What type of gas do you put in your GX
01-05-05, 09:03 PM
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a short article on this topic in the Jan. 5, 2005 Wall St. Journal.
Low-Octane Liability?
By JONATHAN WELSH
Staff Reporter of THE WALL STREET JOURNAL
January 4, 2005; Page D6
Editor's Note: The first installment of a weekly column answering readers' questions about automobiles. Questions can be e-mailed to mecar@wsj.com.
Gasoline prices have been high – over $2 a gallon in some parts of the U.S. – for over a year but most drivers still haven't been moved to trade in their big, fuel-thirsty vehicles for more economical models. But that doesn't mean the cost of filling up hasn't gotten consumers' attention. A number of readers say they've been saving money by using regular unleaded gas instead of the higher-octane fuel specified in their owners' manuals. With the highest-octane premium fuel often costing about 30 cents more per gallon than regular, the savings can be significant. But motorists wonder whether there's any danger in using lower than the recommended octane.
Martin Arnold of Trumbull, Conn., says his BMW 330i "runs great on regular grade, no knocks no pings. But am I doing permanent damage to the engine or shortening its life?"
The short answer is no. Even manufacturers like BMW and Volvo say low-octane gasoline won't cause the harmful "knocking" that threatened engines 20 or 30 years ago. Knocking occurs when fuel is burned or "detonated" prematurely in the engine. On modern cars, electronic engine controls include devices called knock sensors that constantly adjust the vehicle's ignition to prevent knocking, regardless of the fuel grade.
There is a downside, though. When car companies publish performance data like acceleration and miles-per gallon, the figures are based on how the car performs using the recommended fuel grade. Cars that are designed to run on premium or "high test" may not perform as well on lower octane. The difference probably won't be noticeable in everyday driving, but a BMW spokesman recommends that drivers do a little math: Compare the car's fuel economy with different fuel grades to determine the cost per mile for each octane level. We've tried this a few times, though not with any BMWs so far, and have found that using regular fuel costs less
Braking vs. Downshifting
A question from Steven Rubin of Manhasset, N.Y., is sure to spark debate: "When going down a long hill in a car with an automatic transmission, is it better to ride the brakes down the hill or downshift and let the engine /transmission slow the car?"
Manually shifting an automatic transmission to a lower gear during long descents makes sense because it allows the engine's resistance to control the car's speed. One would certainly downshift if driving with a manual transmission. Indeed, several vehicles, mainly pickup trucks used for towing, now have automatic transmissions with "logic" that helps them sense hilly terrain on which it is best to "hold" a lower gear for long periods. The absence of such a system leaves drivers to do the shifting themselves. But like most things, too much shifting can be overkill.
I recall my college roommate's father shifting his Dodge Omni from high gear down to second (most automatics had just three gears back then) whenever he slowed to stop at an intersection. It drove my roommate to distraction in part because he believed his dad's effort to "save the brakes" was just wearing out the transmission. It's true that shifting an automatic transmission manually all the time can increase wear, and while a brake job costs about $300, transmission repairs typically cost $1,000 or more.
Low-Octane Liability?
By JONATHAN WELSH
Staff Reporter of THE WALL STREET JOURNAL
January 4, 2005; Page D6
Editor's Note: The first installment of a weekly column answering readers' questions about automobiles. Questions can be e-mailed to mecar@wsj.com.
Gasoline prices have been high – over $2 a gallon in some parts of the U.S. – for over a year but most drivers still haven't been moved to trade in their big, fuel-thirsty vehicles for more economical models. But that doesn't mean the cost of filling up hasn't gotten consumers' attention. A number of readers say they've been saving money by using regular unleaded gas instead of the higher-octane fuel specified in their owners' manuals. With the highest-octane premium fuel often costing about 30 cents more per gallon than regular, the savings can be significant. But motorists wonder whether there's any danger in using lower than the recommended octane.
Martin Arnold of Trumbull, Conn., says his BMW 330i "runs great on regular grade, no knocks no pings. But am I doing permanent damage to the engine or shortening its life?"
The short answer is no. Even manufacturers like BMW and Volvo say low-octane gasoline won't cause the harmful "knocking" that threatened engines 20 or 30 years ago. Knocking occurs when fuel is burned or "detonated" prematurely in the engine. On modern cars, electronic engine controls include devices called knock sensors that constantly adjust the vehicle's ignition to prevent knocking, regardless of the fuel grade.
There is a downside, though. When car companies publish performance data like acceleration and miles-per gallon, the figures are based on how the car performs using the recommended fuel grade. Cars that are designed to run on premium or "high test" may not perform as well on lower octane. The difference probably won't be noticeable in everyday driving, but a BMW spokesman recommends that drivers do a little math: Compare the car's fuel economy with different fuel grades to determine the cost per mile for each octane level. We've tried this a few times, though not with any BMWs so far, and have found that using regular fuel costs less
Braking vs. Downshifting
A question from Steven Rubin of Manhasset, N.Y., is sure to spark debate: "When going down a long hill in a car with an automatic transmission, is it better to ride the brakes down the hill or downshift and let the engine /transmission slow the car?"
Manually shifting an automatic transmission to a lower gear during long descents makes sense because it allows the engine's resistance to control the car's speed. One would certainly downshift if driving with a manual transmission. Indeed, several vehicles, mainly pickup trucks used for towing, now have automatic transmissions with "logic" that helps them sense hilly terrain on which it is best to "hold" a lower gear for long periods. The absence of such a system leaves drivers to do the shifting themselves. But like most things, too much shifting can be overkill.
I recall my college roommate's father shifting his Dodge Omni from high gear down to second (most automatics had just three gears back then) whenever he slowed to stop at an intersection. It drove my roommate to distraction in part because he believed his dad's effort to "save the brakes" was just wearing out the transmission. It's true that shifting an automatic transmission manually all the time can increase wear, and while a brake job costs about $300, transmission repairs typically cost $1,000 or more.
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