Premium vs. Regular

[Peter7307]

Went through this same exercise myself some time ago and found there was some FE gain when using premium BUT the cost differences meant the amount spent was actually increased.

I can't recall the exact details but the variances were small and the experiment was definitely not scientifically executed.

Cheers , Pete.

[Nate]

Premium will give you negligible gains in fuel efficiency, however, it is theoretically possible to make up for the cost difference and get better MPG. Most cars can't detect whether it's premium or 87 in the tank, so the fuel map remains the same, which means the same amount of fuel is pumped into the engine. The higher octane fuel results in a horsepower boost, but if you could remap the ECU to sip less gas when premium is in the tank, it would be probably be okay to run leaner since high octane gas is more potent and will give you the same horsepower 87 gives you while burning less of it. Also, and don't quote me on this, but I remember hearing that premium fuel is going to have a cleaner and more complete burn. But like CFG said, don't do that if you have a hot air intake, unless you want to grenade your engine.

[thebluelightbulb]

I have heard of some manipulating the fuel map to make the burn leaner. when i first bought the car, i thought that i could run lean by easing up on the gas, but that's only true in honda engines, not the L91. without modification i get constant 14.3:1 burn at all throttle ranges. i have heard about swaping the MAP sensor to move the fuel map, but i'll have to wait until i can get my hands on a Chilton's manual. i don't even know where the sensor is. the easiest thing i can do is get a K&N air filter. Perhaps that's the cheapest way to get a gain, right now.

[MazdaMatt]

I bought a K&N filter... don't bother. Pure marketting hype. Replacing a filter is also much easier than the process of cleaning the K&N

[thebluelightbulb]

true. i forgot that you had to clean it.

[some_other_dave]

Originally Posted by Nate

...high octane gas is more potent and will give you the same horsepower 87 gives you while burning less of it....

Not true. In fact, the only reason that 91+ would ever give you more power in a given car is because the engine advances the timing and/or increases the boost to take advantage of the more detonation-resistant fuel. It does this by listening to the knock sensor--if there's no knocking, it can advance the timing (or increase boost).

In some cases, in fact, higher-octane fuel has (marginally) less energy per volume than regular-grade fuel! But in all cases that I know of, any difference is small enough to be "in the noise".

The only thing that the "octane" number of a fuel tells you is its resistance to pre-ignition. Nothing more, nothing less. It doesn't have noticeably more energy, it doesn't burn significantly slower, it just doesn't light itself off as easily. I've been schooled by several petro engineers on that subject over the years...

Now, if your engine is built for high-octane fuel, you will generally get better performance and economy by using high-octane fuel. Note that many modern engines are built to deal with regular-grade fuel, but also to take advantage of higher octane when it is available. That's one of the advantages of running a knock sensor. Some older cars can take advantage of higher-octane fuel by manually advancing the timing (to a point!), but it isn't universal by any means.

Some people have been able to get better FE out of running higher-octane fuel. Some have shown a big enough improvement that the extra cost is worth it. Some have not shown any improvement at all. The best way to tell on your own car is through carefully instrumented and rigorous testing.

-soD