Lower octane gains MPG'S?

I always understood lower octane was more volatile than higher octane.

It is a misconception that higher octane has more power than lower octane. In reality, higher octane requires more energy to ignite it. Similar to diesel fuel, but not that extreme.

Some racing engines use octane up to 100+ because their compression is a lot higher. The higher pressure causes more heat, which can cause pre-detonation (pinging), if the octane is too low.
Higher octane requires more heat to ignite.

 

There is no advantage to using an octane higher than that for which your engine is designed. As the previous poster pointed out, it doesn't ignite as easily as a lower octane, so there may be negative effects.

Tom

 

energy content

Testing in Huntsville revealed that three brands, Shell, Exxon and Chevron, have more energy in their 87 octane gas than other brands. Shell 93, 89 and Texaco 93 are low energy content fuels.

GOOD LUCK!
Bob Wilson

 

An Octane question

Here's my question: Will burning one tankful of high-octane fuel damage a new Prius?

I'm currently on the third tank of gas in my new car; when the gauge got down to one pip, I pulled into an unfamiliar station, thinking it would be the usual self-service place. Instead, an attendant ran out and asked in broken English if I wanted a fill-up. I said yes, adding: "regular please."

When he handed me my card receipt, the attendant mumbled "I'm sorry, sir." I didn't realize what he meant until I looked at the receipt: I had been charged $3.29/gallon (for high-test), instead of the $3.09-per-gallon regular that I thought I was buying.

Anyway, my engine has just reached the break-in point, and I've been happily watching my mileage climb from 39.4 mpg for the first tank to 45 or so for the second. Now, on my third tank, I'm getting 49.6 to 48.4.

Is it the octane, the engine getting broken in or my greater experience driving a Prius that's responsible for the great mileage? And, once again, will I damage this wonderful car by burning high-octane for one tankful?

Thanks,

--Joe

 

High-octane gas won't hurt anything except your fuel economy. Worst case scenario is that it takes a tank or two of regular gas after this one for the fuel maps to optimize back to 87 octane. Mechanically the car can handle it as well as any other pump gas.

Too bad the town of Bolton doesn't allow self-service gas pumps. Stupid ordinance, predictable results. (I'm a Bolton expatriate.)

 

Don't blame Bolton for this one

Hey Brick:

Thanks for the encouraging words about my not damaging my new car.

How surprising to find an ex-Boltonite in South Carolina (are you a refugee from the collapse of DEC and the "Massachusetts Miracle"?).

Actually, the station in question was on Morrissey Blvd. in Dorchester. Most stations in that part of Boston are self-serve. It was just my bad luck to hit this one--never again!

Anyway, thanks.

--Joe

 

Hi edthefox5,

In many places the octane additive is Ethanol. Ethanol has a much higher octane than regular gas, but also much less fuel energy. This explains the difference you see.

If an engine was designed to accomadate the increase in octane, its possible to recover more energy from the fuel. With a 100 % ethanol fuel, and car designed to take full advantage (increased compression ratio or higher turbo waste gate setting) the mileage decrease is reported to be less than that which would make the ethanol cost uneconomic.

Unfortunately, no cars marketed in the US right now have this capability. The next gen Prius prototypes are reported to have a turbo, which could be controlled to accomadate fuel Octane.

On a side note, gas dropped 10 cents a gallon here in Chicagoland, in a week. And Diesel went up 50 cents a gallon. I presume that means we are now running on winter blended fuels. The gas is probably cheaper as it now has 10 % ethanol. And my mileage tends to confirm that, although the multiple 20 degree F startups, and the one 10 degree F startup probably has something to do with it, besides the tire pumping snow out the way on the part of one trip.

 

If the buck or two is worth it to you, you might want to consider disputing the difference with your credit card. If nothing else, the nasty letter that the gas station will get from the credit card might get them to be a little more careful next time.

I was always under the impression that higher octane gas takes longer to burn, and is therefore less explosive than lower octane gas. Again, this is necessary in a high performance engine bu it does rob power.

 

87 Octane

I also think that one may get more mpg in using 87 octane. With the Prius engine, it is not highly recommended to put 91 octane and with fuel economy in mind. I am on my 4th tank and had always used 87/Shell. So far so good and averaging 46 mpg - mostly interstate ride.

 

The only reason I could see a Prius needing 91/93 octane is if it is a VERY high milage engine with lots of carbon buildup on the pistons which would could cause preignition.

 

octane and mileage not related

My understanding is: The octane rating of a fuel has nothign to do with the energy density of that fuel. Octane relates to the fuels resistance to ignition. The higher the octane, the higher the resistance. This is why higher octane fuels resist preignition (burning before it's supposed to) and detonation (burning too fast and uncontrollably). Preignition can raise engine temperatures and reduces power output and efficiency. Detonation often results in pinging and knocking noises, which are actually caused by the shock wave from the sudden explosion hitting the piston and head. Detonation damage to an engine is caused by too many repeated sudden explosions, which can actualy lead to holes in the pistons if allowed to continue. Most modern cars have ping and knock sensors that the computer uses to detect detonation, and the computer then changes things such as engine timing to reduce the detonation before real damage occurrs. If you have an engine that is finiky about octane then higher octane fuels may allow the computer to run more aggressive settings which will allow your motor to produce more power. So in this case the additional power doesn't come from a more energy dense fuel, but from the fact that its resistance to preignition and detonation allows the computer to run more aggressive engine settings.

So in summary, depending on the chemical makeup of the fuel, any given fuel may have a high or low energy deisnty regardless of its octane. However depending on the engine, higher octane fuels may allow the computer to run more aggressive and more efficient settings giving you more performance. The prius appears to be designed for lower octane fuels, so it's doubtful that higher octane alone would provide any benefit at all.