gasoline octane and motors

My sister asked me this question yesterday and I couldn’t answer it. She drives a Mini Cooper S whose manual asks for premium gasoline. What with prices and all, she filled up the other day with a normal variety of gasoline. She asked if this would harm the engine as we were driving, the only thing I could think of was to ask her if she was getting engine “knocks” and she said no. Any doper with technical knowledge about this? thanks

I am not a mechanic.
However, all the engine management systems that I know of today include knock sensors that will retard ignition timing in the event of premature combustion. So limited use should pose little or no problem. She may however notice slightly less power (or peppiness) and possibly reduced economy depending upon how aggressive the anti-knock system responds.

However, knowing that all senors have a threshold below which the system is not ging to respond, and given that manufacturer specifies premium fuel I personally wouldn’t make a habit of using low octane fuel if I intended to keep the car for a long while. Besides, the minicule savings just isn’t wortht the potential monster repair costs. Assuming 20 cents a gallon difference and only 25mpg average, that still amounts to paying just $120 more on a $2200 annual fuel bill.

If the manufacturer says that high octane gas is required for the engine to run properly, then the consumer should put high octane gas in their tank. The engine will develop problems overtime if these instructions are not followed.

If the engine does not explicitly require high octane gas, putting higher octane gas in the tank will provide no benefit.

Will it hurt the engine? It can, but with a modern car like the Mini, probably not. It will affect performance, however.

I found a really good article in Car and Driver that explains it far better than I could.

And less than $3 per tank (the Mini has about a 13 gallon tank).

I’ve always wondered what it is about gas prices in particular that make people obsess over savings that are a drop in the bucket. Skipping premium when the car calls for it. Driving several miles further to a station that’s a few cents a gallon cheaper. Buying a new car “for better gas mileage” when the difference is a mile or two a gallon (and the cost of the car is far greater than the lifetime potential savings over the old one).

$3 will buy you a nice coffee at Starbucks, but if you can afford to have a Mini Cooper S, I think you can afford to spend an extra $3 per tank…

Some of the octane stuff puzzles me too. My limited understanding is that its purpose is to slow down the fuel’s “burn” so that it can have a longer, smoother power stroke.

I just bought a truck, and the manual says regular gas “can be used, but 91 octane is preferred”. I don’t know how to interpret this, so I’m sticking with regular 87 octane for daily drives, and I fill the tank with 91 before long trips pulling heavy trailers. I’ve never owned a truck this small, so maybe this is normal for half-tons.

FWIW: Dodge 1500 quad cab, 5.7 hemi.

Obsessing over gas prices and then driving at 75 MPH or way over other speed limits. The list goes on.

It’s normal for Hemi engines. They’re high-compression designs and regular low-octane gas tends to knock. Under low load such as bopping around town and going to the grocery store, the anti-knock sensors can cope and not hurt performance too badly.

Octane rating is how compressible the gasoline is before it spontaneously combusts. If you use 87 octane in a car designed for 91 octane, the gasoline can spontaneously combust just from the compression alone, which means it will explode before the spark. This can be very bad for the engine. Most modern cars have knock sensors that fiddle with the timing and prevent you from doing any major damage, but it’s still not a very kind thing to do to your engine.

If you use 91 octane in a car designed for 87, generally speaking you typically are only wasting your money since you are paying for gas that won’t spontaneously explode at pressures that your engine doesn’t reach anyway. If your car is designed for 87, it may actually run worse on 91 than 87, and one of the reasons is that the higher octane fuels often burn a bit slower than the lower octane fuels. The main purpose of the octane rating isn’t burn rate, though, it’s compressibility.

My Cadillac is the same way. It allows 89 or higher and recommends 91 or higher. If I put in 87 it will start to ping which is bad for the engine. I tend to use 89 because the mileage difference I get between 89 and 91 is smaller than the cost difference, so it is cheaper per mile for me to use 89. Since your truck can use 87, 89, or 91 you might want to check what mileage you get with all three and find out which for you gives you the best cost per mile.

Higher octane fuels do not necessarily contain more energy, so filling with 91 before pulling a heavy trailer doesn’t necessarily buy you anything. The actual energy content of that 91 octane may be higher or lower than the energy content of the 87. The octane rating alone doesn’t tell you which it is.

For example, take a bunch of 89 octane fuel and add alcohol to it. You can easily bring it up to 91 octane doing so, but each gallon of fuel will now contain less energy since alcohol has only about 60 percent as much energy per gallon as gasoline. You end up with a higher octane rating, but your mileage will go down and you’ll have less torque for your trailer pulling. Note however that this isn’t necessarily how they made your particular 91 octane fuel, so YMMV, literally.

Putting regular gas in a car that calls for premium is false economy. Yes, you save $3-$4 per tank, but fuel economy is worse on regular. The engine management system will retard timing advance to prevent pinging on regular and that makes the engine less efficient.

I once did a test to satisfy my curiosity. I had rented a Buick Park Avenue (supercharged V6 engine) that recommended premium. Filled it with regular and started a 1500 mile cross country trip. It got 23 mpg (10.2 l/100km) on that tank. Filled it up with premium and it jumped to 27 mpg (8.7 l/100km). After that I stuck to premium and it got between 27 and 28mpg on every tank all the way to California.

You are all missing the point. Whether the woman gets any additional fuel economy or saves $3 per fillup is irrelevent. Do you think people who drive $35,000 cars ($35,000 subcompacts, no less) are going to be bankrupted by an extra $3? I’m sure she audits her expenses every months and makes a note of it. :rolleyes:

The UNDERLYING problem your sister is facing is a deep seated feeling of helplessness in the face of rising fuel costs, the feeling that she can’t do anything to control her own destiny. Using regular fuel instead of the manufacturer’s recommendation is a way for her to lash out and make a statement. Technical explanations will not satisfy her, they’re not even that factually informative anyway - as you can see from the C&D article, there are many factors that affect that affect how the engine deals with low octane fuel. If she drives like a granny and never goes over 3,000 rpm she may never get any engine knock. One of our cars specifies midgrade 89 octane fuel, we always use regular because we drive it very gently and we live at a substantial elevation that reduces the effective compression ratio on a naturally aspirated engine. It’s fine. There are cars where the manufacturer specifies 93 Octane AKI only, a grade that isn’t actually available in many parts of the US.

What your sister needs to do is get one of these. The data logging feature should be able to tell her in real time whether the knock sensor is being triggered and also her ignition timing, and will potentially give her the ability to reprogram her ignition timing tables to suite her particular fuel needs. That way she will have an answer for sure instead of internet hearsay. Yes, spending $1,000 on additional capital expenditure to potentially save $3 per fillup may seem excessive, but if she was really worried about the costs of fillups, she wouldn’t have bought the Cooper S, and yet here we are. Rapists will probably also save money in the long run by visiting prostitutes instead of raping people - what they/your sister is actually looking for is control.

My Mazdaspeed3 calls for 91 octane but all we have around here are 87, 89, and 93. Usually the 89 octane is 10 cents more than 87 and the 93 octane is 20 cents more.

I routinely put 89 octane in. As mentioned before, you might have less power available but really how much are you driving with the pedal floored anyway?

True, but increasing the load on the engine can increase the likelihood of engine knock, so switching to a higher octane for trailer pulling makes sense.

IF your engine has high enough compression to warrant higher octane fuel. Otherwise it’s a waste of money.