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Comment by baby | Hacker News Reader

I never understand what gas I'm supposed to get with my car. There's 87, 89, and 91 I believe? But I just don't understand why so many choices.

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What you are supposed to do is look in the Owner's Manual. It should tell you what octane number gas you should get, plus a lot more. For example, in the manual for mine (a 2006 Honda CR-V) it says this:

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Your vehicle is designed to operate on unleaded gasoline with a pump octane number of 87 or higher. Use of a lower octane gasoline can cause a persistent, heavy metallic rapping noise that can lead to engine damage.

We recommend using gasoline containing detergent additives that help prevent fuel system and engine deposits.

In addition, in order to maintain good performance, fuel economy, and emissions control, we strongly recommend, in areas where it is available, the use of gasoline that does NOT contain manganese-based fuel additives such as MMT.

Use of gasoline with these additives may adversely affect performance, and cause the malfunction indicator lamp on your instrument panel to come on. If this happens, contact your authorized dealer for service.

Some gasoline today is blended with oxygenates such as ethanol or MTBE. Your vehicle is designed to operate on oxygenated gasoline containing up to 10 percent ethanol by volume and up to 15 percent MTBE by volume. Do not use gasoline containing methanol.

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Dealing with the octane number is easy. Just don't use gas with a lower octane than your manual calls for. You can use a higher octane if you want, but it won't really do anything other than cost more. You'll get the same performance, wear, pollution, and mileage as you would with the octane number given in your manual.

To further clarify, the average street car is designed to run on the ethanol which is sold everywhere, but some engine configurations need a higher octane fuel in order to combust the fuel completely. I had a buddy who rebuilt an engine with a compression ratio that required 91 octane, or he'd get knocking. This was during the late 80's, and more than one station around these parts had pumps selling 95 octane (it might have even been 100) for the real hot-rods.

This whole comment thread is surprising to me because I haven't seen sub-90 octane rating in ages - I have even just checked, and one of the main petrol companies in Poland sells only 95, 98, and special extra variant of 98.

USA, they have all that weird stuff there...

It’s complicated, as that number measures different things in different countries: https://en.wikipedia.org/wiki/Octane_rating#Measurement_meth...

As per usual, the US uses a different scale (AON) than the rest of the world (RON).

> some engine configurations need a higher octane fuel in order to combust the fuel completely

This is incorrect. Higher octane ratings mean a fuel can withstand more compression before detonating. Pre-detonation, or "knocking", where an engine experiences detonation in the cylinder before top dead center can cause severe damage, so higher octane is used with higher compression engines. Lower compression engines see no benefit.

If the fuel pre-ignites, proper flame propagation will be thwarted, and the fuel will not be burned as thoroughly, leaving residue, and exhausting (albeit tiny amounts of) fuel. Sure, I put the cart in front of the horse, but I can always count on HN to point out the pedantic.

Reading this again, you're right, it was a little pedantic. This is one of those topics I feel is just rife with misinformation, so I tend to jump on explanations that don't seem correct at first glance.

I'm sure this is accounted for, but octane levels do degrade if you don't use your car often. So it can be good to put a higher octane fuel in your car.

The instruction manual should say. But in my experience, many (most?) cars have a sticker inside of the door that you flip open to access the gas cap.

Here's a random example I found: https://i.imgur.com/nDkwl0i.jpg

It doesn't actually say "octane", but note that it says "MIN.(R+M)/2", and (R+M)/2 is what you'll see on the pump too, as in this example image: https://img.hmn.com/fit-in/900x506/filters:upscale()/stories...

(This is for the US. I would assume other countries are similar.)

There are different ways of calculating octane and that's why they specify the method.

It seems like in the US, 89, 91 are reasonable octane numbers (from reactions to this post). Around here, I've never seen stuff that low - here, it's Euro95 and Euro98. I've been told a real long time ago that the 95 and 98 in there were octane numbers.

Anyone any idea if octane numbers are calculated differently in US vs EU? Or if someone lied to me? Or if the US is really using much lower-grade petrol than is Conon in the EU?

They are calculated differently. US numbers are 8-12 lower than EU for the same petrol.

Yes, they're calculated differently. This covers it pretty well:

https://en.wikipedia.org/wiki/Octane_rating#Measurement_meth...

TLDR, the US and Canada use "(R+M)/2" (also called Anti-Knock Index), and much of the rest of the world uses RON (Research Octane Number).

If you zoom in closely on the first image in my comment above, you can see that the sticker has both. It says "MIN.(R+M)/2 87 Regular" and "MIN.RON 91 Normale".

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Tangentially, you will see lower-octane fuel in specific parts of the US. Denver, Colorado is nicknamed "the mile-high city" because its elevation is about 1 mile (1.6 km) above sea level. In many parts of the US, you will see octane ratings of around 87, 89, and 93 at the pump, but in Denver you might see 85, 87, and 91. The thinner air reduces the need for high octane.

I suppose something similar must happen in Europe, but it may not be as common or well-known. From what I can tell, non of Europe's large cities are at that high of an elevation.

In my experience in germany, gas is sold as is from the tank, so even at 1200m you'll get Euro95 or Euro98.

Higher octanes are designed to prevent preignition of the fuel in engines that are supercharged/turbocharged or use higher compression ratios. Preignition is when the fuel combusts before the cylinder is at the "ready" point in its cycle (wherever that is for your car, with variable timing and all that).

As others have said, your manual will tell you, but a good rule of thumb is if you don't have a high performance vehicle or an engine with a turbo or supercharger, 87 should be fine.

Excuse me, but honest question - are there new vehicles sold without compressor?

Yes, though depending on your part of the world there may be more or less. Mazda MX-5 is one example.

Yes. Toyota hybrid engines are not turbocharged, as that's more efficient.

Higher octane means the gas has a better ability to resist knocking, which is spurious ignition not caused by the spark plug. Knocking can damage your engine if it happens often. Turbocharged cars have a higher tendency to knock, so they generally need high-octane gas. If you put low-octane in a turbocharged car, most modern engines will detect the knocking and detune the engine automatically to prevent damage, but that means you're not getting the power or efficiency the car was designed for.

So use high-octane in turbocharged cars and low octane for non-turbocharged cars. (If you use too high an octane in your car, it won't hurt anything but your wallet.)

If you're racing a turbo car you might need close to 100 octane, which cannot easily be bought but it can be made by adding toluene to your gas tank (google it).

Other than that check your owner's manual.

Some states have different levels. In Colorado 85 is common. I assume it has something to do with the air pressure and oxygen content of the air perhaps?

85 is functional at elevation, but definitely still not preferable at elevation.

Works best with carbureted engines, which are obviously few and far between these days. You shouldn't put 85 octane in a modern fuel injected engine period.

The method of fuel metering the car uses does not necessarily have any connection with what octane you should use. It's all about the engine design. If we are going after rules of thumb, I would say that if the car is turbocharged, it will most definitely run better with higher octane. Although my turbo VW can run with regular fuel, it will just change timing and make less power.

I have a 78 Fiat with a carburetor however, that definitely needs 91 or higher.

VW is seemingly, in my experience, an outlier. Their small displacement turbocharged engines will run fine on 87, but really like 91 (or E-15 88). My '14 Jetta with the 1.8T got better power and fuel economy on "the good stuff."

This is an oddly specific hardline stance to have.

When you have 85 octane available at 7,000 ft and your car is engineered to run with 87 at sea level, you'll be fine.

One of the reasons for this is that for a NA (normally aspirated) engine, i.e. not turbo charged or super charged, the lower atmospheric pressure results in a lower compressed pressure (versus sea level) as the piston rises toward the top, which means a lesser chance of pre-mature detonation (knock) due to pressure, which is what higher octane prevents. So therefore you don't need as high octane at higher altitudes.

Unfortunately that isn't true with turbo or super charged engines, which are more and more popular. I really miss the 93 (and even 95) octane fuel I could get in Boston. I use a octane booster in my car here in Colorado to help offset the lower octane pump gas, and allow my turbo charged engine to make more power (by not having to pull timing to accommodate the lower octane gas).

Your car’s manual will say.

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