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Octane And Performance

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On: Mon, Oct 3, 2011 at 2:47PM | By: Peter C Sessler


Octane And Performance

I am always looking for ways to get better mileage, so I thought, perhaps the gas I’ve been using makes any difference, so I decided to try premium gas in my 1998 Neon commuter special.

I have always wondered if using premium would make any difference in my mileage. According to an oil company executive I know, the octane rating of a gasoline can be adjusted during the “cracking” process as the gasoline is being made—and at no or negligible cost.

I’ve read different opinions on gas mileage; the most common one is that, as long as your engine is tuned and in good condition, switching to a gasoline with a higher octane rating will have no effect on performance, power, or mileage. After some lengthy research on the Internet, I believe this to be true. Yet, why does my Neon now get 3 mpg more than it did before? When first bought the car, I got 30.5 mpg on the highway (75-80 mph, keeping up with traffic on I-75). Switching to Mobil 1 synthetic oil added another 1.5 mpg for a total of 32 mpg. And now I’m up to 35 mpg. What gives?

The answer is quite simple. All engines today have complex engine management systems that adjust the timing, fuel mixture, and so forth for optimum performance and emission output. If the manufacturer designed an engine to run optimally at 89 octane, increasing the octane rating to 93 wouldn’t make any difference, because the engine management system won’t know the difference between the two octane grades.

However, switching to a lower grade, such as 87, is something that the engine management system would definitely know about. A lower octane gasoline will cause an engine to knock. This knock will be picked up by knock sensors that are typically part of an engine management system, and, in order to avoid engine damage, the system simply retards the timing. This stops the knocking but does have the detrimental effect of lowering performance and fuel efficiency. When 89 octane is once again used, the management system advances the timing to the maximum amount possible for an 89 octane fuel.

I believe that this is what’s happening with my car. Although the owner’s manual states that 87 octane is all my Neon needs, the engine management is actually calibrated to run at optimum levels with the higher octane gas. The only way you’ll know with your car is to try a higher octane gas and see if your mileage increases. If it does, then you’ll know that your engine is designed to run optimally with premium. If it doesn’t make any difference, switch back to whatever you were using before.

I know that my wife’s Toyota Avelon’s owner’s manual says that the car is supposed to take premium gas 93, yet the car will run with regular 87. This car begs to be tested, as long as mileage increases—and I plan to try premium.

So as long as I can get the premium at a price that is cost effective, it will be worth it. Obviously, if premium is priced way higher than regular, it won’t pay. Stay tuned!




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