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  #13  
Old 02-18-2011, 02:31 AM
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Re: V8 --- what grade of gas to use?

All modern engines have knock sensors and when the onboard computer detects knocking it will either retard timing and/or richen the air/fuel mixture slightly to correct for the problem.

If knocking is detected then richening the air/fuel mixture by adding a little more fuel will affect your fuel economy slightly and reduce power also.

That being said, it is only under heavy load where this is likely to occur. If you drive your car around like a "normal" person all day then higher octane fuel is waste of money. If you are towing a lot of weight, climbing steep grades, and putting the pedal down then a higher octane fuel might yield more power because the computer won't need to retard the ignition timing (less power) or richen the air/fuel mixture (also less power).
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  #14  
Old 02-18-2011, 06:40 AM
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Re: V8 --- what grade of gas to use?

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Originally Posted by Technosavant View Post
Not correct. The engine itself can be designed to require higher grades of fuel. It comes down to the amount of compression in the engine- the more the volume of air in the cylinder is compressed, the more prone to preignition it is. Boyle's Law and all that- temperature and pressure are the same thing; compress a mixture, you will raise the temperature. More volatile fuels (lower octane) WILL preignite in very high compression engines. There's a reason many forced induction engines require premium fuel- high compression.

The more compression, the more efficient the engine, but that comes at a price. You can squeeze a few more HP out of an engine by increasing the compression, but that will result in preignition with low grades of fuel. This is why race engines generally require VERY high octane (100+) fuel; they're set up to get the most power possible out of a given engine size.

You are correct that the electronic engine controls can adjust timing on the fly to reduce preignition or to take advantage of less volatile fuel, but that isn't the full story. Usually the engine computer will dial things back to prevent damage. However, if the manufacturer intended for lower grades to be used to give maximum performance out of the engine, a higher grade of fuel will NOT give more power. The engine controller is not capable of just cranking things up like that. It will go to what it is designed (both the engine combustion chamber and the computer) and no more.

Putting a higher grade of fuel into a vehicle than it is intended to require is wasting money; anywhere from ten to thirty cents per gallon, depending on the grades involved and the price differentials. If you're putting premium into a Grand Cherokee on a 20 gallon fill-up (I'm assuming you don't run it all the way dry), that's somewhere between $2 and $6 every tankful. Not going to break you, but it adds up.

Again in two words - NO HARM to use higher octane and try yourself, the rest is yap-yap having no relations to what I said. There is no evidence of any such wonderland designs in this or any other car, book said "ACCEPTS 87" for V6 and that is all. It is minimum requirements, same as for example you could run this program on your computer with min 256mb of RAM installed or any other min requirement.
Using higher octane is always better to the any gasoline engine. If you are saving money or having no money or just cheapo that is your personal problems having no relation to the question. Personally no needs of for me as I said, my driving is mostly heavy traffic, in this case who cares what gas is in tank?

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Old 02-18-2011, 02:52 PM
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Re: V8 --- what grade of gas to use?

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Originally Posted by Breitling65 View Post
Again in two words - NO HARM to use higher octane and try yourself, the rest is yap-yap having no relations to what I said. There is no evidence of any such wonderland designs in this or any other car, book said "ACCEPTS 87" for V6 and that is all. It is minimum requirements, same as for example you could run this program on your computer with min 256mb of RAM installed or any other min requirement.
Using higher octane is always better to the any gasoline engine. If you are saving money or having no money or just cheapo that is your personal problems having no relation to the question. Personally no needs of for me as I said, my driving is mostly heavy traffic, in this case who cares what gas is in tank?

Higher octane is only better in engines that require it to prevent pre-detonation, contrary to popular belief it is not more volatile than lower octane gasoline. The manual for the Hemi says that 87 is ok, but 89 is recommended. So use 89 unless you are at 4,000 above sea level or higher in which case you can use 87. Bottom line, use whatever grade is recommended, anything higher is simply throwing your money away.
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Old 02-18-2011, 03:13 PM
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Re: V8 --- what grade of gas to use?

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Originally Posted by vikingdave View Post
You are completely mis-informed about higher octane always being better. Higher octane is only better in engines that require it to prevent pre-detonation, contrary to popular belief it is not more volatile than lower octane gasoline.
Trust me, its not going to work. We all tried to knock some sense into him the last gasoline thread and it didnt work lol.
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Old 02-18-2011, 03:27 PM
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Re: V8 --- what grade of gas to use?

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Old 02-18-2011, 03:30 PM
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Re: V8 --- what grade of gas to use?

There is a higher BTU content, potential energy, in 87 octane compared to higher octane fuel. An 89 octane fuel is actually a blend-at-the-pump mix of 91/93 and 87 to yield the 89 rating. The Hemi in the WK2 is designed to maximize performance with 89 octane fuel and has the ability to run the lower octane albeit at a marginal performance reduction (may or may not be noticeable). The octane rating is resistance to detonation.

Some engines are designed to maximize power with higher compression and require premium. i.e. 91/93 octane. These engines CAN run 87 octane, but the ECM will be retarding the timing and adjusting the fuel mixture to avoid detonation which reduces the engine's power potential often to noticeable levels. Some engine designs have combustion chambers that reduce/eliminate hot spots so that a higher compression, i.e. 10:1 or 11:1, can be used with 87 octane fuel like the Pentastar.

For modern computer-controlled engines, running with a higher octane than what an engine is designed for yields zero improvement in performance; it could even be argued that power/economy may be marginally reduced due to the lower BTU content of the higher octane fuel. The contrary point is if an engine experiences detonation running on the fuel for which it was designed; if a particular Hemi had detonation running 89 octane, then it would behoove the owner to run 91/93 if that rectifies the problem. A possible explanation for this occuring would be either poor fuel quality at a particular station or the gas station owner is cheating.
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Old 02-18-2011, 03:31 PM
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Re: V8 --- what grade of gas to use?

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So what are you saying? fill our Jeeps up with Jet A? Yeah that will work haha
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Old 02-18-2011, 03:40 PM
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Re: V8 --- what grade of gas to use?

I fill mine up with the mid-grade.. whatever octane that may be. Here where I live, it's 87.. up in the mountain towns, it could be 85.
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  #21  
Old 02-18-2011, 03:45 PM
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Re: V8 --- what grade of gas to use?

So if only 87 or 91 are available at the station, which should I choose?
I have been using the 91.
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Old 02-18-2011, 03:51 PM
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Re: V8 --- what grade of gas to use?

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Originally Posted by metalaxe View Post
So if only 87 or 91 are available at the station, which should I choose?
I have been using the 91.
If you are at an altitude of more than 4,000 feet the 87 will give the same performance that 89 normally would (at this altitude). If you are at sea level it's a tougher call, the 91 is more than you need but the ECU will dial the engine timing back a bit on the 87. Find a different gas station.
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Old 02-18-2011, 04:06 PM
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Re: V8 --- what grade of gas to use?

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Originally Posted by metalaxe View Post
So if only 87 or 91 are available at the station, which should I choose?
I have been using the 91.
T'were it me, I would run the 91.

As breitling says, higher octane won't hurt. You just won't get any benefit over 89.
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Old 02-19-2011, 04:58 PM
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Re: V8 --- what grade of gas to use?

mid-grade 89 3.31 a gallon.
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