87 Octane vs 91 Octane - Fact or Myth?
03-21-2004, 11:32 AM
#26
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After trying all available brands of gas in my area (Inland Empire in Southern California) and all 3 available octanes, I've come to the conclusion that gas brand matters more than octane rating. I run 87 regular in my 8 (6mt) and the car runs great. I use Shell gas exclusively now (will use Chevron in a pinch) and although Shell regular is more expensive than discount gas, it outperforms them all at any octane. I found Arco gas, even their premium, to be the worst....it was the cheapest...in every way. Costco gas didn't work very well either; Sam's Club gas was ok but not great.
Bottom line? Don't bother with cheap gas at any octane. If you haven't tried it yet, try a couple tanks of Shell regular and see how it works for you. It may very well be that different cars might need 89 or 91 octane because of production variances in construction, but the average RX8 should be just fine with Shell regular; and you'll save money too.
Bottom line? Don't bother with cheap gas at any octane. If you haven't tried it yet, try a couple tanks of Shell regular and see how it works for you. It may very well be that different cars might need 89 or 91 octane because of production variances in construction, but the average RX8 should be just fine with Shell regular; and you'll save money too.
03-21-2004, 04:36 PM
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Ole Spiff,
I'm using 87 octane Shell too. If I may make one more suggestion ....... if you decide on Shell and want to save some $$$, consider getting a free Shell credit card which will save you 5% off Shell purchases and 1% of your non-Shell purchases can be used towards you Shell purchases. You can usually find applications at the stations or check Shell's web site.
rx8cited
I'm using 87 octane Shell too. If I may make one more suggestion ....... if you decide on Shell and want to save some $$$, consider getting a free Shell credit card which will save you 5% off Shell purchases and 1% of your non-Shell purchases can be used towards you Shell purchases. You can usually find applications at the stations or check Shell's web site.
rx8cited
03-25-2004, 02:22 PM
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Well, I'm a new RX-8 (automatic) owner and am getting 12.5 mpg in the city (L.A.)! I've been using the recommended high octane fuel (at $2.35 a gallon!!!) and think I should be bankrupt by next winter.
I'm going to try Shell 87 octane for 2 tanks in a row and see what the heck happens. Fingers crossed. This 12.5 is very upsetting.
Granted, I only have 400 miles on the car and they say the engine gets better with miles... But, please, 12.5?
I'm going to try Shell 87 octane for 2 tanks in a row and see what the heck happens. Fingers crossed. This 12.5 is very upsetting.
Granted, I only have 400 miles on the car and they say the engine gets better with miles... But, please, 12.5?
03-25-2004, 02:34 PM
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Originally posted by misterdc
Well, I'm a new RX-8 (automatic) owner and am getting 12.5 mpg in the city (L.A.)! I've been using the recommended high octane fuel (at $2.35 a gallon!!!) and think I should be bankrupt by next winter.
I'm going to try Shell 87 octane for 2 tanks in a row and see what the heck happens. Fingers crossed. This 12.5 is very upsetting.
Granted, I only have 400 miles on the car and they say the engine gets better with miles... But, please, 12.5?
Well, I'm a new RX-8 (automatic) owner and am getting 12.5 mpg in the city (L.A.)! I've been using the recommended high octane fuel (at $2.35 a gallon!!!) and think I should be bankrupt by next winter.
I'm going to try Shell 87 octane for 2 tanks in a row and see what the heck happens. Fingers crossed. This 12.5 is very upsetting.
Granted, I only have 400 miles on the car and they say the engine gets better with miles... But, please, 12.5?
there was one member a while ago who had a temp sensor issue, and the engine wouldn't come out of cold-start mode and he couldn't get better than 12mpg or something.
you may be having a malfunction of something. see your dealer about it if it persists and is very consistantly low.
10-08-2004, 08:14 AM
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Originally Posted by pp13bnos
I guess I'm out of the norm for running higher octane. 
I talked about it in anouther thread. But, I'll give a breif sum of what happend. I dyno'd my 6spd Rx-8 with around 4-5 gallons of 110 oct. unleaded fuel. I put down 161rwhp. My buddy dyno'd right after me, in his 6psd, with 91 octane. He put down 154rwhp. Bolth of us had around 2,500 miles. Bolth with 6spds. My car was Red with the app. package...his was silver without. Perhaps the wing added the extra rwhp? I guess we'll never know.....
And btw, my car made peak power right at the rev limiter, and his made it around 1k rpms less than mine. CJ
I talked about it in anouther thread. But, I'll give a breif sum of what happend. I dyno'd my 6spd Rx-8 with around 4-5 gallons of 110 oct. unleaded fuel. I put down 161rwhp. My buddy dyno'd right after me, in his 6psd, with 91 octane. He put down 154rwhp. Bolth of us had around 2,500 miles. Bolth with 6spds. My car was Red with the app. package...his was silver without. Perhaps the wing added the extra rwhp? I guess we'll never know.....
And btw, my car made peak power right at the rev limiter, and his made it around 1k rpms less than mine. CJ
we need to remember the purpose of a dyno. it is a tool to measure horsepower. you could take two identical cars and they will have a different hp number. the correct way to use a dyno is: take your car and make a baseline run and note the number. make the changes you're going to make, then run it again and note the number. if it increased good, if it decreased, then find out why.
i go round and round about this with my motorcycle friends as well.
on point of octane, the octane rating only means the point in which it detonates. if you use a gas with a higher octane than needed, the engine is not detonating efficiently.
my .02, but i could be all wet.........since it rained 5" last night.
yamajj
Last edited by yamajj; 10-08-2004 at 08:17 AM.
10-08-2004, 11:20 AM
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With regards to dyno readings, keep in mind that mass production cars and engines are not by any means perfect. There is a variable called "tolerance" which means an assembly can be within a certain percentage of blueprint spec and still be acceptable for production purposes. This is going to produce some cars that are better than others off the same assembly line....by a small margin. Every once in a while you'll get a car that gets lucky and is VERY close to blueprint; it will have great power and performance; and on the other side of that coin you'll get one that's unlucky and is pretty far off and those are usually considered a "lemon."
This means you're going to get a variance of power and performance abilities from one car to the next. The only way to "fix" that would be to spend the time and money to "blueprint" your engine yourself; which is the process of bringing it to exact blueprint spec. Theoretically this would give you an engine that produces exactly the power rating that the blueprint calls for; anything less than blueprint will give you less power. This is why some cars have slightly more power than others that have come from the same assembly line.
Manufacturers could tighten their tolerances but that would slow things down during production and the price of the car would go up significantly. This is why hand-built race cars are so much more expensive than a mass-production car; it's not only the components that cost more, it's the amount of time and labor involved to make it exactly to spec.
So based on all the above, a 5 to 6 horsepower difference between one 8 or the other is normal and doesn't mean there's anything wrong with the cars or the testing method. This also means some cars will not respond well to 87 octane because they are probably on the edge of tolerance, while others will respond very well because they are closer to spec. Apparently I got lucky; my car runs great with it. Others claim pinging and knocking and a loss of performance. Try it yourself and if it works, great. If not, then go to the next higher octane until your car runs okay. But as I've stated earlier, get a good brand of gas; not cheapo gas. I recommend Shell here on the west coast of the U.S., but there may be an equally good brand in other areas...but go with a good brand; don't get cheap gas!
This means you're going to get a variance of power and performance abilities from one car to the next. The only way to "fix" that would be to spend the time and money to "blueprint" your engine yourself; which is the process of bringing it to exact blueprint spec. Theoretically this would give you an engine that produces exactly the power rating that the blueprint calls for; anything less than blueprint will give you less power. This is why some cars have slightly more power than others that have come from the same assembly line.
Manufacturers could tighten their tolerances but that would slow things down during production and the price of the car would go up significantly. This is why hand-built race cars are so much more expensive than a mass-production car; it's not only the components that cost more, it's the amount of time and labor involved to make it exactly to spec.
So based on all the above, a 5 to 6 horsepower difference between one 8 or the other is normal and doesn't mean there's anything wrong with the cars or the testing method. This also means some cars will not respond well to 87 octane because they are probably on the edge of tolerance, while others will respond very well because they are closer to spec. Apparently I got lucky; my car runs great with it. Others claim pinging and knocking and a loss of performance. Try it yourself and if it works, great. If not, then go to the next higher octane until your car runs okay. But as I've stated earlier, get a good brand of gas; not cheapo gas. I recommend Shell here on the west coast of the U.S., but there may be an equally good brand in other areas...but go with a good brand; don't get cheap gas!
10-08-2004, 10:55 PM
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The main difference between octanes is how fast they burn. The higher the rating, the slower the fuel burns. There is NO power to be gained by running a higher octane than what the factory recommendation is. The factory knows what octane will work best in conjunction with an engines timing.
The octane rating system was developed to standardize the anti-knock quality of gas based on, not the performace level or quality of the gas. The idea that higher octane is better perfoming is pure marketing B.S. All octanes go through the same refinement process and contain the same addatives and detergents. If a manufacture says to run 87, running a higher octance is NOT needed, unless there is a problem with your engine, or if you have advanced your timing more than a few degrees. Infact, depending on the engine you may lose HP by doing this. Of course this means that you should never run a lower octance than recommended because it very well could cause knock. I believe that is where all the confushion begain about running higher octane as being safer.
I just got through all of this for the 2nd time in my college automtive courses (no I didn't fail a class, they cover fuel in 2 different classes).
The octane rating system was developed to standardize the anti-knock quality of gas based on, not the performace level or quality of the gas. The idea that higher octane is better perfoming is pure marketing B.S. All octanes go through the same refinement process and contain the same addatives and detergents. If a manufacture says to run 87, running a higher octance is NOT needed, unless there is a problem with your engine, or if you have advanced your timing more than a few degrees. Infact, depending on the engine you may lose HP by doing this. Of course this means that you should never run a lower octance than recommended because it very well could cause knock. I believe that is where all the confushion begain about running higher octane as being safer.
I just got through all of this for the 2nd time in my college automtive courses (no I didn't fail a class, they cover fuel in 2 different classes).
Last edited by s13lover; 10-08-2004 at 11:15 PM.
10-08-2004, 11:15 PM
#33
Since my above-referenced thread has mysteriously dissapeared, I will post up what I have written before:
Gasoline engines of all types are driven by the same process - combustion.
Combustion is the rapid release of energy from a fuel - in this case gasoline.
A finite quantity of gasoline contains an equally finite amount of energy, which can be released by combining it with a specific quantity of oxygen at an absolute ratio.
1 kilogram of gasoline contains 43 megajoules of energy that can be released via oxygen at a ratio of 1:15.179 (a ratio referred to as "stoichiomentric"). That is about 41,700 BTUs. Plenty.
The trick with an internal combustion engine (ICE), whether it be rotary or other wise, is to control that combustion in space and time.
We do this by causing the combustion process to occur in the combustion chamber at a precise time and OVER a precise quantity of time to convert that heat into torque.
To effect this level of control, we must take a fixed quantity of space (about 650 ml in the case of the 13b-MSP), fill it with a quantity of gas and air as proscribed by the above ratio, compress it to a precise degree and ignite it at precisely the right time as to cause the maximum pressure increase resultant from that combustion to occur at the time of maximum delta for the combustion chamber's swept cyclic volume (that is to say at the precise moment that the combustion chamber is starting to get bigger again after it just got done getting smaller to compress the charge).
The beauty of this process is that it can occur completely independent of any change in factors in the outside world - temperature, pressure, altitude, pollution, humidity, whatever - as long as we can be
assured that these conditions inside the combustion chamber are constant.
The problem is, we can't.
Because the process of getting a fixed volume of O2 into the combustion chamber at a proscribed density (meaning temperature via Avagadro) is complicated by the fact that this air is supplied by the available atmosphere, we are straddled with the effects of varying density on the combustion charge.
What that means is we must compensate for the volatility of gasoline as it responds to the varying charge densities. At differing charge densities, the amount of energy necessary to start the combustion process and the time it takes to complete the combustion process changes in a not so linear fashion.
So, what we do is vary the amount of fuel we add and ignite the process on a adjustable schedule based on what information we can obtain about the conditions of the air going into our ICE.
What we measure in the case of the RX-8 to know these conditions are these:
Air Flow
Intake Air Temperature
Barometric Pressure
Coolant Temperature
Throttle Position
Eccentric Shaft Position
By computing all of these measurements together, the engine control unit (ECU - sometimes called PCM for powertrain control module) can determine APPROXIMATELY the density of the air charge in the combustion chamber at any given time. It is a shame, really, that there is no way to measure the density directly or we could forgo all of this.
Two factors that can't be measured by the above methods are important to the whole equation as well.
First is volumetric efficiency (VE) or the amount of air, as a percentage of maximum, that the engine actually ingests as a result of the physics of mass and inertia. This number is fixed to some degree and changes at different RPM.
The other is the latent temperature of the actual combustion chamber as a result of the combustion cycles that proceed the cycle under scrutiny at that moment. This changes as a result of RPM as well, but it is also tied to 'load' or the increase of RPM over time as a proportion to charge density.
What that leaves us with is a very crude measurement of the total charge density.
How do we compensate for that?
By conservative 'hedging' on the bet that is ignition timing through advancement and retardation of the onset of the spark and by introducing elements into the gasoline that seek to stabilize its volatility. That is what octane is for and how much it affects the combustion process is measured by various methods including, but not limited to, Research Octane (RON), Motor Octane (MON) and the Anti Knock Index (AKI - and average of the RON and MON numbers). Unfortunately for the average motorist, many other ingredients are added to the fuel we use to affect its environmental impact that are not directly computed into the AKI. Ingredients are added to lower the boiling point and vapor point, reduce the hydrophilic nature of gas and reduce the amount of oxides of nitrogen after the combustion process. Many, if not most, of these ingredients change the combustion process in ways that may not be consistent from sample to sample. They also alter the total energy content of the fuel itself.
As it has been mentioned previously in this thread, having the ignition process starting at the wrong time (especially too soon) is a bad thing and can (especially in the case of the rotary ICE) quickly destroy a motor. So what is done, more often than not, is to err on the side of safety and bracket the combustion process with extra fuel and start the ignition with a slightly delayed spark. What this does is lower the temperature of the intake charge and insure that the combustion process is slower and later than optimal and never faster or sooner. Raising the AKI of the fuel used will accomplish the same thing but since the manufacturer of the vehicle can't insure that the fuel used will always have the proper AKI to achieve this or won't contain additives that adversly affect the ignition onset, they don't depend on their octane recommendation alone.
What is done by "tuners" then, is to take into account this margin of error and dial some of it out for more power which can be achieved by charge composition that is closer to optimal. To achieve this, they depend on the operator to use fuel that takes up the slack in AKI and remove some of this extra fuel and spark retardation.
Really, that is all there is to it. How good a tuner can be is dependant on his or her ability and knowledge with respect to the events within the combustion chamber in question. Unfortunately, that knowledge usually comes at the price of some blown-up motors.
Gasoline engines of all types are driven by the same process - combustion.
Combustion is the rapid release of energy from a fuel - in this case gasoline.
A finite quantity of gasoline contains an equally finite amount of energy, which can be released by combining it with a specific quantity of oxygen at an absolute ratio.
1 kilogram of gasoline contains 43 megajoules of energy that can be released via oxygen at a ratio of 1:15.179 (a ratio referred to as "stoichiomentric"). That is about 41,700 BTUs. Plenty.
The trick with an internal combustion engine (ICE), whether it be rotary or other wise, is to control that combustion in space and time.
We do this by causing the combustion process to occur in the combustion chamber at a precise time and OVER a precise quantity of time to convert that heat into torque.
To effect this level of control, we must take a fixed quantity of space (about 650 ml in the case of the 13b-MSP), fill it with a quantity of gas and air as proscribed by the above ratio, compress it to a precise degree and ignite it at precisely the right time as to cause the maximum pressure increase resultant from that combustion to occur at the time of maximum delta for the combustion chamber's swept cyclic volume (that is to say at the precise moment that the combustion chamber is starting to get bigger again after it just got done getting smaller to compress the charge).
The beauty of this process is that it can occur completely independent of any change in factors in the outside world - temperature, pressure, altitude, pollution, humidity, whatever - as long as we can be
assured that these conditions inside the combustion chamber are constant.
The problem is, we can't.
Because the process of getting a fixed volume of O2 into the combustion chamber at a proscribed density (meaning temperature via Avagadro) is complicated by the fact that this air is supplied by the available atmosphere, we are straddled with the effects of varying density on the combustion charge.
What that means is we must compensate for the volatility of gasoline as it responds to the varying charge densities. At differing charge densities, the amount of energy necessary to start the combustion process and the time it takes to complete the combustion process changes in a not so linear fashion.
So, what we do is vary the amount of fuel we add and ignite the process on a adjustable schedule based on what information we can obtain about the conditions of the air going into our ICE.
What we measure in the case of the RX-8 to know these conditions are these:
Air Flow
Intake Air Temperature
Barometric Pressure
Coolant Temperature
Throttle Position
Eccentric Shaft Position
By computing all of these measurements together, the engine control unit (ECU - sometimes called PCM for powertrain control module) can determine APPROXIMATELY the density of the air charge in the combustion chamber at any given time. It is a shame, really, that there is no way to measure the density directly or we could forgo all of this.
Two factors that can't be measured by the above methods are important to the whole equation as well.
First is volumetric efficiency (VE) or the amount of air, as a percentage of maximum, that the engine actually ingests as a result of the physics of mass and inertia. This number is fixed to some degree and changes at different RPM.
The other is the latent temperature of the actual combustion chamber as a result of the combustion cycles that proceed the cycle under scrutiny at that moment. This changes as a result of RPM as well, but it is also tied to 'load' or the increase of RPM over time as a proportion to charge density.
What that leaves us with is a very crude measurement of the total charge density.
How do we compensate for that?
By conservative 'hedging' on the bet that is ignition timing through advancement and retardation of the onset of the spark and by introducing elements into the gasoline that seek to stabilize its volatility. That is what octane is for and how much it affects the combustion process is measured by various methods including, but not limited to, Research Octane (RON), Motor Octane (MON) and the Anti Knock Index (AKI - and average of the RON and MON numbers). Unfortunately for the average motorist, many other ingredients are added to the fuel we use to affect its environmental impact that are not directly computed into the AKI. Ingredients are added to lower the boiling point and vapor point, reduce the hydrophilic nature of gas and reduce the amount of oxides of nitrogen after the combustion process. Many, if not most, of these ingredients change the combustion process in ways that may not be consistent from sample to sample. They also alter the total energy content of the fuel itself.
As it has been mentioned previously in this thread, having the ignition process starting at the wrong time (especially too soon) is a bad thing and can (especially in the case of the rotary ICE) quickly destroy a motor. So what is done, more often than not, is to err on the side of safety and bracket the combustion process with extra fuel and start the ignition with a slightly delayed spark. What this does is lower the temperature of the intake charge and insure that the combustion process is slower and later than optimal and never faster or sooner. Raising the AKI of the fuel used will accomplish the same thing but since the manufacturer of the vehicle can't insure that the fuel used will always have the proper AKI to achieve this or won't contain additives that adversly affect the ignition onset, they don't depend on their octane recommendation alone.
What is done by "tuners" then, is to take into account this margin of error and dial some of it out for more power which can be achieved by charge composition that is closer to optimal. To achieve this, they depend on the operator to use fuel that takes up the slack in AKI and remove some of this extra fuel and spark retardation.
Really, that is all there is to it. How good a tuner can be is dependant on his or her ability and knowledge with respect to the events within the combustion chamber in question. Unfortunately, that knowledge usually comes at the price of some blown-up motors.
10-08-2004, 11:35 PM
#35
Originally Posted by s13lover
Stoichiometric ratio is actually 14.7xxx:1
Maximum power tends to occur at 14.7:1.
Stoichiometry refers to the point at which there will be no uncatalyzed gasoline molecules.
10-09-2004, 12:02 AM
#36
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Interesting article on this subject. I would think the RX8 is more similar to the M3.
http://www.caranddriver.com/article....&page_number=1
http://www.caranddriver.com/article....&page_number=1
10-09-2004, 11:51 PM
#38
Registered User
Damn... I need a drink... maybe I'll try some Bacardi Black, or would I get drunk faster with Bacardi 151?
Ha! You guys kill me!
Just for *****-&-grins, try the mid-grade! LMAO......
Yep, gonna **** off many folks with this post. Sorry? Hell no...
Ha! You guys kill me!
Just for *****-&-grins, try the mid-grade! LMAO......
Yep, gonna **** off many folks with this post. Sorry? Hell no...
10-10-2004, 12:31 AM
#40
Kaiten Kenbu Rokuren
The stoichiometric ratio is in fact 14.7:1.http://www.epa.gov/otaq/epg/keyterm.htm
The very first text box. The click the hypertext "stoichiometric" It is understood in chemistry that for gasoline, 14.7:1 is the level for stoichiometry. But I would for my own reasons like to know where you got 15.179:1. Thanks
The very first text box. The click the hypertext "stoichiometric" It is understood in chemistry that for gasoline, 14.7:1 is the level for stoichiometry. But I would for my own reasons like to know where you got 15.179:1. Thanks
10-10-2004, 09:37 AM
#41
I always thought the optimal A/F ratio for power in an N/A (piston based) engine was 12:1 through 13:1...with a slight bump in richness @ the torque peak of the engine. (This of course slightly varies engine to engine)
I was under the belief that the optimal mix for "mileage" was 14.7:1, which is why most vehicles try to maintain that mix in closed loop fuel modes.
I was under the belief that the optimal mix for "mileage" was 14.7:1, which is why most vehicles try to maintain that mix in closed loop fuel modes.
10-15-2004, 10:13 PM
#42
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Stochiometric is shitty anyway...
Combustion is never 100% efficient anyway...
It gets even lower with more overlap.
I believe that 13.5 with an N/A engine is good, and even lower with more overlap, when unbunrt fuel goes straight through the exaust... Combustion is the same, but more fuel is dumped into the exhaust, without ever seeing any spark.
BTW, my all motor 2.4 neon likes 110 octane better that 91 pump gas...even when not pigning... maybe the race gas just burns better... then again, 11.5:1 is not the 10:1 we've got in the '8...
Chris
Combustion is never 100% efficient anyway...
It gets even lower with more overlap.
I believe that 13.5 with an N/A engine is good, and even lower with more overlap, when unbunrt fuel goes straight through the exaust... Combustion is the same, but more fuel is dumped into the exhaust, without ever seeing any spark.
BTW, my all motor 2.4 neon likes 110 octane better that 91 pump gas...even when not pigning... maybe the race gas just burns better... then again, 11.5:1 is not the 10:1 we've got in the '8...
Chris
10-15-2004, 11:08 PM
#43
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Im glad some of you guys aren't engineering the fuel injection system! 
Air/Fuel ratio is based on pounds of AIR to pounds of FUEL but carbs/injectors are metered (jetted) by volume so changes in fuel can change A/F ratios. A/F Ratios range from about 2:1 for NitroMethane to about 16:1 for gasoline, with 14.7:1 considered the stoichiometric or chemically correct ratio under perfect conditions with normal (non-oxygenated) gasoline (hexane). Gasoline A/F ratios for best power tend to be in the 13.25:1 - 13.75:1 range.
When you get confident with that, take a stab at the correct fuel air ratio for a hydrogen/helium slurry scramjet injection system with respect to altitude and ram air pressure. :D
heheheheheh
Air/Fuel ratio is based on pounds of AIR to pounds of FUEL but carbs/injectors are metered (jetted) by volume so changes in fuel can change A/F ratios. A/F Ratios range from about 2:1 for NitroMethane to about 16:1 for gasoline, with 14.7:1 considered the stoichiometric or chemically correct ratio under perfect conditions with normal (non-oxygenated) gasoline (hexane). Gasoline A/F ratios for best power tend to be in the 13.25:1 - 13.75:1 range.
When you get confident with that, take a stab at the correct fuel air ratio for a hydrogen/helium slurry scramjet injection system with respect to altitude and ram air pressure. :D
heheheheheh
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