Do mixed octane fuels average out?

[CookingWithGas]

My car’s manufacturer recommends 91 octane. All the pumps around here have 87, 89 and 93. Do octane ratings average arithmetically, or is there some qualitative difference? If I mix 89 and 93 do I get gas that is rated 91? Or is that like putting blue marbles and yellow marbles in a bag and expecting to get green marbles?

I have been using 89 and my understanding is that modern cars will electronically adjust the timing to prevent knocking, although this may have a slight impact on performance.

(PS I found that it’s a little cheaper to mix 87 and 93 in the necessary proportions to get 91, rather than mixing 89 and 93 half/half. Oddity of gas pricing.)

[Tride]

Yes. Most gas stations only have 2 gasoline tanks–high octane and low, and they mix to create the mid-grade. You can google your car and see if it actually benefits from higher octane–Car and Driver for one has done some dyno tests.

[Dead_Cat]

Nothing to add to the first reply (except to say I agree with it) but had to point out the outstanding username/post combo.

[Riemann]

@Dead_Cat

had to point out the outstanding username/post combo

It’s so apt that if today were April 1st my hypothesis would be that it’s a clever joke.

[Ruken]

@CookingWithGas

I have been using 89 and my understanding is that modern cars will electronically adjust the timing to prevent knocking, although this may have a slight impact on performance.

I can’t tell you whether this is true but if you’re at higher altitude, you can often get away with a lower rating. While 87 is the lowest in most states, I’ve seen 85 in the mountain west. Here are the typical offerings by state:

List of U.S. states by standard octane ratings

Looks like “premium” is 91 or 93 most places.

[Schnitte]

Just adding that the octane scale was actually defined in such a manner as to make averaging work. A fuel with an octane rating of 92 has the same knocking resilience as a hypothetical blend of 92 % octane, 8 % heptane, and so on. The scale extends beyond 100 (for fuels that are more knock-resistant than pure octane), but the idea of averaging out blends was built into the design of the scale.

Oh, and I agree that the thread title/username combo is hilarious.

[CookingWithGas]

@Dead_Cat

outstanding username/post combo.

@Riemann

if today were April 1st my hypothesis would be that it’s a clever joke.

@Schnitte

thread title/username combo is hilarious.

Then there was this:

Does rinsing beans reduce their gas-producing qualities?

My wife insists on rinsing beans before cooking with the argument that it reduces the beans’ gas-giving properties (even canned beans, which don’t otherwise need to be rinsed). I said that beans give you gas because we do not have the enzymes to digest the type of starch in the beans, therefore allowing our gut bacteria the chance to eat it, and they produce the gas. The starch isn’t just on the surface, it’s throughout the whole bean. But she insists. What’s the straight dope? I could find onl…

[Tride]

Interestingly, a buddy was having problems with his Ram 1500 cutting out when it was hot and the dealer recommended lower octane fuel and it seems to have solved the issue. From the C&D article the F150 Ecoboost loses quite a bit of power with lower octane fuel, but the 1.5L turbo 4 in our CR-V does not, so we run it on 87. And in some cars (Demon 170, notably) you want E85 for maximum power.

[thelurkinghorror]

In my experience at about 4000 ft elevation it’s 87/89/91. If I go out to around 6000 ft it’s 85/87/89. I don’t think I’ve seen 93 in years, even at sea level. Though there are the “performance” places which have other settings for 100 octane or separate pump for E85.

[FinsToTheLeft]

@thelurkinghorror

In my experience at about 4000 ft elevation it’s 87/89/91. If I go out to around 6000 ft it’s 85/87/89. I don’t think I’ve seen 93 in years, even at sea level.

I’m in Toronto at 250’ and our standards are 87/89/91 or 92. Some stations offer 93 as a 4th grade for people who think it’s “better”.

[Omar_Little]

Unless your vehicle requires (not recommended) that you use a premium gas, you are better off using regular unleaded. If you start out using premium gas even if it isn’t required, your fuel injectors get used to the premium gas and won’t operate as effeciently if you switch back to a lower grade.

There is a current approx. $0.80 difference per gallon between regular octane and premium octane. You would need to see a 4 MPG improvement in your performance to make it economically indifferent. You won’t.

[FinsToTheLeft]

@Omar_Little

If you start out using premium gas even if it isn’t required, your fuel injectors get used to the premium gas and won’t operate as effeciently if you switch back to a lower grade.

Isn’t that a bit anthropomorphic? Fuel injectors don’t care what’s going through them.

The Engine Computer Module should be continuously monitoring the cycle and advancing or retarding ignition as needed. The aggressiveness of the ECM to get the best compression without knocking will of course vary, but after a couple of tanks of lower octane it should be running like the higher octane was never there. Further, if the engine is not designed for higher octane it’s never going to try and take advantage of the higher compression the increased octane allows for.