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Is premium gasoline really necessary?

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#1 of 39 OFFLINE   Jeff Cooper

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Posted September 25 2007 - 01:53 PM

I was reading an article a few weeks ago about the top 10 money saving tips, and one of them said something to the effect of "Despite what the manufactuers would like you to believe, your car will run just as well off of regular unleaded vs. the more costlier premium octanes." Since gas is so high priced these days, I was wondering if this was really true. I have a car that the manual clearly states to only use premium gasoline in, but would certainly like to save some extra bucks at the pump. However, I definately don't want to cause any damage to my car that would greatly overshadow the savings of the gas.
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#2 of 39 OFFLINE   KurtEP


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Posted September 25 2007 - 02:01 PM

From what I've heard, most normally aspirated engines can adapt to lower octane fuels without too much problem, the engine management will adjust to it and the only result will probably be to lower the horsepower. On the other hand, I've also heard that turbocharged engines are the exception to this. Apparently, a heavily turbocharged engine, like the Subaru WRX, can be damaged pretty easily by running low octane fuel when it is pushed hard. Of course, I'm neither a mechanic or an automotive engineer, so you can take this with a grain of salt.
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#3 of 39 OFFLINE   John Dirk

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Posted September 25 2007 - 02:32 PM

Kurt is correct [and refreshingly humble]. Octane is a measure of the point at which fuel ignites, also referred to as its "flash point". The higher the octane, the higher the flash point, or point where the fuel will ignite. The efficiency of a gasoline powered engine is highly dependant upon timing. Without getting into excessive detail, there is a specific cycle to engine operation. The cylinder must rise up into the cylinder head, compressing air and fuel along the way. Then a spark is added from the ignition system [modern cars no longer have carburators] at just the right time. The timing belt and engine computer regulate this operation. If the octane level is too low, then the fuel may ignite [flash] too soon, forcing the cylinder head back down before it was able to complete its compression stroke. When this happens an audible "knock is produced, the engine is robbed of horsepower, and ultimately suffers excessive wear. Most mass produced cars compensate for the variance in octane levels of commercial fuels by having the engines computer adjust the timing accordingly. Since turbocharged cars have much higher compression ratios by nature, they can still experience engine knocking with lower octane fuels.

The bottom line is this. Try the lower octane fuel in your car. If it cannot adjust to it, you'll know very soon because the knocking is easily detected.

For what it's worth, I drive a $60,000 [not bragging, it's 6 years old Posted Image] Lexus and it has the same ridiculous "Premium Fuel Only" label. I use 87 octane fuel almost exclusively and my car runs as smooth as butter.

If you need further anecdotal evidence, consider the rental industry. They know we're not going to put premium fuel in their cars, yet they continue to rent cars that clearly call for it. Remember when they used to rent standard transmissions? They quickly realized this was not a good idea.Posted Image

I am also not an automotive engineer or mechanic, but I do love cars almost as much as Home Theater, and I'm fairly confident that the information presented is correct.

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#4 of 39 OFFLINE   Steve Schaffer

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Posted September 25 2007 - 04:18 PM

It really depends on the particular engine in question. Most V6 Toyota, Lexus, etc. recommend premium but will run fine on regular, these aren't turbocharged nor are they particularly high output engines for thier displacement. Anything turbocharged or anything naturally aspirated that gets really unusually high horsepower for it's engine size (i.e. the 180hp 1.8 liter in some Toyota models) should not even try regular. Any modern engine management system will have knock sensor(s) to detect pre-ignition and retard spark timing to minimize engine damage. In the case of stuff like the V6 Camry and such one can get away with regular all the time but high output engines should only be run on regular when premium is not available and should be driven very gently until premium can be added. Conversely engines designed and meant to run on regular can actually suffer if premium is used--starting may be harder and idle speed may be too high to compensate for engine miss due to too high a fuel volatility. A Prius will actually kick on the check engine light if run on premium.
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#5 of 39 OFFLINE   Benovite


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Posted September 25 2007 - 04:58 PM

The gauge by which I usually go by in regards to what particular octane I use for my cars is this; 1) How old is the car? 2) Does the car make noises, particularly when accelerating? 3) How does the car idle? If you have any older car(and in this day and age with technology moving as swift as it does)older meaning 10 years or older, you would put in the higher octane gasolines. For example, is your car made in the new century/2000's? Go for 87 octane, you should be good unless there's already been some engine work and such. If your car was made in the 90's, upgrade to 89 octane. Depending on your engine and the wear and tear you might wanna go for 91. Wait is 91 even an octane level? I forget, anyway you get the point. Furthermore, as has been pointed out, if your car makes knocking sounds especially while accelerating, that's a reasonably clear sign that the gas you're using is crap. Upgrade, either to a better gas station or octane amount, preferably the latter. Lastly, if your car idle's kinda rough that's also a sign that you should put in 89 if not 91 octane. Go for the best juice and your car will love you for it.

#6 of 39 OFFLINE   Jay H

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Posted September 25 2007 - 11:47 PM

I have the same 2.5L turbo engine in my Subaru Outback XT as in the WRX and it calls for premium fuel. I know and have done so myself put the midgrade in and from what I've heard, you'll lose some HP and lose a bit of gas mileage so it really makes it awash so I put in Premium 95% of the time, especially since I always try to fill up in NJ, which has some of the cheapest gas prices in the nation, especially when compared to our neighbor in NY.... Especially for the mileage I do, my cost savings would be pretty minimal between premium and regular considering I bike to work. Jay
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#7 of 39 OFFLINE   Philip Hamm

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Posted September 26 2007 - 01:09 AM

As has been stated, it depends on the car and engine in question. My advice would be to find an online forum devoted to your car and search there, the question has almost certainly been answered. Some cars really require premium to run well. My wife's car has a little supercharged I4 which puts out amazing power for it's size and gets tremendous gas mileage. It says in the manual that it "requires" premium. I've tried 89 and 87, and the results with that car are - it really requires premium! It will run on lower grades, but the gas mileage is horrible and it runs like crap. Sometimes manufacturers have ECUs which will advance or retard the ingnition timing based on sensors so that you'll get better performance out of premium, but they will run just fine on regular. One thing's for sure - if the manual says 87 octane, putting anything better than that in is a complete waste.
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#8 of 39 OFFLINE   Jeff Cooper

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Posted September 26 2007 - 05:06 AM

Thanks for the replies. My car is a 2003 Mercedes C240, I don't believe it's turbocharged or anything. I also understand that this is not a auto experts forum, and was just wondering what people's experiences were. I may give it a try for a tank or two and see if I notice any knocking or performance issues. The price of gas varies wildly in my area. Right next to my home it is 29 cents per gallon (!) more expensive than the Costco 5 miles away. I try to swing by Costco on my way home from work to refuel.
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#9 of 39 OFFLINE   Philip Hamm

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Posted September 27 2007 - 02:43 AM

If you have a Mercedes it probably really does need premium, like my wife's car. Be sure to track your gas mileage carefully. European brands generally need premium because in Europe there is no gasoline lower than something like 90/91 octane, so that's what they engineer for.
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#10 of 39 OFFLINE   Paul Padilla

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Posted September 27 2007 - 04:18 AM

It's usually best to go by what the manufacturer recommends, or if your budget is really eating at you, at least split the difference between regular and premium if you have that option. (To this day I can't get used to any unleaded gas being referred to as "regular". Time flies.)

Manufacturers say a lot of things...some of it over compensation. Changing oil every 3K miles is far too frequent unless you are driving in a severly dusty environment on a regular basis. Miles of dirt roads, etc. The heat of the engine turns the oil black, but there is no breakdown in viscosity until closer to 10K. Of course changing every 3K causes no harm.

This is also referred to as detonation and definitely causes damage. One small point though...the cylinder cannot be forced down on the compression stroke. The crankshaft has to continue it's revolution first, but the important part is that the force of the cylinder coming up is fighting against the explosion. (Squeeze your nose shut during a sneeze for a demonstration. Posted Image ) However the knock doesn't have to be pronounced or even easily audible for it to accumulate damage.

This is splitting hairs a bit, but the issue with higher performance engines being susceptible to this problem is attributed to the temperature surrounding the cylinders being higher and prematurely igniting the fuel. The added octane guards against that. The big misunderstanding about premium gas is that it is somehow better or cleaner. The feeling is that one can get more power or mileage by using premium but that's not strictly true. A high performance engine (Super or Turbo charged, primarily) can lose performance by using lower octane (plus the detonation problem) but an average engine doesn't gain anything.

One brand vs. the other may have different additives, but the only benefit of premium is guarding against detonation. I had a friend with a classic Corvette hardtop convertible ('54 I think) that would knock on anything less than a tank full pumped up with an additive called 104 Plus. It raises 15 gallons of gas to a mimium of 104 octane. His engine was in perfect condition but simply ran very hot.
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#11 of 39 OFFLINE   Scott L

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Posted September 27 2007 - 05:40 AM

This isn't the best example but it shows that higher octane in performance cars does make a difference:

Posted Image

Note that they are pushing the engine to its limits on the dyno. If you usually just sit in rush hour traffic or coast on local roads then you shouldn't have anything to worry about.

#12 of 39 OFFLINE   Benovite


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Posted September 27 2007 - 06:00 AM

German cars demand the highest grade alcohol, I mean gasoline. Posted Image

#13 of 39 OFFLINE   Jason Harbaugh

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Posted September 27 2007 - 06:26 AM

How about places like Colorado where our regular is 85, and premium is 89? Is the lower octane because of the altitude?

#14 of 39 OFFLINE   Jeff Cooper

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Posted September 27 2007 - 06:40 AM

Wow, that's interesting, I never knew it was different in different regions. In southern California it's 87/89/91 for regular/plus/premium.
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#15 of 39 OFFLINE   Ed Moxley

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Posted September 27 2007 - 09:55 AM

If I put regular, in the Ford Ranger I had (V6), it would spark knock so bad, it would drive you nuts. Bad performance overall. It would, and did, run fine with the mid range gas. My bass boat's outboard motor requires premium gas. The new ones will run on regular, but the older ones like mine ('91) will get carbon build-up so bad, and harm it. Having work done on an outboard motor isn't cheap either........
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#16 of 39 OFFLINE   JohnRice


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Posted September 27 2007 - 10:44 AM

I have a 94 Oldsmobile Bravada, which is your standard GM SUV. It's engine (4.5L Vortek V6) is built more for power than performance and the manual actually says to use octane 94-98. I have always used regular, which is 85 here in Colorado, but as pointed out, is 87 In Connecticut where I bought it. It runs just fine, and I actually get a bit better mileage with 85 than 87. My folks have an Eagle Vision, which definitely has a more performance engine, and if it gets 85 it knocks.

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#17 of 39 OFFLINE   Clinton McClure

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Posted September 27 2007 - 12:53 PM

I couldn't agree more. My 2000 Toyota Celica GTS has a 1.8L 180hp engine which requires premium. (91 octane I believe the manual calls for.) I tried stepping down once with a little 89 octane plus grade and it felt like I was running with a burned out cylinder or a bad spark plug. The hp seemed to be cut by almost 1/4 and the idle was rough.

#18 of 39 OFFLINE   Todd Hochard

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Posted September 27 2007 - 01:49 PM

Looking on the web, the 2003 C240 has a 10.5:1 compression ratio, so it may want the higher octane. My 2002 Acura TL-S has a 10.5:1 V6 (260hp) and back in 2004, after Hurricane Charley, I had to use 87 for a couple of weeks, due to the scarcity of gas. It didn't fuss too much, but I could tell the power was down. I track mileage, and for those couple of tanks, I was at 17! mpg. The long-standing normal was 21-22mpg. So, even though my car did OK with it (I could hear the pinging from time to time), the mileage would have killed any savings.
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#19 of 39 OFFLINE   Nimrod



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Posted September 28 2007 - 01:57 AM

I drive an SS Impala which is Super Charged = 93 octane Posted Image

Also the Factory Supercharged cars run hotter, so you should change your oil morePosted Image

#20 of 39 OFFLINE   Philip Hamm

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Posted September 28 2007 - 03:47 AM

This is true for many lawn mower engines as well.
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