Is higher octane gasoline more or less fuel efficient?

This article was on the front page at Yahoo today. The article advocates a lower octane for better fuel efficiency (cost while using the car, either miles or idling.) However, one of the first comments below the article says that raising the octane (through fuel additives) will actually reduce cost.

However, this article states that they are essentially the same. “Octane” is just the threshhold at which fuel combusts through compression rather than the spark from the spark plug. Lower octane means less resistance to pressure and a greater likelihood of combustion by pressure.

My friend believes the “higher octane, better mileage” theory. However, in the 80’s, I did a series of tests using both 87 and 92 octane, and in every single test, the lower octane resulted in a greater number of miles traveled per tank.

So which is it? Lower mileage, higher mileage, or equal mileage?

If it’s more resistant to detonation, it can be compressed more and yield more power.

A high-performance engine designed to actually take advantage of 92 Octane will yield more power than one that is optimized for 87 Octane. If your engine is best on 92, it might run on 87 just fine, while yielding slightly less power (and mileage).

In practical application, the higher octane fuel yields more power. The question for efficiency is whether you care whether your car makes 298 HP or 290, and whether the power and potential gains are worth the investment. If pre-detonation (knock) occurs, you have to calculate the cost of going down in octane, if you care about engine longevity/damage (damage which might yield lower mileage).

High octane fuel should be more efficient, as it can be compressed more without igniting. This leads to more complete combustion. I believe the energy density of high octane fuel is also higher. However, you will only see a benefit if your engine is designed to run with it.

Was that a carburettered engine though? Most in the 80s were. In that case it wouldn’t have made proper use of the higher octane fuel. Modern fuel injected engines have an oxygen sensor, which enables them to run reliably at close to the point of knocking (fuel igniting through compression). This increases efficiency, as the fuel is mixed more throughly with the air.

Octane says absolutely nothing about energy content. The octane rating only tells you how compressible the gas is before it spontaneously combusts.

Engines are designed for a specific octane rating. If you use too low of an octane rating for your engine, the gasoline may combust before the spark ignites it. This is very much not good for your engine, and you can actually ruin your engine because of it. Most cars these days have a knock sensor which can detect when this is happening and it will adjust the timing of your engine to prevent damage, though your gas mileage will go to hell in a handbasket.

Generally speaking, if you use a higher octane than what your engine specifies all you are doing is wasting money. You are paying for gasoline that won’t spontaneously combust at pressures far higher than what are produced inside your engine.

In terms of actual energy content, higher octane gas can often surprisingly be lower in energy content. Want a higher octane gas? Add alcohol to it. Alcohol has only about 60 percent of the energy of gasoline, but it has a much higher octane rating.

Engines vary. The engine in my Cadillac can tolerate gasoline that is 89 octane or higher. Put in 87 and it will run very poorly. Put in 89 and it is happy and runs well. Put in 91 or 93 and it gets slightly better mileage, but not enough to justify the extra cost. Our Toyota Camry though doesn’t behave the same way. Put in 87 and it runs like crap (that much is the same - the car is designed for 89 or better). Put in 89 and you get good mileage. Put in 91 or 93 though and the mileage actually starts to drop. It doesn’t go down as much as if you put 87 in, but it definitely goes down instead of up.

The oil companies don’t help matters either. They advertise higher octane as “supreme” as if it is somehow better gasoline. It isn’t. It may be better or it may be worse, and the octane rating alone won’t tell you which it is. Often, the higher octane gasolines burn slower, which may make them run very poorly in your particular car.

The long and short of it is that you should use exactly the octane rating specified in your car’s users manual. If it gives a single number (like 89) then use it. If it’s like my Cadillac and it says something like 89 or better, then you can experiment and see what your cost per mile is with the options that your car allows.

I agree with engineer_comp_geek.

The octane an engine needs to run properly is determined by the engines compression ratio. If you increase the CR, you will make more power. Most people, unless they are gearheads, will have no clue what the ratio is, so they should use what the owners manual says to use. For example, my CR now is 10.6:1 so 93 octane is required. Before I rebuilt the engine it was 9.4:1, and was perfectly fine on 87 octane, which was what the owners manual said to use. If I tried to run 87 in it now, I’d most likely blow the motor. The motor is making more power now, but it is because of the increased CR, not the octane of the fuel.

Higher octane fuel, in and of itself, has no effect on how efficient and engine is.

Just to add, a high millage engine might get carbon buildup on the pistons, effectively raising the compression ratio. In this case higher octane fuel might help the engine, but only temporally. This is why you some times hear people say the higher octane made their engine perform better.

If higher octane fuel is not required by your engine (see your manufacturers suggestion), you will see no benefit from using it…unless you think paying more per gallon at the pump is a benefit.

Yep carbeurated. '87 Honda Civic CRX HF. The tests were usually run in 5th gear, 55-60mph, under ~1500 rpm, relatively flat freeway. The most I got was around 60 mpg from 87 octane, 92 octane was usually under 45 mpg.

This is the point my friend uses which seems to confuse the issue: if higher octane burns slower, it should result in higher mileage. My impression though was that energy output was higher because it burned faster for the same volume.

I’m done some more digging and you’re right. An Octane rating of 95 means it has the same ignition point as a ratio of 95% Octane (C8H18) to 5% Heptane (C7H16). If you incease the octane rating by actually increasing the percentage of octane, it will have a higher energy density, as it is composed of larger molecules on average. However, additives have more effect on changing the octane rating.

Some high octane fuels will have a higher energy density and others will not.

Yes, but running at a higher compression ratio is generally more efficient, as combustion is more complete. Higher octane fuel allows you to run an engine at a higher compression ratio.

As I said above, fuel-injected cars can benefit from higher octane fuels as they as fitted with a knock sensor and can advance or retard the ignition point to the optimum point (within limits).

In the UK, petrol with 95 and 97 octane rating is sold. My car is designed to run on 95, but I’ve heard that power and economy is better on 97. I’ve put 97 in a couple times and I did seem to get slightly better mileage (measured with a trip computer). However, the difference wasn’t enough to make up for the higher cost.

It’s not a matter of faster or slower burn. It is about when the fuel will spontaneously combust.

Compression ratio determines how much air and fuel will burn for a given volume.

Also, I think you’re thinking of the knock sensor. However, the knock sensor will reduce power(by retarding spark advance) if it senses detonation.

But compression is determined by the pistons, cylinder heads and camshafts. In other words, it’s set from the factory. It’s not some thing you can easily change.

Therefore, most people should use the owners manual, because the factory knows what octane the engine was desinged for.

Agree.

Doesn’t advancing or retarding the spark change the compression ratio slightly as the engine runs? Or am I misunderstanding something?

The compression ratio is a function of the piston travel distance. The spark timing only affects when the fuel is ignited in the cycle, not the compression ratio.

Exactly, I guess this was what I was trying to say. Higher octane fuel just prevents the air-fuel mixture from being ignited too early.

1 Older engines had a fixed spark and valve timing. They would run as well on the lowest octane that avoided engine damaging detonation.

2 About 20 years ago, the manufactures introduced knock sensors. That meant using a more efficient spart timing if the octane was high enough and protected the engine for lower octane. I never saw much difference in my 92 Grand Am with the 10:1 compression Quad 4 between grades. Some cars have variable valve timing now adding a new factor. The compression ratio will stay the same, but if the intake valve stays open longer, you will have a higher charge and higher pressure. This thread contains too much outdated information.

3 Some additives raise the octane while lowering the heat content per gallon. The up to 10% ethanol common now is a good example. For better millage, go to the paint store and buy some tolulene.

Compression ratio is mechanically fixed by the volume of the cylinders (cast into the block) and the volume of the combustion chamber (cast into the head). You can change it by altering the engine block or cylinder head (= huge job, buncha bucks, engine essentially dismantled). You cannot change it by varying the ignition timing or the valve timing.

Now, ignition timing has a significant effect on engine performance, and is advanced or retarded automatically to correspond to engine speed and load. In the old days, this was done mechanically with centrifugal advances (speed) and vacuum advances (load). Nowadays it’s done electronically by the computer reacting to input from various sensors.

Not sure what you mean by fixed spark (timing). Fixed valve timing, yes, but even Model T’s had variable ignition timing (spark advance). On them it was via a lever the driver moved, as opposed to the later automatic methods I mentioned above.

Fixed spark within in the RPM and vacuum advance curves. At a given RPM and vacuum, you always had the same advance. Now it can vary with octane too.

I have a VW GTI with a turbo charged engine. It is suppose to take 91 octane gas (premium). However, you can run the car on regular (87 octane) or mid-grade (89 octane). The problem is you lose a bit of performance and some gas mileage as the knock sensors adjust adjust the engine to use the lower octane fuel. The car doesn’t knock, but it feels sluggish and gas mileage is down by about 10% on regular fuel.

If I use mid-grade, the difference is minimal, but still somewhat noticeable.

Here’s my gas mileage:

Premium fuel: 31 to 32mpg
Mid-Grade: around 30 mpg
Regular: 27 to 28 mpg

Remember, my car is suppose to take premium. A car that’s suppose to take regular fuel will see no benefit from putting premium in.

I find myself wondering which answer provides the most miles/barrell of oil, and the least amount of processing cost per gallon of resulting fuel. The answer for the world, rather than the answer for the individual motorist.

Tris