I'm finding I am getting petter performance with 91 vs 87 but my mileage is about the same. I only have 400 miles on it though so it may change once its broken in

 

yes it does. search for octane and you will find usufull resources.

 

The lower the octane rating or the higher the percentage of additives such as Ethanol, the lower the combustion value of the fuel. The higher and more complete the combustion, the more fuel is burned and more power is developed. The higher the available power, the less fuel you need to feed to the engine to make up for incomplete combustion. So one value hinges upon the other.

However, no octane rating will make as much difference in fuel economy as how you drive, where you drive and what the barometric conditions are when you are driving. Therefore, your day to day driving from point A to point B is always going to be quite different from tankful to tankful and, IMO, would be difficult to tie down specifically to the octane rating of the fuel being consumed. Unless your consistent route is similar to that used to accumulate mpg data, you're more likely to use more fuel now and then and less then and now.

The car's computer will adjust the firing of the cylinders to whatever octane rating the engine senses. In possibly 5k worth of driving with the same brand of fuel, filled at the same seller over more or less identical driving conditions, you might be able to see a slight difference in the fuel economy of a higher octane rating. Chances are, it would still be very difficult to tie that difference only to the octane rating.

Try a few tankfuls when you're going to be doing "average" routes and using consistent driving techniques while using the instant fuel econmy gauge of the 500 to judge if you really notice the difference between octane ratings.

 

The lower the octane rating or the higher the percentage of additives such as Ethanol, the lower the combustion value of the fuel. The higher and more complete the combustion, the more fuel is burned and more power is developed. The higher the available power, the less fuel you need to feed to the engine to make up for incomplete combustion. So one value hinges upon the other.

You mean the Higher the octane, not lower, lower octane has more combustion value. Ethanol boost octane, E85 is around 102 to 105 octane but mpg is lousy.

 

it's usually good practice to use the lowest octane rating that doesn't cause the engine to detonate or the computer to retard the timing. theoretically, that would also give you the best mileage out of that engine. apparently, the magic number for the 500 (non-abarth) is 91...


s3aturnr

 

For economy that's probably true most of the time - not so with our ancient A6 wagon (using 87 made the car so gutless you had to floor it all the time and there went any savings). For my 500, the difference in spunk is quite evident to me and so my car gets 91 (93 not available out my way). I gave what I think were 2 fair attempts at using 87 and the car just was not fun enough for me...

 

If you intend to keep your Fiat 500 for a long time,I would suggst that the most important things that you can do is to use 91 octane gasoline and keep on schedule for oil changes.Other than that,enjoy driving your car.

 

The gas lid: View attachment 4758

Does it really make a difference which octane to what your MPG?

I ran a couple of tankfuls of 87 a while back. I found as did at least one other here that the mileage didn't change much, if at all, but the 'performance' or 'perkiness' of the engine was much more evident when running on 93 octane which is what most stations here in Florida have for Premium. Therefore I decided to switch back to Premium as a matter of course.

 

I switched to 91, and I have noticed a difference in the idle of the engine, and specifically the purr of the engine when downshifting and coasting (in gear, without applying the gas). Seems the engine operates as it should with 91.

 

I've never been around a vehicle which could flex to E-85. I had no idea the octane rating was that high. However, "lower octane has more combustion value" isn't the truly correct answer either. It's the ehtanol which is degrading the fuel's ability to burn as, sad to say, petroleum based fuels still have the highest energy bang for the amount sent into the combustion chamber. It's rather well known that ethanol actually decreases power and mileage. But that octane rating seems preposterous for something that doesn't burn well!

Allow me to amend my earlier statement to read, "The lower the octane rating of the gasoline without additives such as Ethanol, the lower the combustion value of the fuel."

With ethanol, this seems to be a different ballgame.

 

higher octane fuels burn slower and have more additives/agents
lower octane fuels burn quicker and have less additives/agents

 

As i am reading through this thread, another question arise...

Is it better of using the same brand all the time or it makes no difference at all?

 

1) Octane has NOTHING to do with "combustion value."

2) Octane has NOTHING to do with burn rate.

3) Octane has NOTHING to do with amount of additives or agents. Although most brands names DO put in a better quality additive package in their premium fuels. But this is NOT required.

Octane ONLY has to do with the fuels ability to resist detonation. PERIOD.

BTW, in the US we buy fuel by AKI (Anti Knock Index) not by octane, at least since the early 70s. AKI is the average between the RON (Research Octane Number) and the MON (Motor Octane Number). The two are measured using different parameters of things like engine temp, intake air temp, oil temp, etc. MON is a more stringent test and gives lower ratings. There is NO specific relationship between RON and MON. However, in Europe (which puts RON on the pumps), they found that high performance engines run on longer high speed runs had engine failures, even though the RON was proper. But it was found that the fuels that caused this problem had a low MON. So the US AKI rating helps prevent this.

Ethanol does raise the AKI of the fuel, but its main purpose (other than a disguised farm subsidy) is as an oxygenate. MTBE was removed due to concerns over leaking tanks contaminating ground water. So ethanol was proposed as a substitute. The use of oxygenates is to reduce air pollution. It used to be that MTBE was only added to gasoline in the winter. But later, they just added it all the time. Now, ethanol is mandated in most areas for all gasoline.

What you seem to be calling "combustion value" is more typically energy content per volume. In the US BTU per gallon, in metric countries, MJ per liter. There is no specific relationship between AKI and energy of a fuel. Although many higher AKI fuels are lower energy content per volume (alcohols especially).

Some energy content of common fuels (MJ per liter):

Regular gas 47
Premium gas 46
Ethanol 31
Methanol 20
Diesel 48
Vegetable oil 38
Biodiesel 40
Aviation Gasoline 47

As for gas mileage and AKI, everything I have seen, shows that most modern cars get better gas mileage with higher AKI fuel. Typically enough to off set the increased price. But YMMV. So test in your car, and your driving to see. You will need to keep accurate records, and do it over several tanks to average out differences in driving for various tanks of fuel.

As for additives, all the top brand gasolines have additives to clean injectors and reduce intake valve and combustion chamber deposits. According to several petroleum engineers, they all also leave some deposits of their own. So they recommend switching gasoline brands, not using just one. Being engineers, they do things like switch brands every 2000 or 5000 miles (the time it supposedly take for a given gasoline to remove the existing deposits and start leaving their own). I just switch depending on what is convenient and inexpensive. I DO run high quality name brand gasoline in my good vehicles. Not the convenience store brand or off brand station gas. Many car companies recommend or require Top Tier gasolines.

Info on Top Tier brands, see here:

http://www.toptiergas.com/

Top Tiers brand list:

http://www.toptiergas.com/retailers.html

 

A couple of other things, but the Edit function timed out.

There is no way to know if your car is reducing power due to low AKI fuel (other than if it feels really slow), so you have to rely on other information. But realize, the only time the car should be reducing power (ignition timing or other methods), should be at pretty much full throttle. The old idea of run the lowest AKI (or in those days octane) fuel that does not knock is no longer true due to electronic engine management. And some high performance cars can benefit from even higher AKI, above normal pump gas. My E46 M3 gains power up to about 96 AKI, which is 50/50 mix of pump 93 AKI and unleaded race gas at 98 AKI (Sunoco GT100). But if you think your gas prices are high, try unleaded race gas at the track. IIRC, the last time I bought some, it was $8 per gallon. And on the track I get about 4 MPG.



There may be other issues with engines and ethanol. I race SCCA in Spec Racer Ford. Our engines are a stock, sealed, Ford Escort 1.9L (early 90s engine). With the ethanol based fuels, we started seeing broken pistons, not typical detonation, but broken top ring lands. Going from our normal 87 AKI to 93 AKI did not stop this from occuring. But going to alcohol free 93 AKI stopped this from occuring. I have not heard of any particular issues with street cars, but makes me wonder what might happen with E15.

 

Jebus. Thank god somebody who knows what they are talking about came along. I was laughing my *** off at this thread. Very good posts. I can see why people think octane "slows" the burn rate even though that's not technically correct.

I want to add for those of you wanted to examine your mileage, you must run through a FULL TANK of ONE TYPE OF FUEL before doing each calculation. You are not going to get accurate results by driving 150 miles with 87 in the car, then filling the tank with 91 and driving another 200 miles. Run from full to empty with one grade of fuel.

 

i am very thorough about how i drive, where i drive, and my mpg's. i am still averaging right at 45 mpg's for the life of my car. ive tried several fillups with 91/93 octane but cannot find any measurable increase in mpg's over the same routes and driving style vs the 87 octane.

 

45 MPG is fantastic! You should be happy about that!

What's your city/highway ratio?

 

You need to do at least 2 - 3 tanks in a row. The first full fill up and run tank down to get rid of the majority of the other fuel. And for the computer to learn the parameters with the new fuel. Then a tank or 2 to actually check mileage.

And as I said, YMMV, so you have to test YOUR car and YOUR driving to see how it works.

 

How accurate do you think the AMPG or IMPG readings on the dash cluster are?

 

Typically not that accurate.

Log the actual miles and gallons between fill ups.

There are some apps for smart phones that you can use to gather the calculate the data. Or online Fuelly (can access from smart phone).

 

My EVIC readout overestimates my average mileage by about 1-2 mpg. It's close enough that I like to keep it on to keep an eye on how my mileage is doing, but I still crunch the numbers every time I fill up to get a more accurate reading.

 

Not really. There is some variation in the fuel density in the tank, but the fuel coming from the pump is pretty much at the same temp, as the tanks are underground.

 

Hi,

I believe the temperature of the fuel in the storage tank at a gas station may only addresses part of the issue. Specifically, the ambient temperature of a car's fuel storage system (including the fuel tank and potentially the filler hose and other piping) will also impact how much gas that the car's fuel 'tank' will hold. The differences between the ambient temperature for a car's fuel tank and piping for a car on a hot 100+ deg day, where the car has been sitting for many hours in direct sunlight will likely be a fair bit different than for a car early in the morning, which may have sat all night in the dark.

As such, I believe that there will likely be differences in the amount of fuel that you can pump into the car' in these two situations, related not only to the thermal expansion of the tank and piping but also the heating of the fuel due to the temperature differences between the temperature that it has been stored at underground and the current ambient temperature in the car's gas 'tank'.

 

Oh! This week, following ~ most of the time ~ the eD recommended shift schedule and burning 93, my mpg averages per day were: 44.8, 45.4, 46.6, 45.9, and 42.1. I live downtown a work 16 miles from home.

I'm still not getting OST numbers, but I'm very happy with the results.

 

Funny you should mention me. Since I have had the car (Apr 11 2012) I have been running 89 and I just checked while in Kelowna BC and its at 44.9 mpg or 5.2L/100kilometers city and highway. On our vacation and this trip which has been at least 80% mountains (7 & 8% grades) my mileage is a 5.1L/100 kilometers or 45.1mpg. On the trip home I will see a final number of 4.9L/100 kilometers (2400K's/1400 miles) and for strictly mountain driving I think it's outstanding.

I will once again see low 4's/100 kilometers when I am travelling north/south/east of Calgary as I have seen those numbers already. We leave for the flatlands Sunday am thank God. Gas here is approx 1.34/L for the 89=OUCH lol OH roughly 5.40 a gallon.^^^^^^

 

If you put 8 gallons of cooler fuel into the tank, the fuel that was in the tank and the tank itself will be cool.

In the old days, if you topped up the car to the very top, and waited, the fuel would start to heat and spill over.

If you are "filling" with only a few gallons, that might be a problem, but if you run down to 1/4 tank remaining, you will not see much difference from this.

And how often do the air temp go from 100 - 30 in a week? So if you are doing comparisons tank to tank, the ambient temps will be in the same range.

 

If you put 8 gallons of cooler fuel into the tank, the fuel that was in the tank and the tank itself will be cool.

And the cooler fuel will warm (and expand) as part of the heat exchange process.

In the old days, if you topped up the car to the very top, and waited, the fuel would start to heat and spill over.

If you are "filling" with only a few gallons, that might be a problem, but if you run down to 1/4 tank remaining, you will not see much difference from this.

It will be a difference, however.

And how often do the air temp go from 100 - 30 in a week? So if you are doing comparisons tank to tank, the ambient temps will be in the same range.

I believe that I had suggested the temperatures going from maybe 75-80 in the morning to over 100 late in the day, for a 20-25 deg or so swing, and not a 70 deg swing.

 

Hi,

I think we might be talking about different situations here. If, as you appear to indicate in your last post, you tell a pump to give you X gallons, then it will give you X gallons +/- a very, very small amount (due probably more to the inherent accuracy limits of the pump) since the temperature of the underground storage may not be changing all that much.

However, I believe that if you fill up your tank one day, and then drive the car normally for Y miles, and then measure the amount of fuel you put back into the tank at your next fill up, that amount of fuel that you just put into the tank may not be the same as the amount of fuel that you burned driving those Y miles since the ambient temperature of the car, etc will have a bit of an impact on how much fuel you can put into the tank. As such the time of day that you 1st filled your car and the time of day that you are refilling the car my likely play a role in trying to calculate your mpg in this way.

 

If you put in 10 gallons of fuel into a 10 gallon tank and run the car. The car actually burns fuel by the pound. So after X miles you will burn Y pounds. When you fill the car again, you will fill fuel at the same density each time you fill, so if you burned the tank down to 1 ounce, you would fill with 10 gallons - 1 ounce.

And BTW, gas pumps are accurate to around 1 cubic inch (0.55 ounces) per 5 gallons. I watched the local weights and measures people test some pumps one day. They fill a calibrated 5 gallon container to a line, and check the pump reading. More than 2.5 cubic inch difference (about 1.5 ounces) the pump will be rejected.

If you burned say about 1 gallon of fuel, the fuel in the tank would be a different density and you might see the change you are mentioning. But if you run down to about 1/4 tank every fill up, it will be a small difference.

For every five-degree (Celsius) change in temperature, figure about a 0.5-percent change in density.

So if one day you filled from 300 miles of driving (40 MPG) at 0C, then did it another day at 40C, the density change is about 4%. Or the about 2.5 gallons of fuel would grow by 12.8 ounces, or 1/8 of a gallon. But seldom does a car go from 0C to 40C in a single tank.

But then again, in that same time frame, the tires have worn, so the rolling radius is less, so the miles you have driven are not accurately depicted by the odometer. And this assumes you actually check and set your tire pressures on a regular basis.

 

If you put in 10 gallons of fuel into a 10 gallon tank and run the car. The car actually burns fuel by the pound. So after X miles you will burn Y pounds. When you fill the car again, you will fill fuel at the same density each time you fill, so if you burned the tank down to 1 ounce, you would fill with 10 gallons - 1 ounce.

And BTW, gas pumps are accurate to around 1 cubic inch (0.55 ounces) per 5 gallons. I watched the local weights and measures people test some pumps one day. They fill a calibrated 5 gallon container to a line, and check the pump reading. More than 2.5 cubic inch difference (about 1.5 ounces) the pump will be rejected.

If you burned say about 1 gallon of fuel, the fuel in the tank would be a different density and you might see the change you are mentioning. But if you run down to about 1/4 tank every fill up, it will be a small difference.



So if one day you filled from 300 miles of driving (40 MPG) at 0C, then did it another day at 40C, the density change is about 4%. Or the about 2.5 gallons of fuel would grow by 12.8 ounces, or 1/8 of a gallon. But seldom does a car go from 0C to 40C in a single tank.

But then again, in that same time frame, the tires have worn, so the rolling radius is less, so the miles you have driven are not accurately depicted by the odometer. And this assumes you actually check and set your tire pressures on a regular basis.

I think the question here is what is limiting the amount of fuel that you are putting in. Specifically, if someone were to either fill their tank until it cuts off, run the car, and then later refill the tank until the pump cuts off again, I do not believe that you are necessarily putting the same amount into the car as you burned off if the ambient temperature during each fueling is different, because that ambient temperature will impact many things including the warming of the fuel as it goes from the cooler storage tank to the warm car gas tank, as well as the impacts that a higher ambient temperature will have on the cars gas tank (and piping) as well.

As such, trying to calculate your mpg by dividing the miles that you have driven since your last refueling by the amount of fuel that you just put into your tank will likely include some discrepancies due to potential differences in the ambinet temperature of the car at the time of each fueling.

Or in other words, if I were to take a car that had been sitting all day in relatively hot (100+ deg weather) and fill its tank, I would probably wouldn't come up with the same amount of fuel put into the same car if I had filled its tank earlier in the morning, after it had been sitting in relatively cool air all night.

PS. I was under the impression that fuel pumps are typically positive displacement pumps (which pump a specific volume per unit time rather than a specific weight per unit time). Also, it's my understanding that the accuracy of a gas station pump is required to be within 0.3% per NIST Handbook 44 Section 3.30. For a nominal output of 5 gals this would equate to 5 gal * 128 oz/gal * 0.003 = 1.92 oz.

 

This thread has been very informative. I personally have been running 93 pure gas no ethanol in my abarth. Except when I left the state to attend the FFO, mileage suffered but still had the pep for the mountain roads.

 

This thread has been very informative.

I wish I could say the same, but most of the recent posts have just gone right over my head. The gas stations where I live offer 87, 89, and 93 octane - I started off with a few tanks of 87, then switched to 93 to see if I could note any difference. I don't see any appreciable change in mileage, but I do think the car performs better - it's smoother in acceleration and shifting, for sure - so I'd like to keep using it. I assume that using a higher octane than is recommended (since the Shell stations around here don't offer 91) won't hurt the car?

 

Speaking of which, any input of those octane booster you put in every tank of gas?