Premium vs. Regular gas

[elkroeger]

I thought some of you would find this interesting. They kinda gloss over the point I was always told - that higher octane gas is slower burning, and thus prevents knocking, so if your car isn't knocking, then use regular. Personally, I've never once purchased premium gas for any engine I've owned.

 

Hijacking my own thread: I like the quote "It gives you more gas mileage". I don't know how English teachers get up in the morning....

 

[szurszewski]

Hijacking my own thread: I like the quote "It gives you more gas mileage". I don't know how English teachers get up in the morning....

 

 

Drinking is a popular option, but personally I'm still in bed...

[roadscholar]

Hijacking my own thread: I like the quote "It gives you more gas mileage". I don't know how English teachers get up in the morning....

 

 

Drinking is a popular option, but personally I'm still in bed...

 

 

For decades I've been using mid grade in Mercs and Bimmers even though they recommend premium because of the high compression engines, but they also have knock sensors and can retard timing if needed. Unless you're racing or doing track days it's not necessary.

 

 

 

[Joe Frickin' Friday]

They kinda gloss over the point I was always told - that higher octane gas is slower burning, and thus prevents knocking, so if your car isn't knocking, then use regular.

 

A bit of an oversimplification. There are two ways a fuel-air mixture can combust:

 

• Deflagration: This is what happens when you light the mixture at one point, and a flame front propagates through it at subsonic speed. the speed at which the flame front moves through the mixture is limited by heat transfer from the just-burned layer of mixture to the about-to-be-burned layer of mixture. "Laminar flame speed" is the speed as measured in a quiescent mixture, and for hydrocarbon fuels it's surprisingly low, a couple of feet per second (you can see just how slow that is in
  ). Turbulence stretches and distorts the flame front, increasing its speed by a huge margin; turbulence happens to scale nicely with engine speed, this is why you can idle an engine at 1000 RPM while also being able to make torque at 18,000 RPM with just a bit of spark advance. Even with such a very rapid burn, it's still subsonic; you don't get knock this way.

• Detonation: This is what happens when the mixture is so unstable (owing to a combination of chemistry, temperature, and/or pressure) that it can be ignited by a sound wave, which then develops into a supersonic shock wave. Detonation is what bombs do, and it's why they're so destructive: it's not so much that they have a lot of energy, it's that they release all of their energy in an extremely short period of time (the detonation velocity within a block of TNT is somewhere around 4 miles per second). When the fuel-air mixture in your engine is compressed and heated and held under those conditions for any appreciable time, it becomes unstable and susceptible to detonation. The knock you hear is that shock wave bouncing back and forth in the combustion chamber after the detonation happens. On the first impact with the cylinder wall, the shock wave bears extremely high pressures and temperatures and can cause localized surface damage if it's intense enough and/or goes on long enough, especially since it generally happens in the same location over and over again.

So if you're an engine designer, you want good fuel economy. This requires high compression, and the optimum amount of spark advance. For a given total engine displacement, you'd also prefer to have a few large cylinders instead of a bunch of small cylinders, since the former means less total combustion chamber surface area for heat loss. But high compression means very high peak temperatures/pressures at max load, and large cylinders means it can take a long time for the flame front to burn the entire mixture; you may end up detonating the last part of the mixture before it can deflagrate. You can retard the spark to dial back the peak pressures/temperatures, but this will compromise economy and max rated power under those conditions. So instead you specify fuel with a higher octane rating. The chemistry of high-octane fuel is such that it tolerates elevated temperatures/pressures for a little bit longer before transitioning to detonation, giving you just a little bit more time to finish combusting via deflagration; under normal combustion conditions (see “deflagration” above), the flame speed isn't appreciably different from low-octane fuel.

 

I actually told a lie in the previous paragraph. You don't want good fuel economy, you want good cost economy: you don't want high miles per gallon, you want high miles per dollar. Honda could probably make their Fit and Civic get better miles per gallon if they redesigned the engines to make the best use of high-octane fuel, but then owners would be stuck with a higher dollar cost per gallon, and therefore a higher cost per mile; the economics wouldn't work out. So they optimize such engines to get the best fuel economy they can while using low-octane (cheap) fuel.

 

Engines that are tuned to squeeze a bit more performance out of them will include a higher octane recommendation. Generally speaking, you can get away with low-octane fuel for them just for cruising around. Cars hardly require any power to cruise at a steady speed: I once saw a Ford Explorer doing 80 MPH on a chassis dyno simulating normal aerodynamic drag force and rolling resistance, and it only needed 40 horsepower to hold that speed. Knock sensors these days will retard spark timing if they hear knock, so if you're using lower-than-recommended octane, usually the worst that will happen is you don't make full rated torque when you put your foot to the floor.

 

In some cases the car specifies a minimum octane requirement. In these cases, I'd be wary of using something much less than that rating, as it may exceed the system's ability to alleviate knock. No guarantees, but if you're not hearing it knock when you put your foot down, things are probably OK.

 

At high altitudes, the atmosphere is less dense, so peak pressures/temperatures in the combustion chamber will be lower than the same speed/load while operating at sea level. This means the octane requirement is reduced. You don't usually see this mentioned in the owner's manual, but what you do see is that in high-altitude locales, you can't find 93 or even 91 octane fuel. And that's OK, unless you take a tankful of that stuff down to low altitude again. For our honeymoon, my wife and I rented a car in Las Vegas and drove to the Grand Canyon, Bryce Canyon, and Zion before returning to Vegas. It was a Lincoln LS, and ISTR it wanted 91 octane. Well, we filled up the tank at Bryce Canyon (elevation 7600 feet); I don't remember what octane I bought, but I know it was less than 90, possibly as low as 85. And then we headed back to Las Vegas (elevation 2000 feet, temps in the upper 90s). The car was knocking like a mofo even under modest acceleration. I filled it with high-octane stuff somewhere outside of town, diluting the crap in the tank and restoring quiet, non-destructive operation. Whew.

 

[Chris K]

Thank you Professor Mitch for an explanation the even I can understand.

[Sonor]

You Da Man Mitch!! Thank you. That clears up a great deal of misunderstandings that I have had in the past.

[chrisd]

Whew is right. And yes, thanks professor. Another terrific post. I feel smarter now. I just hope the feeling lasts.

[Rougarou]

Big difference between "Required" and "Recommended",.....recommended is "meh, you might want to use this for the best performance but you don't really need to",....required "you better use this 'cause the engine was designed to use it".

[realshelby]

I do think one could, if measured carefully over a few tanks of each, prove an improvement in power and mileage using non-ethanol gasoline over E10. Not much, but I think it is there.

 

There can be a difference in fuels. You could buy Sunoco 260 years ago at the pump. Don't know if it is still around, but Sunoco also had 94 unleaded. While the 260 was leaded and meant for high compression engines of the day, there still is a difference in these fuels beyond just octane. They are formulated to make POWER. Not so much to clean the engine, burn cleaner, or those type of things the fuel we buy today is good at. These type of fuels and the race car fuel I buy today have more ENERGY per pound than pump gas. There is no way on earth to benefit from running them in a daily driver, cost is too high.

 

I pay just under $10 gallon for Sunoco 110 octane leaded. Run it in ALL my weedeaters, trimmers, chain saw, edgers. They never have carburetor issues! But I also put it in my Shelby on occasion. While it will run fine on 93 pump gas, without touching anything the power difference on the race fuel is there.

 

A lot of people thing Diesel engines are very efficient. They are, but the fact is that the fuel they use is the reason they have huge torque numbers and good economy for the work performed. Diesel fuel has a lot more energy in a gallon than gasoline. And its "octane" would be very, very, low!

[Gregori]

My own semi-scientific testing on fuel economy has given different results with different vehicles.

I always note the mileage and fuel at every fill up, and will test different grades of fuel over 3 or more consecutive tankfuls, under comparable driving conditions.

I had a V8 Ford van at one point that got under 10 MPG on regular gas, but right around 15 MPG on super. So it was actually much more economical to run the more expensive fuel. The V6 truck I have now averages about 16 MPG on regular, and almost 19 on super. So at current fuel prices ($2.37/$2.77) the cost per mile difference is pretty negligible. (Having the correct tire pressure makes more difference.) My Porsche gets *slightly* better fuel economy on super - not enough to justify it based solely on cost, but the car does seem to run a little better on high test. (Which is completely without any data, is purely subjective, and likely a figment of my imagination.)

However, In the Subaru, the fuel economy economy didn't change, no matter what I ran, so the cheaper the gas, the better.

And, of course - YMMV.