Octane Rating With Gas Prices

[SCOTTFISH]

With the gas prices like they are and can be, I have to ask a question. What, if any, would be the long time effects of using the midgrade or low grade octane? Any suggestions on how to fight the high prices. I know that where I live is probably not the highest place on gas right now, but at $2.05 per gallon, does anyone have any idea on how to beat it? I have thought about going to the lower grade and using octane boost, but I don't have enought data yet to determine whether it would even save enough to fool with it.

[Ali Al-Chalabi]

Depends on the compression ratio of your car. Use what is recommended.

[neanderthal]

as long as you dont operate the engine under load or rev hard, if is normally ok to use a lower octane in your engine.

if your car is a high performance car, or turbocharged, then i say stick with the recommended octane.

[yosshimura]

Premium here is about $2.20-$2.25 ... and just got my w124 last week. I have been putting premium in , but will be interesting to see what the general opinion is.

[suginami]

I would not recommend using a lower octane rating than your car requires.

Ever.

Should you do so, your engine's anti-knock sensor will retard the timing to prevent detonation, which can damage the piston crown.

Retarding your engine's timing robs your engine of power, negating any benefit you might be trying to achieve.

[Ali Al-Chalabi]

In addition if your engine retards the timing to prevent detonation, then it will not be harnessing as much of the gasolines energy content as it can and thus gas mileage will suffer.

[Duke2.6]

So what make, model, year car are you talking about. Most cars built in the last ten years have detonation sensors, and most OEs have gotten away from the dire warnings in the owner's manual about using less than the recommended octane.

Case in point. The 2005 C6 Corvette. The new LS2 6.0L 400 HP V-8 has a quoted 10.9:1 compression ratio. The fuel recommendation is "premium recommended but not required".

If you use regular the detonation sensor will pick up detonation and the ECU will select a less aggessive timing map and adjust it from there as required. This may be noticeable to sensitive drivers as less power, particularly at the low end of the rev scale.

Nearly any car with a detonation sensor will operate without signficant detonation on regular fuel even if the recommendation is premium. but the later the model year, the more sophisticated the algorithms to "tune" the engine to the octane number it is consuming at the instant.

Duke

Duke

Thanks Duke for asking the year and model.

Once the timing is retarded when knock is sensed, not only will power suffer, but also fuel consumption will increase.

Even though gas prices are higher, the cost difference between regular and premium is still the same. So, the percentage difference between regular is lower. Using lower octane fuel will cost about 10% less, but your fuel mileage will suffer.

Since you didn't tell us what year and model you're driving, what I said is assuming that you have a mid eighties or newer car with a knock sensor.

Sorry Paul, but the reason that the ignition is retarded upon sensing knock is to PREVENT engine damage, particularly piston tops.

Have a great day,

[Duke2.6]

Fuel economy may or may not be different. For aggressive stop and go driving fuel economy may suffer because of spark retard. On a long freeway cruise it may not be noticeable.

You bring up an interesting point about fuel prices. The price difference between regular and premium seems to be about 20 cents regardless of price. Back when it was a buck a gallon premium fuel was 20 percent more expensive. Now, at two bucks a gallon, premium is only 10 percent more expensive.

Duke

[Kestas]

On a related note, my manual specifies using 91 octane. Around here we typically have available 87, 89, and 93. Will the knock sensor take advantage of the extra two points of octane over 91, or does the spark efficiency max out at 91 and anything more is wasted?

[beevly]

($0.20/gal extra) x (20 mpg) x (200 mi./week) = $2.00 per week extra. Your mileage may vary, but for that paltry amount why not just do what the manual says and burn premium?

Kestas,

As long as the octane is adequate to prevent knock, any added octane beyond that does nothing for you, even if it were 100 octane. You might very well get by with 89, but I personally won't try it to save ten cents or so per gallon.

Have a great day,

[afmcorp]

on my 87 i've never used anything but 87 octane regular and just about all of our gas gets cut with meth alcohol. and after 30k very rough miles not one ping. the only thing i've done to that car's engine is to change oil every 3k miles. engine runs just fine.

[KermitF]

Question for Larry Bible.

Larry,

If I fill up with 50% 89 octane and 50% 93 octane, would I not meet necessary octane requirement of 91?

(Yeah, I realize the extra time/thought required to accomplish this exercise, and may well not do this, but am curious as to whether the formula is valid.)

Thx,

Kermit

As mentioned, the current ECU's can handle the lower grade fuels at a performance cost. I have noticed the difference in my 98 E320, however, the cost difference between the fuels is minimal.
I would recommend staying with the premium fuel and look for other ways to increase your milage like keeping your tires at the recomended pressures (I would say the most important and often overlooked) , lower a bit your cruising speed, avoid hard acceleration, and keeping the engine well tuned.