wow Pat, I'm surprised...down here, all of the Shell stations are almost always $.10 higher than everyone else...

 

There is no more energy in a gallon of 91 octane gas than there is in a gallon of 87 octane gas. Higher octane gas simply is more resistant to expoding at higher temperatures. So in higher compression ratio engines (such as the X-fire's), the owner's manaul calls for 91 octane fuel. The higher the compresion ratio....the higher the heat caused by the compression process. Low octane fuels may explode (ping) just from the heat of compression.....for instance .....diesel fuel might be described as having a super low octane rating ( explodes much easier than gasoline....that's why it's used by terrorists in bombs!)...diesels don't need spark plugs except to start the engine I think.....they fire just from the heat of compression.

So, higher compression ratio engines call for premium.....most nowadays have computerized "Knock-sensors" and will adjust timing if you put lower octance fuel in.....BUT.....you won't get the maximum horsepower from the engine.....

The whole purpose of engines designed with higher compression ratios......you're shoving more AIR & FUEL into the volume of the cylinder....before you light the match......power is determined by how much 'Stuff" you can blow up.....to put that into perspective ....you can have 2 engines with the same displacement...say 3.2L (like the X-fire).....if one has a 9.0 compression ratio & the other a 10.5 ratio.....you are shoving more fuel-air mixture into the 10.5 one on each firing = more power!!

If you put 87 in....you will either get premature detonation (ping).....or, the engine's sensors will adjust to dumb down the process to prevent the ping.

 

I find Shell cheaper. It now cost me about 15 cents less. Reason being is the Shell Master card with the 5% cash rebate and 1% rebate on all other purchases.. It works if paid in full monthly...cheers Gary

 

Informative reply, Chris. My buddy, Smitty, was a Merchant Marine and spent many tours hauling gas. He always told me to buy the cheapest gas you can find because it's basically all the same. He delivered to all the major brands and the only difference was what they put into it at their tank farms.

I've used 87 octane in many of my V-8's including my company car. They always performed well with no problems. I have a Hemi in my Dodge Quadcab but I use 89 octane in that. Occasionally, I use premium and I can actually feel a slight difference in performance. I've only used premium in the XFire, either 92 octane Gulf or Ultra 93 Sunoco. It would be hard for me to put a lesser octane in my XFire. I just don't feel like I want to try a lesser octane in that car.

Bob

 

Our Shell station used to be a Crown and they undercut everybody by at least a nickel a gallon. When they switched the station to Shell they kept the prices and even go so far as to price mid level and high test the same price. Bunch of nice guys run the place, they always say hello Mr. Pat and they took care of my son one night when he forgot his wallet. Nice people.

 

I use Mobil 93. The Mobil station is less than 2 miles from my house.
I would prefer Shell, but there are none in my town. There are some ~10 miles away though.

 

so far we agree and you are correct.

Correct again, at least to some extend

Now here is the section where you either did not understand what is going on or you did not do your homework. Let me fill you in: Gas (or petrol for our british friends) has a flashpoint of -40F, and the flashpoint of diesel is ABOVE 200F (I don't have the exact number of the top of my head right now) Flash point is the temperature where you can still ignite fumes above liquid. From that alone you can see that diesel is much harder to ignite. There are a few more (lesser important) parameters but the most important one is the flashpoint.

The next problem is that even if you have a flammable mixture of correct proportions a tiny spark as we have the gas engine would not ignite diesel fumes. There is just not enough energy in that spark.

Diesel gets ignited because the compression of a diesel engine is a MINIMUM of 18-19 and when you compress air fast it heats up. So when the diesel is injected the whole air diesel mixture is at well above 500F and that ignites the diesel.

The whole procedure is just very different to how gas engines work.

Correct again

The formula Higher compression = more power is not wrong for the most part, but it is also not correct. There are a number of other parameters that come into play here. (see above the comment about diesels) Diesels have MUCH higher compression and they have even turbo chargers, yet do they have MUCH more power? It depends. Like I said there is a host of other factors to be considered. Flame speed being one of them, energy content another, and then how they all work together.

Somewhat correct: In a static engine that has a high compression ratio you CAN get premature detonation (most of the time you have the still hot spark plug as ignition source) but again it depends. In the crossfire engines, while it is not recommended (and you will loose 2-5HP) it will not damage the engine. Please note though that if you run it hard, frequently hit redline, accelerate hard, you will be better off with 92-93 Octane. If you just have the crossfire because it looks kewl and very seldomly hit redline or accelerate hard the regular stuff is just fine.

 

I fill 93 Octane, but I don't see why 91 would be any worse....

 

Shell user here as well. Premium only. Premium in Virginia is about 20 cents higher on average than mid-test. I use Shell because of top tier gasoline which I learned about through the forum a while back in a similar thread.

http://www.toptiergas.com/index.html

Shell used to be about 10 cents more expensive than the 'bargain gas' stations around my college in Fredericksburg, VA such as Wawa. However since prices have been going up Wawa gas has become evenly priced with Shell in Fredericksburg. Not only that but near my home in Northern VA (20 min. from DC) gas prices are now equal to the prices in Fredericksburg. Prices in Fred used to be significantly cheaper than NoVA up until 3 months ago or so.

 

Mobil 93 Octane or Sunoco 94 Octane. Nothing under 93 octane.

 

Sadly, the best we can get out here (without adding race gas) is only 91 octane....

 

Where did you get your 2-5 hp number from? Or is that sort of a....back-of-the-buttcheeks sort of number?

 

Yes. This is a number that is as you put it so eloquent "back of the butt cheeks" I guess what I am trying to say and was unable to: "if you use regular in X-fires, unless you drive it very hard you will not experience pinging and it will not damage the engine, but you will loose a minute (1-20 HP) power.

IF OTH you drive it hard, then by all means use premium. (Or super for our EU friends). Another thought: If you run regular on a regular basis then changing the spark plugs to one step colder may be a smart idea. Just an additional thing to keep detonation from occurring.

 

I use Shell premium. I have their MC and get a rebate on gas. Just put the 4th tank full in yesterday in my new 08 roadster autostick. Last tank was 27.89 MPG. This is slightly better than my 06 base stick that I traded. I just might have drove the other one a little harder.LOL

 

Apleschu.....thanks, I was hoping someone with a better understanding would clear it up a bit. So if you were to guess about something like an octane rating for diesel....what would it be?

Grey ghost, how much did you have to put with your '06 to buy the '08?

 

Shell V-POWER 99 OCTANE WITH NOS OCTANE BOOSTER RACING FORMULAR (UPTO 7 POINT INCREASE)

 

There is no octane rating for diesel. Just does not work. (you can't measure distance in gallons either LOL)

For diesel the rating is called cetane, and if there is any comparison, at all then you can roughly say cetane is the exact opposite of octane, but then again, even that is a VERY weak and wrong comparison. The cetane number of diesel fuel goes up the easier the fuel is ignited AND the more energy content the fuel has. In gas(petrol) there is no reference to energy content, in cetane/diesel there is. In essence a cetane of 50 is very good fuel, has good flammability and high energy content, 40 and below is getting iffy, but modern diesel engine will use that as well, (just use a bit more of it because the energy content goes down) Kind of like the gas/alcohol mixture. But since diesel is an oil to begin with the engines are much more relaxed in terms of which fuel they want.


But bottom line: You CANNOT compare octane to cetane they are two different measuring schemas.

 

Since I have worked for Shell for all of my professional career. I use nothing but Shell.. Since there's absolutely no better gas

On a side note, octane rating is not a rating of how combustable a gas is or how much compression it can handle before ignition, rather octane is an alkalyde C8H18. Being able to handle compression before premature ignition is a property of octane.

 

Premium BP/Amoco makes my car run the best from my experiences. I used to run Shell but I don't like how none of their stations around here accept debit at the pumps, and it's also the most expensive.

 

you do know that a 7 point increase means it goes from 99octane to 99.7 octane right? that octane booster is crap. get something like torco or klotz octane booster. the klotz would raise it from 99 octane to 101.5 octane with just one ounce per gallon.