Gasoline Octanes

I'd really appreciate if you guys could take a moment of yours for me.

I bought a 2003 Pontiac Grand Am (24K mileage) recently. In the manual I
read that, 87 octane gasoline is recommended for my car. I've never gone cheap on the gas and always bought 93 octane for my old car. So did I for my new car at first tank, too. Then I read the manual and Iswitched to 87, right after I hit the bottom of my fuel tank.

I'm not comfortable. I just don't want to be happen anything to my engine because of a few cents. That amount of a money is not an issue. I just want to do the right thing for my car. Also I think I feel more vibration and knocking while I'm idling. I don't know, I've had so many bad memories with my old car, and maybe I'm just being paranoid.

I've been reading a lot of things on the internet. So, I'm aware of what the octane is and what it does. But, I'm not able to get a honest and open recommendation for the octane choice. Some sites recommends to use basic, some recommends higher.

I'm from Florida:
Hot wheater and low altitude, means requires higher octane.
High humidity, means requires lower octane.
Over 15K mileage, may mean requires higher octane.
V6 3.4 liter engine, does that mean requires higher performance engine? Higher octane??

Questions, question, questions... Don't want to take time too much of yours. Please give me your suggestions.

Thanks.

i use 89 or 91 octane mostly. i have a 91 grand am se.
i just got it recently at 73K miles...

the original owner was told to use 87 octane.

the manual of course just says to use at least 87...

i live in wa state. a bit cold now... not sure how that would affect the
octane recommendation.

i'm curious what the experts say.

thanks.

Your car is calibrated to run on the lowest octane fuel available.
Using a higher octane fuel could cause hard starting conditions due to the fact that higher octane fuels evaporate faster, therefore the combustion chamber is not recieving a sufficent amount of fuel.

Thanks for the quick responds. Let me give you some quotes from the websites I've been reading. I understand, they are all the gasoline brand websites, but still they're worth to be considered I guess.

In addition thanks again GMMerlins!

http://www.shellus.com/products/product_info/gasoline_faq.html

** My car’s owner’s manual recommends regular grade (87 octane index) gasoline. Is that the gasoline grade you recommend?
Yes. We recommend you start with the gasoline grade recommended by the manufacturer of your vehicle.

** Then why are two other octane grades offered?
A) Some car manufacturers recommend higher grades (midgrade - Shell Plus - or premium - Shell Premium), for higher performance engines. For best performance and engine protection, these cars should use the higher recommended octane grade.

Higher performance engines can have octane requirements exceeding the antiknock quality provided by regular grade gasoline, which is why the manufacturer recommends higher octane gasoline. Using low octane gasoline deprives the engine of performance, and may lead to engine damage.

B) As an engine accumulates miles, its octane requirement increases, sometimes to the point of needing a higher octane grade than that initially recommended for the engine.

We know that as engines accumulate miles, their octane requirement increases, and often this increase does not exceed the octane quality for which the engine was designed. But if the engine begins to ping or knock when accelerated hard, climbing a grade, or towing a trailer, the next higher octane grade will likely satisfy the engine’s new octane requirement. Studies have shown that about 60 percent of all vehicles are satisfied by regular grade gasoline. Midgrade satisfies about 80 percent, and premium nearly all engines.

C) Many modern engines have control systems that keep the engine from knocking. Some of these engines can benefit from using higher than recommended octane grade gasoline.

Many of today’s engines have knock control systems. These systems are designed to sense knock before you can hear it, and adjust spark timing to eliminate the knock. If the gasoline used does not satisfy the engine’s octane requirement, the system will retard spark timing. Retarding spark timing causes some loss in fuel economy, and reduces maximum power. Using higher grade gasolines can often restore lost engine performance when knock control systems cause retarded spark timing.

** Are premium grade gasolines ever harmful to my engine?
No. Some higher octane gasolines contain higher octane components that have poorer volatility and these can cause poorer cold start and warm-up performance, but they will not be harmful. In addition these heavier components have more energy and they can give improved fuel economy. Shell gasolines are carefully formulated to give high octane performance without compromising on cold start performance or fuel economy.

*****
http://www.chevron.com/prodserv/fuels/gas_qanda/api_octane.shtml

Q. What determines my car's octane requirements?

A.*Your car's octane requirements are mainly determined by its basic design. In addition, variations in engines due to manufacturing tolerances can cause cars of the same model to require a different octane of several numbers. Also, as a new car is driven, its octane requirement can increase because of the buildup of combustion chamber deposits. This continues until a stable level is reached, typically after about 15,000 miles. The stabilized octane requirement may be 3-6 numbers higher than when the car was new. Premium or midgrade fuel may be advisable to prevent knock.
Other factors also influence your car's knocking characteristics:

Temperature - Generally, the hotter the ambient air and engine coolant, the greater the octane requirement.

Altitude - The higher the altitude above sea level, the lower the octane requirement. Modern computer-controlled engines adjust spark timing and air-fuel ratio to compensate for changes in barometric pressure, and thus the effect of altitude on octane requirement is smaller in these vehicles.

Humidity - The drier the air, the greater the octane requirement. The recommendations that vehicle manufacturers give are for normal- to low-humidity levels.

Your engine's spark timing - The octane requirement increases as the spark timing is advanced. Both the basic setting of the spark timing and the operation of the automatic spark advance mechanisms are important in controlling knock. In some computer controlled engines, the spark timing can only be changed by replacing modules in the computer. If they are equipped with knock sensors, these computer controlled engines have the ability to retard the ignition temporarily when a sensor detects knock. This temporarily reduces the octane requirement and may also temporarily reduce vehicle performance.

Method of driving - Rapid acceleration and heavy loading, such as pulling a trailer or climbing a hill, may result in a greater octane requirement. Stop-and-go driving and excessive idling can increase octane requirements by causing the buildup of combustion chamber deposits.

Malfunctions of emission control systems - An improperly functioning emissions control system can affect the octane requirement by changing the air-fuel mixture or by not providing dilution gases through the exhaust gas recirculation (EGR) system. If a malfunction occurs, your vehicle should be taken to a qualified vehicle service mechanic. Some problems are indicated by warning lights on the driver's instrument panel.


Q. Which octane grade should I use in my car?

A.*Use the recommendation in your car owner's manual as a starting point for selecting the proper gasoline. If you notice engine knock over an extended time and your engine is adjusted correctly, try a higher octane gasoline. Also, higher octane may provide a performance benefit (better acceleration) in cars equipped with knock sensors. Many late model and high-performance (turbo-charged and supercharged) cars fall into this category.

Dear GMMerlin:

I really appreciate you sharing your knowledge with us. Just to be sure, were you talking to me (2003 Pontiac) or to the other friend (1991 Pontiac)?

Also what do you think about the vibration that I feel?

Thanks in advance!

Dear GMMerlin:

I really appreciate you sharing your knowledge with us. Just to be sure, were you talking to me (2003 Pontiac) or to the other friend (1991 Pontiac)?

Also what do you think about the vibration that I feel?

Thanks in advance!

oops. i was wondering who had the 1991 pontiac.. my pontiac is a 2001 grand am.. sorry..

and yes thanks gmmerlin!

My comment was directed at those who own newer cars...mostly vehicles with OBD2..96 and up.

I have found that a good many of vehicles with owners who have complained about difficult starts have 91 octane fuel...once they go back to 87 the hard start problem goes away.
As you see from my profile, I am the proud owner of an 02 GMC Sierra (my wife has an 01 Jimmy)...we only use 87 octane fuel and have had no problems...plus the fact that we both drive about 100 miles round trip to work every day..if we were to use the 91 octane we would go broke in no time.