Premium gas or octane gasoline?


Premium gas better for cold weather start?
Will a higher octane gasoline help cold weather start


If your vehicle does not require premium then using it will only cost you more money.


I think the opposite is true. The octane rating of gasoline is its resistance to ignition. This is why high compression engines require higher octane fuels which don’t ignite as quickly while a lower octane causes preignition which causes knocking. Theoretically, cold weather starting should be easier with non premium gasoline.


It won’t help.

However, what car do you have? What does your owner’s manual say about the required octane?


Absolutely not.


For good cold weather starting, you need fuel that has a low boiling point so it vaporizes easily. This has nothing to do with octane ratings of fuels. Ethanol is a good example, it has a high octane rating, around 100 or so when pure. Go to a drag racing track where some of the dragsters burn alcohol and watch them fire up the engines, a crew member is usually squirting gasoline into the injectors to get the engine to fire. The main reason they sell E-85, 85% ethanol and 15 % gasoline, and not E-100 at gas stations is so cars will start in cold weather.
Octane also has nothing to do with a fuel’s energy content, again let’s look at ethanol, very high octane rating, but only 12,600 BTU’s of heat per pound of fuel compared to 18,500 or so for pure gasoline.
Octane is only a measure of a fuel’s resistance to detonation in high compression engines, it’s a waste of money in low compression engines. It won’t make more power, you won’t get better gas mileage, and it doesn’t make the engine easier to start in cold weather.


@B.L.E nailed it. Years ago higher octane gas did have more energy in it, was made with longer chain hydrocarbons, harder to burn, resisted ignition due to compression. Tetra ethyl lead was added to gasoline for many years as it allowed regular gasoline to resist pre ignition. Since lead was outlawed other chemicals have been used. MTBE (bad stuff) and ethanol have been used. If you could come up with a clever enough delivery system, water could be used to reduce pre ignition. But today gasoline that ranges from regular, mid grade to premium is basically the same stuff with more chemicals added. No benefit to use a higher grade of gasoline than your manual tells you, won’t start better, won’t run better, won’t clean better


Modern vehicles that are properly maintained, and that have a strong-enough battery, do not normally experience starting problems in the winter. Is the OP attempting to compensate for a poorly-maintained engine by using premium gas?

Premium gas will not help a poorly-maintained engine to start, so if the OP is having cold-weather starting problems, then he/she should review his/her maintenance record, and compare it to the vehicle manufacturer’s maintenance schedule. For example–when were the spark plugs last replaced?

If all maintenance is up-to-date, then his/her poorly-starting engine needs the attention of a mechanic, rather than a tankful of premium gas.


How cold is cold for @nhuphuong422? Cold has a different meaning in Fairbanks than it does in Saigon.

One can buy stuff to spray into the throat of the carburetor for quicker starts in really-cold weather (below 0°); it’s mostly ether. Can one do that to a fuel injector? I used to live in places that got that cold. Because I lived way up a dirt road without neighbors (and no electricity to recharge the battery), I’d spray carburetor cleaner into the carb on really-cold mornings to make it more likely it’d start.

Really? I thought the original model of gasoline was that it was octane (8-carbon) and heptane (7-carbon), not longer alkanes.

If longer chains are harder to burn, why has ethanol (2 carbons) such a high octane number?

Because it allowed manufacture of gasoline with a high octane rating without bothering to make it have more octane.

How about a pure octane gasoline? It’d be expensive.

Have you considered nitro?


Nope, that was never the case.


Quoth Wikipedia: ‘Octane rating is measured relative to a mixture of 2,2,4-trimethylpentane (an isomer of octane) and n-heptane.’


That’s the scale that is used with test engines. Gas purchased by the public was never a mix of octane and heptane. It has always been a mix of many hydrocarbons.

A gas you could buy (at Sunoco, back in the day) rated at 100 octane had the same ability to prevent knocking as pure octane. It was NOT made out of pure octane. Likewise, a 90 octane gas acts like a 90%/10% mix of octane and heptane.


Those two chemicals were chosen mostly because they are easily synthesized in pure form in the lab and one has a high resistance to detonation and the other has a very low resistance to detonation. They could have used straight chain (n-octane) for the low resistance to detonation fuel since its octane rating is also around zero but it may have been more expensive to synthesize in pure form.
But octane ( 2,2,4-trimethylpentane is essentially a yardstick against which other fuels are measured.


If your car is hard starting when it’s cold then there’s a problem and it’s not related to the gasoline octane rating.


When I wrote ‘model’ that’s what I meant.

Because that was cheaper than trying to make something ‘pure’. Why has that changed? Diesel is hexadecane (16-carbon) - it must have a really high octane number!

The Imperial on Juan Tabo & Indian School (Albuquerque) sells 104. Never used it.

Yes, lest others lose the ball. The thread has wandered off into the eternal octane controversy.


Not sure I understand what you mean. Nothing was ‘changed’. Gasoline at the pump was never ‘pure’. Ever. Neither is (or ever was) diesel. Both are mixtures of many different hydrocarbons that meet the desired specs.

From wiki:
“Petroleum diesel, also called petrodiesel, or fossil diesel is the most common type of diesel fuel. It is produced from the fractional distillation of crude oil between 200 °C (392 °F) and 350 °C (662 °F) at atmospheric pressure, resulting in a mixture of carbon chains that typically contain between 8 and 21 carbon atoms per molecule.”


+1 to @texases. Gasoline and diesel are the best mix the refiners can produce out of the crude they get. Refineries have to adjust their process depending on the crude they get. Some crude has lighter hc’s some heavier, sulfur varies etc. Most of the time the refineries get the same crude over and over. Nice steady state for their process. Saudi crude may make it to an east coast refinery on a regular basis. Weather, politics or pirates may mess up the supply chain and they may need to get crude from Africa or Mexico. Their process has to shift and while they make gasoline the mix is different from the Saudi crude. It still works in cars, the consumer does not see the difference, but the operators have to be clever and change the blend to end up with something called gasoline (or diesel)
Following is an MSDS for gasoline, the major component is saturated hydrocarbons, then aromatic hydrocarbon and list them as mixture. Same applies for diesel.


If it was a mixture of octane and heptane, it would be absolutely odorless just like methane, propane, and butane.
The only reason you can smell natural gas and propane is because an odor has been artificially added so you can detect leaks.
A lot of the “gasoline smell” is from compounds such as benzine, toluene, etc, which chemists call aromatics, for a good reason.
By the way, pure benzine has an octane rating of 101. Toluene is 114.


@SteveCBT wrote:


Long straight chain alkanes tend to have low octane ratings, n-heptane for example. Highly branched alkanes tend to have high octane ratings, 2,2,4-trimethylpentane for example. Straight chain octane or n-octane has an octane rating of minus 10.
Compact highly branched molecules are tough and not easily fractured by heat. Long spaghetti like molecules are fragile and can easily be fractured by heat resulting in two highly reactive incomplete molecules also known as free radicals.
Also, the shorter molecules have high octane ratings, methane is 120, propane is 105.
Spill some motor oil on a red hot surface and it will usually ignite into flames. In order to ignite methane, the hot surface has to be heated to a bright yellow heat, as anyone who has worked on a furnace using hot surface ignition can attest to. The glow element makes enough light to read by when it’s on.