Riddle Me This, Batman (Preferred Gas)

[waymur]

Here in Canada mid-grade (89) gas unlike some parts of the U.S. is more expensive than regular(87) gas. The owners manual says that the 2.7L recommends 87 octane and the 3.5L and 5.7L prefer drinking 89 octane but 87 octane is acceptable but premium (91) gas is next to a mortal sin. Discounting the potential urban legend that 87/89/91 octane gas is formulated differently in Canada and the U.S. to obtain their respective octanes can anyone point me in the right direction. Mine is the Dodge Magnum RT soooooo...

1) I've gassed up at some discount gas dispensers that have only 87 and 91 octane and they say equal blending of the two is okay since it will give you the midgrade (89) in a pinch. Seems logical but is it?

2) The manual says 87 octane is acceptable but just how acceptable is it? Relating back to 1) can I use it in a pinch because I don't want to do any damage and I don't want to look like an idiot asking for $10 of regular and $10 of premium. It wouldn't be equal anyway because the price is different for each of the grades.

3) What is so bad about using just premium, other than the cost? Is it a Hemi thing because this is the first Hemi I've ever owned?

Thanks for whatever comments or suggestions you can provide.

[henry]

The octane rating is simply a measure of the fuel's pre-disposition to spontaneous compression ignition - the lower the octane rating the more prone it is to firing without spark.
The manufacturer recommends the use of
89 octane for optimum performance. The
use of premium gasoline is not recommended.

I think that the word "recommended" in that last statement in the quote should actually read "required".
There's no real danger in using fuel rated higher in octane - it will still burn the same - you just won't see any benefit over 89 octane.

[Rev.Hammer]

you may have increased emissions.

[1cobrakid]

I ran chealp 87 for in my RT for quite a while and lately I've been getting 89 Shell or BP. It runs a little smoother, maybe faster or not. It really gets better gas milage, like 2.5+ better. I've been getting almost 18 around town the last few weeks. I've never calculate if the increased milage offsets the 15 cent plus cost increase. I don't think anything mechanical will be hurt by changing grades, just performance. Hammer may be right, emissions may get thrown off a little too.

[Mike2thaC]

With today's wonderful world of comoputer controlled vehicles, you can pretty much use any grade of fuel........however, using fuel at a lower grade than what is recommended or required will most likely result in lower performance and fuel economy.

If you car is tuned to require 91 octane, then putting in 87 octance will most likely give you knocking, which your computer will sense, and will in turn de-tune the engine to compensate. If you spent extra money to get a car that is over the base model, and it requires a higher than regular octane fuel, it would be best to use the required octane if you don't want to compensate performance.

[Fred]

Amen!

[dece9410]

If anybody is curious as how the vehicle "de-tunes" itself, it does so by retarding spark timing and adjusting the fuel injection curves. Retarding timing will give you less torque, but will reduce knock (pre-combustion). Adding more fuel will likewise change air/fuel ratio, creating a "rich" condition which also reduces the chance for pre-ignition. That's why you may see a slight decrease in fuel efficiency when you go to a lower octane. The engine is adding more fuel to make up for the improve knock reducing qualities of the higher octanne fuels. This will only work to a certain point and using premium fuel in this car will not improve mileage or performance because the stock computer only works within established perameters. In other words, the stock computer will only advance spark timing and reduce fuel injection to a certain point. This is why we need the aftermarket to step in with some software mods to really adjust the perameters to give us better timing and fuel curves, drastically improving performance, but perhaps neccesitating the need for premium fuel.

[markyneutron]

First tank of gas (factory installed) was crap and burnt up fast. First fill up used Shell 89 mid grade and was very impressed with the improvement in mileage and it appears to be getting better by the tank. I can't give hard numbers but FROM my experience so far I'm guessing that if you use 89 as opposed to 87 you will probably come out even as to the amount of money you spend on gas.

[waymur]

Thanks for the assist all concerning the preferred gas issue.