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Discussion Starter #1
Hello all,

I've noticed a few posts about high-octane gas usage. I thought you might find the following interesting. This information is straight from a Chemical Engineer.

1. The "octane" rating on gas only refers to its ability to RESIST detonation. Basically, the higher the octane, the more you can squeeze it and heat it before it explodes. Very high performance engines use high-octane gas because their high compression ratios and/or blowers necessitate it.

2. High octane gas has LESS energy potential than low octane gas. Putting high octane fuel in your car REDUCES horspower output, because you're using a fuel with less energy potential per gallon.

3. If you FEEL like your car puts out more horsepower when run on higher octane fuel, it may be that your engine is out of tune, and that the higher octane gas is reducing pinging, thereby reducing the amount your electric engine management backs off the timing.

4. Using high octane fuel on an engine that doesn't require it can actually create an engine that's an "octane junkie". The cooler burning high-octane fuel can leave carbon deposits, that effectively change your compression and create hot-spots in your cylinders. This increases detonation, and requires the high-octane fuel to prevent it.

Anyway, this info is from a pretty reliable source. Take it for what it's worth!

Half Shovel
 

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Half Shovel said:
Hello all,

I've noticed a few posts about high-octane gas usage. I thought you might find the following interesting. This information is straight from a Chemical Engineer.

1. The "octane" rating on gas only refers to its ability to RESIST detonation. Basically, the higher the octane, the more you can squeeze it and heat it before it explodes. Very high performance engines use high-octane gas because their high compression ratios and/or blowers necessitate it.

2. High octane gas has LESS energy potential than low octane gas. Putting high octane fuel in your car REDUCES horspower output, because you're using a fuel with less energy potential per gallon.

3. If you FEEL like your car puts out more horsepower when run on higher octane fuel, it may be that your engine is out of tune, and that the higher octane gas is reducing pinging, thereby reducing the amount your electric engine management backs off the timing.

4. Using high octane fuel on an engine that doesn't require it can actually create an engine that's an "octane junkie". The cooler burning high-octane fuel can leave carbon deposits, that effectively change your compression and create hot-spots in your cylinders. This increases detonation, and requires the high-octane fuel to prevent it.

Anyway, this info is from a pretty reliable source. Take it for what it's worth!

Half Shovel
Comment #2 is a misleading generalization. There are vehicles (such as the VW Jetta 1.8T and the VW Golf VR6) that can run on regular, but will get more horsepower and torque with the premium fuel. But I do agree that using premium in an engine designed to use only regular, can result in poorer mileage
 

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Discussion Starter #3
outahere said:
Comment #2 is a misleading generalization. There are vehicles (such as the VW Jetta 1.8T and the VW Golf VR6) that can run on regular, but will get more horsepower and torque with the premium fuel. But I do agree that using premium in an engine designed to use only regular, can result in poorer mileage
The reason the VW "gains" horsepower is that it has an electronic timing profile that will take "advantage" of the increased octane and move the timing a bit more aggressively. The fuel itself has no more energy to give, and in fact a bit less.
 

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Half Shovel said:
The reason the VW "gains" horsepower is that it has an electronic timing profile that will take "advantage" of the increased octane and move the timing a bit more aggressively. The fuel itself has no more energy to give, and in fact a bit less.
elaborating on that, vehicles that require a higher octane rating, like the ones you had mentioned, will most likely lose performance if a lower octane is used. the increase in heat that high compression, forced induction engines have will cause knock (which is detonation of the air/fuel mix before it reaches the combustion stroke) if a low octane is used. The sensors will detect the knock and the ecu will retard timing to prevent damage. This will result in reduced performance and fuel economy.
 

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aws316 said:
elaborating on that, vehicles that require a higher octane rating, like the ones you had mentioned, will most likely lose performance if a lower octane is used. the increase in heat that high compression, forced induction engines have will cause knock (which is detonation of the air/fuel mix before it reaches the combustion stroke) if a low octane is used. The sensors will detect the knock and the ecu will retard timing to prevent damage. This will result in reduced performance and fuel economy.
Great Info
 

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Half Shovel said:
2. ..... Putting high octane fuel in your car REDUCES horspower output, because you're using a fuel with less energy potential per gallon.

Half Shovel
Sorry, but this is still a misleading statement. For example, if you put high octane fuel in a 2004 VW Golf (with the 6 cylinder NA motor), the horsepower output will most definitely NOT be reduced, despite the lower energy content of premium fuel compared to regular. Ultimately, the amount of horsepower produced will be more dependent on how much energy can be extracted from the fuel, rather than the energy content of the fuel. The energy content of the fuel is not the limiting factor, but the "energy extraction" process is. And the energy extraction process is improved with higher compression, more aggressive timing, the appropriate air:fuel ratio, supercharging, etc.
 

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I's a matter of matching the correct timing to the correct octane level (Higher octane = more advance, Lower octane = less advance).
I did a quick search on the vr6 and I noticed that VW recommends premium fuel. The timing is set/optimized to use premium. I also noticed the likely culprit to why premium is needed and that is that it has a pretty high compression ratio, 10.5:1 Most people know that an increase in compression ratio will require an increase in fuel octane for the same engine design. Increasing the compression ratio increases the theoretical thermodynamic efficiency of an engine. The efficiency gains are best when the engine is at incipient knock, that's why knock sensors (actually vibration sensors) are used. Low compression ratio engines are less efficient because they can not deliver as much of the ideal combustion power to the flywheel.
Half Shovel said:
2. High octane gas has LESS energy potential than low octane gas. Putting high octane fuel in your car REDUCES horspower output, because you're using a fuel with less energy potential per gallon.
93 octane has a higher octane and thus resisits igniting more than 87 octane gas. Although when they ignite the create the SAME POWER. It all depends on timing.
 

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well said aws, I was wondering when someone was going to bring up other points to octane levels and requirements. :cheers:
 

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Discussion Starter #9
aws316 said:
I's a matter of matching the correct timing to the correct octane level (Higher octane = more advance, Lower octane = less advance).
I did a quick search on the vr6 and I noticed that VW recommends premium fuel. The timing is set/optimized to use premium. I also noticed the likely culprit to why premium is needed and that is that it has a pretty high compression ratio, 10.5:1 Most people know that an increase in compression ratio will require an increase in fuel octane for the same engine design. Increasing the compression ratio increases the theoretical thermodynamic efficiency of an engine. The efficiency gains are best when the engine is at incipient knock, that's why knock sensors (actually vibration sensors) are used. Low compression ratio engines are less efficient because they can not deliver as much of the ideal combustion power to the flywheel.
.
Very well said!

aws316 said:
93 octane has a higher octane and thus resisits igniting more than 87 octane gas. Although when they ignite the create the SAME POWER. It all depends on timing.
Actually, when they ignite, they don't produce the same power per se. The actual energy "content" of high octane gas is slightly lower than "low" octane gas. Sometimes the power that can be gained by advancing the timing (hi octane) exceeds the power lost by using the lower energy, high octane fuel. That's why it "seems" like high octane gas produces more "power" in some engines.
 

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Why higher octane has less energy

To increase the octane rating of a fuel, additives are used. This is often alcohol, which has way less BTU/volume. If the total has less BTU/unit volume, then it has less "energy". This is the main reason that diesel engines get high mileages, they have a heavier fuel with more BTUs per gallon. Engines burn fuel by the pound, but we buy it by the gallon or litre or whatever, but volume. When I was flying DC-6 aircraft, we had to allow for the temperature of the fuel, because warm fuel (say loaded in Guam) had a lot less energy per gallon than cold fuel (say loaded in Kodiak). We knew how many pounds it took to get to our destination, but we had to tell the fuel truck how many gallons to put into the tanks. Al in Benson, AZ
 

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well i dont know alot obout the chemical stuff of gas but ill say this. Normally in my ford i run 87 octane, but when i mud drag it i fill it up whith 92 becouse i can advanse the timing another 6* which is where the power comes from (running about 18* of advance). But i dont realy see much of a point in running higher octane gas unless your having pining problems.
 

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One more thing you must take into account is the fuel quality itself. The refinery's put out winter and summer blends of fuel. The idea is to raise of lower the rvp (reed vapor pressure) We run into this alot in the marine business. People fill the boats up in the winter time for storage and come spring the boat doesn't run. This is because in the winter they triple oxygenate the fuel to lower the rvp. I have tested winter fuel for octane content and after 3 weeks of storage it went from 87 to 64. It all depends on how long it sits in the gas stations tank.


amacd... were you in the coast guard. not many people even know where kodiak is.
 

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I'm really glad this subject came up...my understanding is that Nissan recommends 87 octane for the Titan...I've always used 89 octane in my '01 Frontier, but when I'd occasionally tow a 4-wheeler to hunting camp, would use 93, thinking it would yield more power, but having read the posts above, I understand octane's effect much better. I'll use whatever Nissan recommends...
 

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Briarman said:
I'm really glad this subject came up...my understanding is that Nissan recommends 87 octane for the Titan...I've always used 89 octane in my '01 Frontier, but when I'd occasionally tow a 4-wheeler to hunting camp, would use 93, thinking it would yield more power, but having read the posts above, I understand octane's effect much better. I'll use whatever Nissan recommends...
According to the brochure, they recommend premium fuel, however, I have read a few posts on this site that have said they ammended it to take regular unleaded.
 

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What I really would like to know is will the Titan "use" the extra octane, as in the 93 flavor by advancing the timing and modifying the fuel curve or is the computer preset for 87 only? Anyone know the answer to that?
 
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