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Old 10-09-2012, 10:48 PM   #201
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Hey, it's just one car and one data point. But it's valid.

You might find it hard to believe that a nuclear-trained geek would be logging miles and gas purchases to calculate his cost per mile, but I think most of this board's members have seen enough of that behavior to accept it.

You and I operate vehicles at very different levels of skill and effort. You're doing a great thread here, and I think it's interesting.

If you want a bunch of sycophantic responses, though, then I fear you've come to the wrong discussion board.
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Old 10-09-2012, 11:01 PM   #202
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Hey, I am sure that like many people here, I would not mind to pay extra for high-octane fuel if I get better MPG.

Now, I remember reading that cars get better MPG if the ignition timing is advanced, but advancing the timing tends to cause the engine to knock, which is very bad. On the other hand, high-octane gas helps with engine knocks.

So, if the above is correct, if I put in high-octane gas, wouldn't I need to advance the timing to get some benefits? Do new cars have controllers smart enough to do that for me?
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Old 10-09-2012, 11:02 PM   #203
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After college, when I started commuting up & down the East Coast with all my possessions stacked in the back, I tried the experiment again. Much to my surprise the MPG was 10% better with premium fuel. It was more expensive, but it actually worked out to less money per mile. Although a full load in that car hammered its mileage, premium gas made a difference worth paying for.
I don't know about the GLC, but many of today's cars will run fine on either regular or premium, but get better mileage with premium. They have knock sensors that automatically detect the "pinging" of predetonation caused by fuel that has insufficiently high octane and then retard the ignition timing when it is detected. All this goes totally unnoticed by the driver. The retarded ignition timing reduces gas mileage (but prevents expensive harm to the engine caused by predetonation). Now, the owner's manuals of these cars usually call for premium gas. Some cars with these knock sensors are designed for regular fuel, but, when something's not right (e.g. a small "hot spot" caused by carbon buildup in the head, etc) will predetonate on regular fuel and run better with premium.

Ever notice that, no matter how high the price of gas goes, that the difference in gas price between grades stays pretty consistent--about a dime per gallon? Seems strange to me.

[edit to add: I crossposted with NW-Bound. Yes, most cars today are smart enough to change the ignition timing in response to fuel of insufficient fuel grade. But, if the car only "needs" regular gas, no benefit will occur from using higher octane fuel.]
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Old 10-09-2012, 11:18 PM   #204
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New cars are computer controlled and can't really be changed. You can buy after market ECUs that replace the OEM one which have different timing characteristics.

The only thing higher octane does is help control knock in certain engines.

The Low-Down on High Octane Gasoline
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Old 10-09-2012, 11:52 PM   #205
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Thanks to earlier posters. Here's something more to ponder.

In my travel up to South Dakota, I saw that a gas station there offered 3 grades of gasoline. The mid-octane one actually cost less than the one with the lowest octane. I kept reading the sign, because it did not make sense. So, I figured that perhaps they mispriced it, and I should just pump the higher-octane yet less expensive gas.

Then, at the next refuel, I saw the same thing again. No way two gas stations would have the same misprice. So, I read the sign carefully and found that the one with the higher octane contained ethanol, while the lower octane one didn't.

Ah hah! I remembered reading that ethanol had less energy than gasoline, and so the lower octane one should cost more and hopefully also gave more MPG. So, I filled up with the higher-priced and lower-octane gas this time.

Thoughts?

Note: I did not bother to figure out the MPG difference between the two tanks of gas, because that would require a lot more driving to average out various factors. The tank was also not empty when refilled.
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Old 10-10-2012, 06:48 AM   #206
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Originally Posted by Nords View Post
Hey, it's just one car and one data point. But it's valid.

You might find it hard to believe that a nuclear-trained geek would be logging miles and gas purchases to calculate his cost per mile, but I think most of this board's members have seen enough of that behavior to accept it.

You and I operate vehicles at very different levels of skill and effort. You're doing a great thread here, and I think it's interesting.

If you want a bunch of sycophantic responses, though, then I fear you've come to the wrong discussion board.
We accept it, but liking it is a big stretch.............
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Old 10-10-2012, 06:49 AM   #207
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Thank you for the compliment...I think.
It was NOT a compliment..........
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Old 10-10-2012, 08:37 AM   #208
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I don't know about the GLC, but many of today's cars will run fine on either regular or premium, but get better mileage with premium. They have knock sensors that automatically detect the "pinging" of predetonation caused by fuel that has insufficiently high octane and then retard the ignition timing when it is detected. ....

Ever notice that, no matter how high the price of gas goes, that the difference in gas price between grades stays pretty consistent--about a dime per gallon? Seems strange to me.

[edit to add: I crossposted with NW-Bound. Yes, most cars today are smart enough to change the ignition timing in response to fuel of insufficient fuel grade. But, if the car only "needs" regular gas, no benefit will occur from using higher octane fuel.]
Just guessing, but I'm not sure they had those knock sensors and automatic timing adjustments back in 1981. Maybe though. Maybe a real car guy/gal can fill us in.

At any rate, I would find a 10% MPG difference hard to explain with premium versus regular. The timing adjustment would only come into play during hard acceleration or lugging. Hard to imagine a 10% delta, unless you were spending a considerable percent of driving under load.

RE: Price delta of regular/premium. IIRC, the additives are a fixed cost, pretty much unrelated to the other costs in gasoline. So it actually makes sense for the delta to remain an absolute, rather than a %. Which means as the cost of gas goes up, premium could be a better deal relative to any MPG increase you might see.

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Old 10-10-2012, 08:53 AM   #209
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So, I read the sign carefully and found that the one with the higher octane contained ethanol, while the lower octane one didn't.

Ah hah! I remembered reading that ethanol had less energy than gasoline, and so the lower octane one should cost more and hopefully also gave more MPG. So, I filled up with the higher-priced and lower-octane gas this time.

Thoughts?
Unfortunately "gas" isn't the same everywhere. Some states like CA mandate their own special blends, which is why their prices are higher. And the blends can change between summer/winter. Taxes may be different on different blends. Also non ethanol gas is harder to find, and people pay a premium for it to run things like lawn equipment, boat motors etc cause ethanol can damage the motor. When I fill up the gas can for the mower I add some of that ethanol treatment product like Sta-Bil.

As long as your car is ok on ethanol, go for the cheaper price. the higher grade won't hurt anything but no benefits
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Old 10-10-2012, 09:05 AM   #210
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Something to consider when thinking about a hybrid is your driving habits. Many times the cost differential for the vehicle is more than savings for MPG. In some case the break even is way more than the life of the vehicle ( 25+ yrs ) and a lot longer than most people keep a car. Consumer reports did piece on this a while back.

Consumer Reports tests small eco models, best only saves $145/year in fuel [w/video]
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Old 10-10-2012, 09:22 AM   #211
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Good point on ethanol blends. Ethanol provides less energy than "pure" gas so mpg will be less all else being equal. In US EPA testing of flex-fuel capable vehicles, mpg ratings for E85 is ~25-30% less than on gas. This suggests ~5% less mpg using 15% ethanol vs "pure" gas.
Re- reg vs mid/premium, 3.5L 6cyl in my late 2001 300M (loved that car) got same mpg on repeated testing over same 15mi commute using mid (Chrysler rec), reg, or premium. OTOH- my last Chrysler 3.8L minivan consistently got 10+% worse mpg dropping to reg from recommended mid-grade. Prem was no better than mid. Keeping some good mpg stats (actual gals vs mi driven calc, not car's mpg computer) can help track your car's efficiency (and promptly ID/fix prob's like bad O2 sens, lightly dragging brake, etc.).

But best fuel $$ saver over long haul is driving habits.
Gas Mileage Tips - Driving More Efficiently
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Top 100 Fuel Economy Tips - Hypermiling Forum
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Old 10-10-2012, 11:27 AM   #212
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It's true that gasoline is not the same everywhere. But as I recall, it was only in South Dakota that I saw the price inversion between the lowest-octane and the mid-octane grades, and it was due to the ethanol blend of the mid grade. Elsewhere, many places just had two grades, and even when there were three, one paid more for the higher octane.

Here's another observation on MPG. The round trip travel between my homes is ~290 mi. long. As we make countless trips over the years, I maintain the same speed, keep the travel time outside of rush hours, etc..., trying to see if I can set a new personal record on MPG.

I should have kept a written record (isn't that what an OCD engineer supposed to do?), but did not. However, I have seen the trip MPG varying from 24 to almost 26MPG with my minivan. I tried to correlate the MPG variations to seasonal changes (A/C or not A/C, heater or not heater, etc...), but have not found a coherent theory. By the way, I use the car computer display, which may not be kosher. That will have to do, as I would have to refill immediately after a trip to know the exact fuel consumption, and I am not that OCD (would drive my wife nuttier!).

Even if the car computer is not accurate, and cannot be trusted for absolute numbers, I should be able to rely on it for a comparative purpose. Yes? Then, what could cause the variations in MPG that I observed?

PS. I refill exclusively at a Costco near my home, using their lowest price grade, which is also the lowest octane. I check tire pressures quite often too.
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Old 10-10-2012, 02:39 PM   #213
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This line from the linked report is misleading:

Quote:
"Unless your engine is knocking, buying higher octane gasoline is a waste of money, too."
If a car has a knock sensor (and most newer cars do) and if that knock sensor detects engine knock, it will retard the timing to eliminate the knock. If this happens, the driver won't know it and will hear no engine knocking and everything will appear to be fine. But the car will get poor mileage and would be getting far better mileage if the owner were filling the car with gasoline of a higher grade. So, it is not true that, just because you don't hear engine knocking you are using the right fuel for your car. You might save a considerable amount of money (through better gas mileage) by using higher octane fuel if your car can benefit from it (e.g. has a high-compression engine or other factors that cause the engine to "need" mid-grade or premium fuel). The owner's manual is the easiest way to know what fuel you should be using.
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Old 10-10-2012, 02:45 PM   #214
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Here's another observation on MPG. The round trip travel between my homes is ~290 mi. long. As we make countless trips over the years, I maintain the same speed, keep the travel time outside of rush hours, etc..., trying to see if I can set a new personal record on MPG.

I should have kept a written record (isn't that what an OCD engineer supposed to do?), but did not. However, I have seen the trip MPG varying from 24 to almost 26MPG with my minivan. I tried to correlate the MPG variations to seasonal changes (A/C or not A/C, heater or not heater, etc...), but have not found a coherent theory. ...
I suspect wind speed differences. You barely notice an average 10-20 mph head wind, but from an MPG view, that is similar to adding 10-20 mph to your speed. Wind resistance is a big component to MPG over ~ 45 mph. Double that delta if you had a head wind one way, and a tail wind the other.

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Old 10-10-2012, 02:56 PM   #215
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Hey, I am sure that like many people here, I would not mind to pay extra for high-octane fuel if I get better MPG.

Now, I remember reading that cars get better MPG if the ignition timing is advanced, but advancing the timing tends to cause the engine to knock, which is very bad. On the other hand, high-octane gas helps with engine knocks.

So, if the above is correct, if I put in high-octane gas, wouldn't I need to advance the timing to get some benefits? Do new cars have controllers smart enough to do that for me?
This is why I said some cars are designed to use high octane fuel. I don't want to write an engineering article here....so I'll try to keep it simple. What's truly important is the cylinder pressure within the combustion chamber at the time the spark plug fires. Advancing (making the spark fire earlier) cylinder pressure means you're exposing the spark to a condition where detonation is more likely. Detonation is bad in a gasoline engine...that's what you call "knock". Gasoline engines are supposed to have a "controlled burn", not an explosion. What I'm trying to say is that there are numerous factors that contribute to the in-cylinder condition that will cause knock...and timing is only one of them (others include valve timing, static compression ratio, humidity of combustion air, EGR function, and so on).

In most modern cars, you cannot advance timing...it is set by the computer. When you see these advertisements for high-performance computer chips you can plug in and make your car more powerful, that's often how they work...by adding more fuel, and advancing spark timing. I recommend against this unless you know what you're doing.

In some cases (again there are a myriad of factors), advancing spark timing can help fuel mileage, but this assumes you don't add more fuel and that you don't advance it so far that you enter the detonation zone. The only way this can be properly tested is on a chassis dynamometer using real time O2 sensor measurement...not a simple thing to do. I've had this done on my Mustang...the cost to get my engine modified to the >725 HP level was over $1,200 for the labor paid to the dyno shop and tuner who did the work.

Gasoline engines like "stoichiometric" A/F ratios of about 14.7:1. Tuners are able to adjust timing in the computer to get the A/F ratio down to a lower number (they call this "leaner", as you're using less air for a given amount of fuel, which burns hotter, resulting in more power). The A/F ratio on my Mustang during WOT (Wide Open Throttle) is about 11.3:1.

Can you buy chips that claim to increase fuel economy? yes
Do they work? Some do
Are they worth all the money you pay for them? Likely not
Are there risks to installing them? Yes, detonation

Hope that helps.
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Old 10-10-2012, 02:59 PM   #216
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How do you feel about running fuel injector cleaner through your engine every 20K or so?
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Old 10-10-2012, 03:02 PM   #217
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I don't know about the GLC, but many of today's cars will run fine on either regular or premium, but get better mileage with premium. They have knock sensors that automatically detect the "pinging" of predetonation caused by fuel that has insufficiently high octane and then retard the ignition timing when it is detected. All this goes totally unnoticed by the driver. The retarded ignition timing reduces gas mileage (but prevents expensive harm to the engine caused by predetonation). Now, the owner's manuals of these cars usually call for premium gas. Some cars with these knock sensors are designed for regular fuel, but, when something's not right (e.g. a small "hot spot" caused by carbon buildup in the head, etc) will predetonate on regular fuel and run better with premium.

Ever notice that, no matter how high the price of gas goes, that the difference in gas price between grades stays pretty consistent--about a dime per gallon? Seems strange to me.

[edit to add: I crossposted with NW-Bound. Yes, most cars today are smart enough to change the ignition timing in response to fuel of insufficient fuel grade. But, if the car only "needs" regular gas, no benefit will occur from using higher octane fuel.]
The bold item above is the "rare" circumstance I mentioned before.

If a car is designed to run on regular fuel, it will NOT advance timing when you put premium fuel in it.

If a car is designed to run on premium fuel, most of them WILL retard timing when you put regular fuel in it.

This is why I say putting high-octane fuel in a car designed for regular will not increase fuel mileage. There are some of you out there that don't believe me and claim you've done tests yourself...then fine...keep wasting your money. For those of you that did tests, did you "control" for all other variables? Was the wind blowing the same way? Was the temperature the same? Was the humidity the same? Did you have a full tank of gas both times you calculated? Was your wife with you once and not the other time? Were you driving from a city that was at an elevation of 1,000 feet above sea level to one that's 1,300 feet above sea level when you tested one way, and then driving "downhill" when you tested the other way?

There is no way you can do an accurate test in your own car. But like I said, there are some out there that just want to believe it...and if you're one of those people...just go on believing.
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Old 10-10-2012, 03:04 PM   #218
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New cars are computer controlled and can't really be changed. You can buy after market ECUs that replace the OEM one which have different timing characteristics.

The only thing higher octane does is help control knock in certain engines.

The Low-Down on High Octane Gasoline
Technically that's not true...but you are thinking the right way. You CAN change new cars...I have re-set the entire timing map in my 2007 Mustang. And yes, you can even do the same thing on a 2012 Mustang. It's not easy, it requires some special tools and VERY specialized knowledge, but it can be done.
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Old 10-10-2012, 03:07 PM   #219
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Just guessing, but I'm not sure they had those knock sensors and automatic timing adjustments back in 1981. Maybe though. Maybe a real car guy/gal can fill us in.

At any rate, I would find a 10% MPG difference hard to explain with premium versus regular. The timing adjustment would only come into play during hard acceleration or lugging. Hard to imagine a 10% delta, unless you were spending a considerable percent of driving under load.

RE: Price delta of regular/premium. IIRC, the additives are a fixed cost, pretty much unrelated to the other costs in gasoline. So it actually makes sense for the delta to remain an absolute, rather than a %. Which means as the cost of gas goes up, premium could be a better deal relative to any MPG increase you might see.

-ERD50
Not sure about 1981...but things were pretty crude back then. They did have computers, but they weren't very smart. Chrysler was the first to try with their "lean burn" computer...the dummies mounted it in the engine compartment right on the air filter housing...the item that vibrates more than anything else in the entire car.
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Old 10-10-2012, 03:08 PM   #220
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As long as your car is ok on ethanol, go for the cheaper price. the higher grade won't hurt anything but no benefits
There is much disagreement on this...and I don't have a strong opinion either way. Some people say the ethanol has a corrosive effect to it and can damage fuel system components over many years. I've not seen convincing evidence either way.
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