Is it bad for an engine to mix fuel grades, for example sometimes give it regular and sometimes premium?
by 
geek860
ANSWERS: 9
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Not so long as you use at least the minimum octane the manufacturer recommends. There's really not a good reason to use a more expensive grade than is called for.
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no it wont cause any problems as long as it will run on regular. if your car has a half tank of regular and you fill it up with premium theoretically your car will be getting in between the two. it wont harm anything. if you put regular in it and it starts acting funny put an octane booster in it that wont harm a catalytic converter.
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If the car runs fine on regular there won't be any SERIOUS problems but it MAY cause the ECU to be confused for a short time when you switch from Premium to Regular. Some ECUs are programmed to set the fuel map and ignition timing near the edge of detonation. Given that Premium is more resistant to detonation the ECU can tweak the timing/maps for optimal power from minimal fuel WITHOUT engine damage. In fact, a stock Subaru WRX can gain up to 30 HP this way. If the car is used to the Premium then giving it Regular may give it a slight ping which will send the ECU into over-correction. The only bad effect is a TEMPORARY loss of power as the ECU re-educates itself. There will be no engine damage or any lasting effects though.
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As long as the combined octane level is greater than the minimum requirement you will be fine. 87 octane gasoline is 87-percent octane and 13-percent heptane. If you mix it with equal parts of 93 octane you will end up with 90 octane. The octane level determines the amount of compression before ignition, with 87 octane requiring the least compression. As long as the compression ratio of your vehicle is compatible with the octane level, it shouldn't result in any engine knocking that could do damage.
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the mfgrs recommendations are listed on the car's door jam
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Mixing octane's won't hurt your engine at all as long as you don't accidentally put E85 or diesel in an engine that isn't designed to use it. Many modern "high performance" engines will run just fine on 87 octane gas, although to unlock their full potential, you need to use 93 octane gas. Most "normal" engines are designed to run on 87 octane gas. Using a higher octane won't hurt them, but it won't make them perform any better either.
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use the octane that the mfgr recommends or you'll just be wasting money on higher grades.
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It won't hurt any thing unless you are using a lower octane grade than the mfgr says you should use. The correct octane is usually listed on the door jam.
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you should use the octane specifically for your car
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