Gasoline Octane Ratings

courtesy of Bob, The Auto Answer Man


Hi,
I have a question concerning the different grades of gasoline. My friend says that the higher grades such as 93 octane do not offer any advantage over the 87 octane. He claims that today's newer cars are designed to run efficiently from 87 octane fuel. He also claims that all grades of fuel contain the same level of detergent, so those claim that 93 octane fuel cleans your engine is true for all grades of fuel. I want to know if I can stop paying 30C-40C extra for the 93 octane fuel.
Mehul M

Well Mehul,
The concept of octane rating is a bit touchy. There are many people that have pre conceived feelings as to what "octane" is. Well true octane is a straight chain hydrocarbon consisting of 8 carbon atoms.

Now that we know what octane is, how does that apply to cars? Well to try and not confuse you too much, octane is a rating. This rating is basically describing the anti-knock rating of the fuel. It has nothing to do with detergents or anything like that. Generally, higher compression engines use higher octane.

Wait, back to antiknock rating. I need to tell you what that is. Knock (or pre-detonation) is what happens when the ignition timing is slightly off on your car. Different fuels have different characteristics and need the timing varied ever so slightly.

Now, how does this effect your car. Well most modern distributorless engines, the computer controls the timing. As long as the fuel that you are using is in the usable range (or adjustment) of the computer, then all should be well. But I must urge you to use the same grade and type and brand of fuel! If you keep changing brands, then your computer will not run your engine as efficiently. Thus, causing you to use more fuel!

Okay, now back to saving money. You are right, lower octane fuel costs 30-40 cents less. Do you need to use it? Well, read your owners manual. It will tell you what the minimum octane rating that you can use in your car. DO NOT GO BELOW THIS!!!! Pre detonation can destroy a valve train in your engine!

Now, there are several ways to increase the octane rating. Some of which also add horsepower to your car. Now it is subtle. Unless you are in harmony with your car (now you might be thinking "What is wrong with this guy, in harmony"?) Yes, the difference you might not even notice unless you are the type of person that can really sense these things (like when a tire is 5 pounds underinflated and it rights heavy in that direction).

Once you have determined what this minimum rating is, try 2 tankfulls and record your starting mileage and the ending mileage. Compare this with the fuel that you currently use. If you get better mileage and it "feels" the same, then go ahead and save your money. If for instance, you notice the car is a little slower when starting and a green light, or a little sluggish in the passing lane, then maybe it's the fuel or maybe you really need a tune up! Describing the tune up procedure would be tooo long to include in this article, but if someone requests a particular problem, I would be more than happy.

Now for detergents. The federal government (EPA) mandates that ALL motor fuel have detergents of some kind in it. Some name brands have more than one and are better than others. These detergents have nothing to do with octane rating directly. Some of them might add to the octane rating but that is not their primary purpose in life.

You could probably save even more money by buying an off brand fuel, but I don't recommend that. There is a reason that the name brand companies did not want this gasoline in their tanks!

To finish off, I must say thanks to all the people that have sent in requests for solutions! Hope that I have helped you. Please send me a response on how things worked out.
Good Luck!
Drive Safely!
Bob, The Auto Answer Man


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