Octane for the Hemi
Joined: Dec 2006
Posts: 10
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RE: Octane for the Hemi
ORIGINAL: Walt
Thanks stump_breakerfor your response. You've got it correct. Its related to the compression ratio of your engine. Sure the temperature is a bit higher with higher octane but thats not was gives you more power. Octane is the compression thatthe fuelcan take. If you run low octane in a high compression engine it will cause a premature explosion in the cyclinder which causes uneven burning and pitts in the cylinder wall. Thats where all the crap comes from. Sure, your safewith higher octane but if your engine isn't designed to force more air and fuel into the mixture to achieve a higher compression then its waisted money and you don't get much more power for your money. I race bikes that have to run at least 110 octane. If I don't it will destroy the engine. If you have remaped your fuel injection with a computer and are allowing more fuel and air into the cylinders I would be careful running 87. You can install an air intake but, once you've remaped your computerfor more horsepower, keep in mind your forcing more air and fuel into the cylinderfor that power which raises your compression, which in turn requires you to raise your octane. Every variableis related to the other. Your debating single aspects of the whole process. Some of you may have mods and some may not. When I race bikes in the mountains I use a different fuel injection map which adjust for lower air pressure. If I don't I will loose a lot of power. Its not magic, its physics and its the same principles with all internal combustion engines.
Thanks stump_breakerfor your response. You've got it correct. Its related to the compression ratio of your engine. Sure the temperature is a bit higher with higher octane but thats not was gives you more power. Octane is the compression thatthe fuelcan take. If you run low octane in a high compression engine it will cause a premature explosion in the cyclinder which causes uneven burning and pitts in the cylinder wall. Thats where all the crap comes from. Sure, your safewith higher octane but if your engine isn't designed to force more air and fuel into the mixture to achieve a higher compression then its waisted money and you don't get much more power for your money. I race bikes that have to run at least 110 octane. If I don't it will destroy the engine. If you have remaped your fuel injection with a computer and are allowing more fuel and air into the cylinders I would be careful running 87. You can install an air intake but, once you've remaped your computerfor more horsepower, keep in mind your forcing more air and fuel into the cylinderfor that power which raises your compression, which in turn requires you to raise your octane. Every variableis related to the other. Your debating single aspects of the whole process. Some of you may have mods and some may not. When I race bikes in the mountains I use a different fuel injection map which adjust for lower air pressure. If I don't I will loose a lot of power. Its not magic, its physics and its the same principles with all internal combustion engines.
The hemi was designed ot run on 89 octane but I'd bet that is at lower altitudes (higher air density). For those of us at higher altitudes (lower density), we'll get lower cylinder pressures and can use lower octanes; we'll also get less fuel mileage because our engines are less efficient with the thinner air.
I've run 87 (mid-grade for me) and 91 (premium) and the ONLY difference has been cost. If I drive the truck in the same manner, the difference in mileage is statistically insignificant.
Joined: Mar 2006
Posts: 41
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From:
RE: Octane for the Hemi
there is a gas station in my area that only has 2 tanks in the ground, low and mid grade are in the same tank so what octane are you really getting and there is one for high grade--what a rip off. but back to the point i have used all 3 grades of gas and i have not noticed the difference in my 2005 ram 1500 with the hemi. My foot is so heavy i dont think it matters with what gas i use---foot to the floor i better go pump some more...
Captain
Joined: Dec 2004
Posts: 679
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From: New Castle Delaware
RE: Octane for the Hemi
No station is allowed to mixmore then one grade of fuelinto one tank.It's a federal law that says no you can't do that. If they are caugt they will be shut down. trust me. One of my jobs whiletesting octane values was to report stations that violated this rule. If what you say were true you'd get the cheaper grade, right.
Joined: Dec 2006
Posts: 10
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RE: Octane for the Hemi
Nope, compression ration is fixed by the bore and stroke. Forcing additional fuel/air into the cylinder raises the cylinder pressure but the compression ration is the same.
So, if you have a barometric pressure of 14.7 lbs and a compression ratio of 10 to one. Your clylinder pressure (assuming 100% efficiency) would be 147 lbs. Now, if you add 8 lbs of boost, you start with 22.7 and get 227.
The ratio is fixed by the bore/stroke of the engine.
So, if you have a barometric pressure of 14.7 lbs and a compression ratio of 10 to one. Your clylinder pressure (assuming 100% efficiency) would be 147 lbs. Now, if you add 8 lbs of boost, you start with 22.7 and get 227.
The ratio is fixed by the bore/stroke of the engine.
