Does Octane affect mileage?

In some other threads other posters have reported lower mileage with higher octane gas. The Owner's Manual specifies 87 for the Prius. I've never seen anything lower than 87 in VA, PA, MD, DC, WV, NC, or TN, and have no idea what the impact of running 85 would be.

 

It is safe to use 85 octane at high altitudes, but less safe to fill up there and drive to low altitudes. Fill up with 87 before going home.

Octane rating - Wikipedia, the free encyclopedia


In the 2004-09 Prius, no benefit was ever shown for high octane gas.

 

I have never been sure that applies to modern fuel injected engines. There is a mass airflow sensor that measures the mass of the air entering the cylinder and a certain amount of fuel is injected accordingly. So maybe the altitude dosen't matter because there will be the correct amount of fuel for whatever mass is measured independent of altitude. To complicate things even more the Prius uses an Atkinson cycle which pressurizes the air in the intake manifold.

I noticed the Wikipedia article was careful to say "It is safe to fill up a car with a carburetor that normally takes 87 AKI fuel at sea level with 85 AKI fuel in the mountains, . . . ." but they did not talk about fuel injection or MAF sensors.

I drove my Gen2 in Nevada and Utah several times and decided to use 87 because I just wasn't sure and a few extra cents a gallon didn't matter anyway.

I'll be back in the mountains this fall with my Gen3 (if it gets here) and I'll have my scan gage maybe the timing would indicate something.

 

Toyota uses a very restrictive paper Air Filter that K&N needs to address. I have a note into the good folks at K&N asking if the Corolla Air Filter #33-2360 will work in the 2010 Prius. They both share the same Atkinson Cycle engine. That would make a difference in our gas mileage. Of all places to make use of a cotton filter and Toyota didn't.

 

Flex fuel vehicles. and it is cheaper, but friends I have talked to say they end up getting less mpg with the cheap stuff.

 

Flex Fuel vehicles run on E85 or a blend of 15% gasoline and 85% gasoline. This thread is discussing gasoline with a octane rating of 85. These are two very different fuels.

To the OP,

From the Wikipedia article linked above:
"A common misconception is that power output or fuel mileage can be improved by burning higher octane fuel than a particular engine was designed for. The power output of an engine depends in part on the energy density of its fuel, but similar fuels with different octane ratings have similar density. Since switching to a higher octane fuel does not add any more hydrocarbon content or oxygen, the engine cannot produce more power."

 

According to a relative who used to blend gasoline at a refinery, all else being equal, the lower octane should have a slightly higher energy content and therefore fuel mileage.

But the consumer is not going to know the blending details of the particular batches being sold at any particular station, to know whether or not all else is equal. And that information came before the widespread ethanol era. Any differences in ethanol content are likely to overwhelm the energy difference she was referring to.

I see it in Rocky Mountain states, in areas that are not within fuel range (for old gas guzzlers) of low elevations. And I happily bought it for my fuel injected pre-hybrids. They always produced better gas mileage there, though that was more due to reduced air drag in the thinner air, and reduced ICE pumping loss, than to the fuel. My hybrid hasn't been there yet.

 

Octane is strictly gasolines ability to prevent knock.
There is a research octane and a motor octane. What's posted on the pump is the two added together and divided by 2.

 

There are only a few refiners in the US. A Shell truck pulls up and they take on a truck full sprinkle some Shell additives and off he goes. The next Mobil truck fills up and they sprinkle some Mobil additives in and off he goes. Next in line is Phillips 76 truck and so on and Scooby doobe do.

We get less mileage because 10% of our fuel comes from corn.

 

You may want to recount as the list of US refineries is 45 pages long. I've seen with my 'lying eyes' multiple refineries:

• Cushing, OK, 2006
• Bartlesville, OK, 2006
• Coffeyville, KS, 2006
• Houston, TX - at least two in town, and two on bay, 2004
• Lake Charles, LA - at least one, 2004
• Delaware, New Jersey, Wyoming, California, Chicago, 1998-2009

This is by no means an extensive list but this nonsense about not enough refineries is inaccurate. The same people making such claims about refinery capacity have other problems with accuracy.

As for the quality of gasoline, I did the experiment:


Folks are welcome to their opinion but the facts and data are different. As recently as June 2009, I had occasion to run an informal test between Shell 87 and 89, E10:

You'll notice it takes more 89 octane Shell to crest the hill than 87 octane Shell.

Bob Wilson

 

This is correct, octane is just a knock preventative. Knock is when the fuel in the cylinder ignites on it's own from the local heat and pressure instead of waiting for the actual spark from the spark plug. This is really bad for engines and performance, so we add octane to make gas less combustible. More octane = gas that can withstand more heat and pressure before spontaneously combusting.

Anyway, for years I avoided the higher price on the higher octane gas. Then I found the joy of turbos, and about variable valve timing.

When a variable valve timing engine senses that it isn't knocking it can advance the ignition timing to take advantage, essentially giving more time for fuel to mix in the cylinder and allowing greater compression. But... this generates more heat. With high octane gas the engine can advance the timing even more efficiently without engine knock, thus getting better mileage.

I live at 10,200 feet and I saw my previous truck improve drastically after I started putting higher octane gas in it. Not enough mileage on the Prius yet to be able to tell. I can't remember the last time I was at an altitude where they sold 91 octane.

 

Hmm... Now that's real data as compared to my "hmm mileage is better on better gas" I wonder if the timing was a bit off on my old engine then?


"Folks are welcome to their opinion but the facts and data are different. As recently as June 2009, I had occasion to run an informal test between Shell 87 and 89, E10:"

 

You live in LEADVILLE CO???

I have long speculated about doing a marathon, tank drive in Leadville to the extent of looking for relatively flat sections for an endurance drive. But in the short term, do you have a section of road, relatively flat, about 10 miles in length that you might do some MPG vs MPH tests?

It looks like 24 south, just after the left turn into the valley and after the intersection with 300 parallels a railroad track. That stretch looks to be a great place for pairs of runs at fixed speeds each way.

Perhaps there is a 6% grade hill you can do some climb tests? Mine have been simple 525 ft. climbs but we can scale just about any size hill. The key is knowing the start and stop places so we can use Google Earth to calculate the grade.

What I'd like to do is compare your MPG vs MPH data points against what I can get at Huntsville altitudes of ~700 ft.:



What we're trying to find out is the relative efficiency of the ZVW30 at ~9-10,000 ft compared to lower altitude performance. It would also be helpful if different grades of fuel could be tested, especially Shell 85 octane versus other Shell grades. Send me a PM and I'll be happy to cover a couple of tanks of gas ... the tests do not use a whole tank but I need to understand the fuel quality effects and Shell may be more expensive than other brands.

Google maps and Google earth have been useful and I've had a chance to survey parts of Leadville. I've spotted a couple of likely routes. Understand, I'm not thrilled about spending a week's vacation on a marathon drive. But if we can get some MPG vs MPH data, I can quickly tell if it makes sense.

Thanks,
Bob Wilson

 

Yes octane affects mileage. If your engine either needs or can take advantage of higher octane fuel "knock sensor equipped and or high compression" you will get better mileage with higher octane fuel. Example Corvette and other high performance vehicles.
If your engine is not designed to use high octane fuel, then using high octane fuel will most probably result in lower mileage.
Why? because the higher the octane, the harder it is to ignite the fuel and the slower the flame front.
Your vehicle will operate best on the fuel it was designed to use. For the Prius, I believe that to be 87 octane. I know the manual say's 88, but that is a mis-print

 

We collaborated and agreed to bring this posting back to the forum in part as a tutorial to share:

The goal is reproducible data so for MPH vs MPG by hopefully using cruise control to maintain the speed. Here are the tradeoffs:

• too short - can lead to sampling errors because the MPG display does not have enough time to average out the reset and average out any errors
• too hilly - can lead to excessive excursions deep into "the red bar" area or battery charging regeneration. The mitigation is to see if a slower speed, to avoid "red bar" or faster to avoid "regeneration." One can also try to use the accelerator to 'hold a speed' with a little slop but this becomes difficult (aka., impossible) to replicate.
• ideal - the power fluctuates within the ECO region and the "ECO" text stays lit

Always record the temperature. We can factor out the temperature effects but only if we have them.

Warm up the car as much as feasible. Earlier tests with the NHW11 Prius indicates a 30 minutes is perfect, 20 minutes is OK but anything less than 15 minutes was not useful. It isn't just the engine but the transaxle and tires that need to warm up.

Because of the high power levels, a steady grade is best to avoid the non-linear effects of engine power settings. You want to have a run-up area that you can set the speed with cruise control and then:

1. START - at a point well identified, say a sign, on the slope
2. TRAFFIC - most of discarded data comes from times when I have to drop out of cruise control due to traffic
3. STOP - at a point just at the top where the slope stops, a mile is good but shorter is OK if reproducible

Accuracy is more important than speed so don't worry about practice sessions or a traffic congestion. Use any aborted runs as 'lessons learned' and pickup the data on another day.

With the baseline, you can test the branded gas, Shell, versus others. One of the advantages of the Prius is the lower fuel consumption allows use to a higher priced but quality fuel without a big hit in the pocket book. Then use your baseline data to evaluate the relative effects of different brands and grades of fuel.

With my NHW11, I have a Graham miniscanner that can record the data via an RS-232 interface to a laptop. I can also use my Gramin nuvi or a USB connected GPS mouse. A laptop with a microphone input can also be used to sample at 40k Hz. These techniques are used for faster polling but then you have this mass of data to reduce. But don't get bent out of shape over the tools.

Issac Newton had pen, paper, protractor and mechanical clocks. Most of my ZVW20 data has been recorded using pen and paper and the instrumentation in the car. I've also used a Garmin nuvi to document speeds and altitude changes.

Don't neglect use of a video recorder. Just set it up, start recording and then drive. Later, you can analyze the video often down to frame by frame.

Sorry:

ZVW30 - the 2010 Prius model
NHW20 - the 2004-09 Prius
NHW11 - the 2001-03 Prius
NHW10 - the 1997-2000 Prius sold only in Japan but some were exported

marathon tank - driving as far as possible on one tank of gas for example to join the 1,000 mile tank club. Our slightly over 12 gallon tank at 20 mph should provide at least 1,200 miles and take about 60 driving hours.
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It is a great tool. No need to reinvent what is readily available.

Bob Wilson

 

Thanks for the information, I'll be back with data after I get some time for the collection.