any high RPM, spirited driving, these are still high compression, modern performance motors. The knock control in the DME will retard ignition timing to compensate for the lower octane fuel, and should save you from motor damage. However, you will have reduced the performance of the motor for a good bit of time, even after you have re-fueled with higher octane fuel until the DME starts to introduce more spark advance once it hasn't had any knock from the lower octane fuel. Hypothetically, a DME reset should start the clock over and force the engine to "re-learn" how to run on the proper fuel.

At the end of the day, how much are you going to save? Lets say you average 20MPG over 2000 miles, you're going to burn 100 gallons of fuel. Say you save $.25 per gallon, you just saved $25. If you average 25MPG (more likely in a highway cruise in your car), the savings are even less.

To me the risk is nowhere near savings that small. The motor was designed to burn Premium Unleaded, and thats all I'd put into it.

Patrick
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Old 06-03-2007, 08:56 PM   #4
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You'll be using about 80 gallons. Where I am, premium is about .10 more and possibly .20 at some places. That's~ 8-16 bucks total difference on your trip.

The box has a knock sensor, however I'm guessing at least one knock must be detected before it backs timing out, therefore, there is some theoretical damage from the one ping. Also, I have no idea how long the DME takes to re-adjust maps.


Given the long trip, there may be sections where you might want to get on it, or you unexpectedly hit a hill while you're loafing around in low rpm's with a drink in your right hand (if you have a manual tranny).


That said, It's a coin flip for saving $8-16 bucks. IMO, if it were me, it's not worth it because I like knowing I can wind it out whenever I want
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Old 06-03-2007, 09:37 PM   #5
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I think your idea is fine. Depending on the route you take, you'll be going through some pretty high altitudes where the less dense air deters knock. Furthermore, in several western states (most notably California, where Porsche sells the most cars) 91 is the highest octane you can purchase. Our cars run fine on 91 24-7.
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Old 06-03-2007, 10:06 PM   #6
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No real savings

On several occassions, I was forced to use regular (low octane) gas on a few highway trips in other high performance, 91/93 Octane recommended vehicles, when high test was not available. Although not a strict scientific test I always seemed to lose 2 - 4 MPG with the lower octane. Since the engine and DME were designed to run with high octane I suspect you operate outside the design parameters and thus less efficiently. Maybe if you ran it long enough the DME would fully adjust to the lower octane and your mileage would improve but maybe not. Since the MPG loss is about 10% of normal the cost savings from low test to high test would need to be about 10% for you to break even. So ... there would not be any immediate $$$ savings.

The anti-knock sensor should protect the engine from damage due to knocking unless you were attempting to drive at or near full throttle and pushing redline during acceleration. But I believe you would see more savings by using your cruise control and avoiding hard acceleration to pass or get up to speed. Driving at the speed limit and using cruise would probably be more effective at improving MPG and thus saving more money than using low test fuel.

ATB,
Tom
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Old 06-04-2007, 06:38 AM   #7
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Saving $10 is that important?

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Old 06-04-2007, 07:48 AM   #8
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Saving $10 is that important?

I've got to be able to afford these payments somehow.. it's been 4 days since I've eaten anything, that last packet of ramen noodles really filled me up.

Just kidding of course.. you guys make some good points, thanks for all the thoughtful replies. I'm going to stick to premium after all, the car's made me smile so much from the last 3 weeks I've had her, might as well give some love back!
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Old 06-04-2007, 08:25 AM   #9
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Probably not a HORRIBLE idea, yet not a GREAT idea either.

You might try to save a buck or two by using MID GRADE. The idiots at the dealer I bought my Box from put $20 worth of mid grade in it. It ran fine, but ran noticeably better when I filled up with premium.


BTW, I drove 1000 miles to Phoenix and then back a little over a month ago. I averaged 31mpg for the whole trip!
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Old 06-04-2007, 08:39 AM   #10
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old link...not saying this is the fuel gospel, but good points

http://www.straightdope.com/columns/041008.html



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Old 06-04-2007, 09:11 AM   #11
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You probably paid at least $50K for a brand new Porsche - and you're worried about saving 20 cents a gallon on gas? The least you can do is put the right gas into it. Buying the car is one thing, actually driving and maintaining it is another thing, and can be quite pricey.
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Old 06-04-2007, 09:18 AM   #12
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Just as an aside, higher altitude, less dense air does not deter knock, but increases its likelihood. This is a phenomenon known as "density altitude" in aviation, and it has to do with reduced aircraft (and thus for us car guys, engine) performance due to high humidity, high temperature, high altitude- all of which cause lower air density and reduced performance.

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Old 06-04-2007, 12:27 PM   #13
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For each 1000 feet of altitude the atmospheric pressure will drop by a little less than 1 inHg (11 kPa/km). An engine that might require 93 octane at sea level may perform at maximum on a fuel rated at 91 octane if the elevation is over, say, 1000 feet.

Octane ratings can vary greatly from region to region. For example, the minimum octane rating available in much of the United States is 87 AKI and the highest is 93. In the Rocky Mountain (high altitude) states, 85 octane is the minimum octane and 91 is the maximum octane available in fuel. The reason for this is that in higher-altitude areas, a typical combustion engine draws in less air per cycle due to the reduced density of the atmosphere. This directly translates to reduced absolute compression in the cylinder, therefore deterring knock. It is safe to fill up a car that normally takes 87 AKI fuel at sea level with 85 AKI fuel in the mountains, but if carried back to sea level the fuel may cause damage to the engine.
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Old 06-04-2007, 12:55 PM   #14
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ejmc11, here's what you do, just have a Starbucks every other day instead of every morning. Take the savings, and buy 93 octane gas.
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Old 06-04-2007, 04:33 PM   #15
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Ah, I forgot one thing in my translation from aviation to cars: forced induction. Many piston aircraft engines are referred to as "altitude engines" and use turbocharging to maintain a constant cylinder pressure as one flies higher and higher, up to the service ceiling of the aircraft (unlike cars, airplanes use forced induction to maintain power at altitude, not make more power). So yes, in the case of a non-forced induction piston motor at higher density altitude, peak cylinder pressures would be lower, and thus you could get away with lower octane fuel.

Either way, when you have a $50k car, I personally find it hard to justify saving $20 on something like non premium fuel. If anything, I'd run premium just for the better additive package.

Patrick

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