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All,

I am going to be heading from Boston to DC in a few weeks for a wedding and was wondering if putting higher Octane gas in the tank will make a difference in total mileage/engine performance on the long trip. If so, which Octane is safe for the engine? I am going to be doing an oil change (full synthetic - Penzoil Platinum and a Purolator PureOne filter - which I have been very happy with since starting these) the week before the trip. I am hoping that the new oil/filter and higher octane gas will boost my MPG on the trip and give the engine a good cleaning.

I have also heard that putting higher octane gas in the Intrepids can cause a knocking noise. Could this pose any risk to my car?

Thanks again to all on the boards.

Joe
:thanks:
 

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i know guys that will religiously only run 93 octane in everything they drive, i personally run 87 in any car that doesn't require higher. as with anything, you get what you pay for. higher octane gas burns cleaner, but if you regularly change around the gas you use and keep up on maintenance, you'll be fine.

btw, octane boosters are worthless, proven.
 

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The Womanizer!
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I use premium here because it doesn't have ethanol. The better gas mileage pretty much covers the difference in price at the pump.
 

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Premium fuels will be LESS PRONE to knocking than lower 'grade' fuels. Detonation (pinging or knocking) is caused primarily when the flame front impacts the surfaces of the cylinder at an inconsistant rate... by slowing the detonation process with higher octane fuels, knocking and pinging from detonation or pre-ignition is lessened.

But, on a counter point, these engines employ a knock sensor that will adjust ignition timing and fuel maps to correct for pinging...

However, the correction for pinging still doesn't release more power that a higher octane fuel can bring (due to the slower combustion). Since the flame front uses more of its force to push the piston, rather than bang into the piston, you get more effeciency out of the higher octane fuels.
 

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Now it's truth time. Running higher octane fuel in an engine that is designed for 87 does nothing except burn money. In some cases it can actually reduce efficiency slightly due to the slower burn characteristics. The only time higher octane provides more power is if the timing is advanced or compression is increased. Neither of which will be happening in a stock Intrepid engine.

The placebo effect is insidious. Most people when they are trying to pay attention to fuel economy subconsciously don't accelerate as hard, or drive as aggressively. That really is a good thing overall, but it creates the illusion you are getting better economy with higher octane. Sometimes it seems like you are getting better economy, but the weather was playing more of a part. Humidity and temperature play a bigger part than the fuel grade. Also, all grades have the same detergents, so you are not "cleaning" your engine by running a higher grade. It's a myth.

Better things to do to are to run proper air pressure in the tires, good maintenance, and most importantly GOOD DRIVING HABITS. I have found I can significantly increase economy just by altering my driving route to work. When approaching a red light, just let off the gas farther back, allow the car to slow, and often times it turns green before you would come to a stop. If you are jackrabbiting to each red light, you are wasting fuel. Just consider your brake pedal your biggest enemy, and take step to use it less. Every time you stop, you waste gas, so just drive routes where you don't stop as much. Don't rush from red light to red light, you won't get yo your destination any faster but you waste a lot more gas.

I have done the high octane fuel mileage test with every vehicle I have owned, running several tanks of each grade, with no difference in economy. I've been driving since the mid 80's and have owned many vehicles and the results have always been the same. People can say what they want, but I have to go with the personally experienced evidence.
 

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eh...I've been told and read on here and other forums that the cars computer will basically change itself on how its running the engine for whatever kind of fuel it has going into it and that just by putting in higher octane gas doesn't mean u r gonna get higher mpg outta the car. The computer compensates for any fuel differences and tries to give the best performance it can with what its fed. I have run a few tanks of premium thru my r/t and noticed exactly no difference. No more or less mpg's or better acceleration or pings or anything. Runs the same no matter what brand of fuel I put in or the octane. I do occasionally use a fuel system treatment to "supposidly" clean the fuel injectors, etc. but I really don't hold any true hope it does what it says. Just a personal choice. My brother says they work, he's a mechanic for GM so who knows....lol...I wouldn't waste your extra cash going premium man!
 

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Now it's truth time. Running higher octane fuel in an engine that is designed for 87 does nothing except burn money. In some cases it can actually reduce efficiency slightly due to the slower burn characteristics. The only time higher octane provides more power is if the timing is advanced or compression is increased. Neither of which will be happening in a stock Intrepid engine.

.... Better things to do to are to run proper air pressure in the tires, good maintenance, and most importantly GOOD DRIVING HABITS.
Aaaah, the sweet sounds of a voice of reason. Thank you Jason. You saved me a lot of typing, my friend.

Check your vehicle's owner's manual, gang. It says so. The R/T is designed to run on regular.
 

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Now it's truth time. Running higher octane fuel in an engine that is designed for 87 does nothing except burn money. In some cases it can actually reduce efficiency slightly due to the slower burn characteristics. The only time higher octane provides more power is if the timing is advanced or compression is increased. Neither of which will be happening in a stock Intrepid engine.

The placebo effect is insidious. Most people when they are trying to pay attention to fuel economy subconsciously don't accelerate as hard, or drive as aggressively. That really is a good thing overall, but it creates the illusion you are getting better economy with higher octane. Sometimes it seems like you are getting better economy, but the weather was playing more of a part. Humidity and temperature play a bigger part than the fuel grade. Also, all grades have the same detergents, so you are not "cleaning" your engine by running a higher grade. It's a myth.

Better things to do to are to run proper air pressure in the tires, good maintenance, and most importantly GOOD DRIVING HABITS. I have found I can significantly increase economy just by altering my driving route to work. When approaching a red light, just let off the gas farther back, allow the car to slow, and often times it turns green before you would come to a stop. If you are jackrabbiting to each red light, you are wasting fuel. Just consider your brake pedal your biggest enemy, and take step to use it less. Every time you stop, you waste gas, so just drive routes where you don't stop as much. Don't rush from red light to red light, you won't get yo your destination any faster but you waste a lot more gas.

I have done the high octane fuel mileage test with every vehicle I have owned, running several tanks of each grade, with no difference in economy. I've been driving since the mid 80's and have owned many vehicles and the results have always been the same. People can say what they want, but I have to go with the personally experienced evidence.
That would be your own testing and that can be flawed. The results are not scientifically done. Get about 9 more people to drive the exact same cars and I'll listen.

Aaaah, the sweet sounds of a voice of reason. Thank you Jason. You saved me a lot of typing, my friend.

Check your vehicle's owner's manual, gang. It says so. The R/T is designed to run on regular.
Do you believe everything you read?

Using higher octane does increase mpg but how much depends on how you drive. I'm a lead foot and see little increase but when I drive intelligently it shows. That's not why I get i though, I get it more for the cleaning effect then the mpg effect. A clean engine is a happy engine.
 

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I run 89 octane in my 300M and even with my lead foot (with a couple of 115 mph high speed runs. Just testing). I am getting 23 mpg city/hwy.
 

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Do you believe everything you read?

Using higher octane does increase mpg but how much depends on how you drive. I'm a lead foot and see little increase but when I drive intelligently it shows. That's not why I get i though, I get it more for the cleaning effect then the mpg effect. A clean engine is a happy engine.
I'm reading this and I don't believe it. :punish:
 

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I'm seeing a few misconceptions running around here..

First, don't assume that because it is higher octane, it cleans better. THAT property is designed by the manufacturer of the fuel, not the octane rating. Octane rating is simply a measure of the fuel's resistance to pre-ignition (by contrast, Cetane rating is a measure of the fuel's ability to self-ignite, and is for CI engines). It has nothing to do with power, or efficiency.

Here's what happens when you use a lower grade of fuel than is recommended: Pre-ignition will occur, either by the fuel/air mixture igniting before the spark plug fires, causing extreme damage to the piston and connecting rod, or the endgasses self-ignite as they're pushed up against the cylinder wall by the flame front before it can ignite them. This is apparent by a knocking noise from the engine.

What happens next is the piezoelectric knock senors feels the pressure waves caused by the knock, creating a voltage signal that the PCM uses to determine the existence of knock. It then retards the ignition timing, lowering performance and lower specific fuel consumption. All bad things, but they help keep your engine alive while you continue to use the poor grade of gasoline.

Putting in a better grade will help to eliminate this. It will not clean your engine unless the brand you buy specifically puts more/better cleaning agents in plus, or premium.

Here is my recommendation. Continue using your normal grade of fuel. Continue driving normally, When you're sitting there, idling, at lights or drive thrus, listen to your engine. SI engines have less of a tendency to knock at higher engine speeds (such as those on the highway), so knock will be more apparent at low idle conditions. Do you hear any knocking, pinging? If so, step up a grade. My Special will occasionally do that with 89, recommended by the manual. I have since switched to 93 and all is well.

Additionally, if you want to have some good cleaning going on, stop buying gas-flavored water and start buying Top Tier fuel. Every company that sells Top Tier fuel sells good cleaning gas at every pump, and they get this rating from GM, Honda, BMW, VW, and Audi. A few examples are Shell, Chevron, Sunoco, Conoco.Phillips, QuikTrip, etc. Search Top Tier at your favorite search engine and you will find the complete list.

Last, something my I.C. Engines professor told me about a couple of weeks ago, is "incipient Knock". Better understood as "borderline knock," this is a condition where the endgasses are expended before the piston moves far away from top center. The benefit is better specific fuel consumption. I'm not sure exactly why this is true, but I know better than question a man who has studied I.C. Engines for the last 30 years, and has sold technology to companies like Ford.

Why do I mention this? Even if you have a bit of pinging and knocking from 87 at idle conditions, I would recommend using it for trips that are entirely highway. The reason is, as stated earlier, is SI engines have less of a tendency to knock at higher engine speeds, such as highway. Basically, that idle-speed knock will not trouble you on the highway, and you can get all the benefits of borderline knock and pay less at the pump. Otherwise, continue to use the next grade up for your regular commute (again, unless it is entirely highway). Use Shell, and you'll have the same cleaning properties as the next grade of the same brand. Same for Chevron, Conoco, etc.
 

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I'm seeing a few misconceptions running around here..

First, don't assume that because it is higher octane, it cleans better...
Robert - That is probably a "belief" held by older members here due to ads that gasoline companies used to run (when you were running around in diapers) to convice the public that the "higher grades" of gasoline had more cleaning additives. I mean - come on - do us old farts not actually still refer to the higher octane fuel as "premium". It must be cleaner, right. Else why would we call it "premium"!!!!

Seriously - I really do think that that is why many people believe that "premium" gasoline has more and better cleaning agents in it. I definitely remember at least one of the companies - I believe it was Amoco - specifically saying that in their TV ads and on decals on the pumps not too many years ago. I actualy remember Corvette owners swearing and declaring that they absolutely *had* to use *only* Amoco gas in their cars because of that.

Now - having said that - I don't know if it was just hype then (i.e., did all gradees of Amoco have the same amounts and types of cleaners in them), and what about today? Do some brands put better and/or more additives in their higher grades of fuel? I don't know. But I really do think people do believe that to be the case (because of the ads of years ago).

Some of the confusion may also have resulted from the transition from leaded gasolines over to non-leaded. During the years of transition, "regular" was the lower octane fuel with lead still in it so that older cars (with unhardened valve seats) could still get fuel that wouldn't damage their valves (at least, that was what we were told - I've seen discussions lately that indicate that that may have not been true), and the premium fuels were unleaded, which everyone knew burned cleaner (and that was true). It was during the later stages of the transition from leaded to unleaded that the ads were touting better or more cleaning additives in the premium unleaded fuels compared to the lower grades (leaded or unleaded).

So you young pups may not have the baggage that the older farts have to unlearn about the cleaning properties of the different grades of fuel. With fuel injection being pretty much universal, it's probably pretty much a necessity that all fuel have similar cleaning properties.
 

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That would be your own testing and that can be flawed. The results are not scientifically done. Get about 9 more people to drive the exact same cars and I'll listen.
Well since my testing has been with over 20 vehicles since the late 80's, and I have known many people with the same results, I'll trust my own results along with a significant percentage of the population. I am sure if this thread persists long enough, you'll get at least your 9 other people who drive Intrepids and actually understand what is happening with their engines and their fuel.


Do you believe everything you read?

Using higher octane does increase mpg but how much depends on how you drive. I'm a lead foot and see little increase but when I drive intelligently it shows. That's not why I get i though, I get it more for the cleaning effect then the mpg effect. A clean engine is a happy engine.
This statement proves my point about not understanding what is happening with your car. It also confirms what I said. When you drive more intelligently. Good fuel economy is as easy as adjusting the nut behind the wheel.

Octane has nothing to do with the cleanliness of your engine. I used to get in conversations with my sister about high octane gas and her little 93 Toyota. She finally saw the light and started using 87 and saw that all she was doing was throwing money at Exxon. They already get enough of our money, no need to give them even more for no other reason than misinformation. It's easy, use what's recommended by the manufacturer. They didn't just dream up what octane to throw in the car, and the real smart people being the ones running high octane. The recommendation exists because that is what they designed the car to run on most efficiently in it's stock as purchased condition.
 

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Also, while most modern cars have knock sensors and will retard the timing when knocking occurs, that doesn't mean those same car's engine programming will actively advance timing to the point of knocking. That theory of running a higher octane and the engine adjusting to it only holds true to the more performance oriented cars. Basic family sedans like ours run on baseline programming. It is very unlikely that our engines along with most vehicles adjust ignition timing for higher octane. If this were the case, all those companies that produce chips and tuners for fuel injected cars would be out of business. There is a reason why you have to actively adjust injection timing and duration along with ignition timing with a tuner or chip if you want more performance, the ecm is running the engine on manufacturers programming and that programming runs within fairly narrow parameters.
 

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Okay all you Old Timer's... Anybody remember "Megatane rated" gasoline? I can't remember the company that touted it, but I do remember the many people were "taken-in" by the advertising and the few people that laughed themselves silly over it.

Also, remember the rumors that floated about for many years that there was only one storage tank in the ground at gas stations with two pipes to the pump, one for "Regular" and the other for "Premium"? This particular rumor was excerbated in the 1970's by the "fact"(?) that when a gas station ran out of fuel, ALL the pumps ran out at the same time! (That one still bothers me because I have seen it happen, twice!... That and that I have never seen a truck deliver to more than one hole in the ground.)
 
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