Gas
- Thread starter CaptJon
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John Herman
New member
At this point I would say any readily available "normal" pump gas from the local gas station, and any of the racing fuels available from your local gas station or the track. No "doctoring" of the fuel is allowed such as octane boosters, etc.
Jon--You can run any Octane level you can find (assuming your car is not registered and driven on the street).
Octane in and of itself offers no power advantage. It is just a measure of a fuel's resistance to detonation.
Higher octane gas literally burns slower (and has less power potential) than some 87 unleaded.
What the club is trying to police is super oxygenated fuels. These are fuels blended with various oxygen bearing compounds (like forms of nitro) that do indeed have a power enhancing capability.
These are expensive and maybe harmful to the grid workers so the club sees the need to ban them. The problem is that certain regular pump gas formulations can trigger the test meters like the banned fuels, so we have confusion in the rules.
Regards.
Octane in and of itself offers no power advantage. It is just a measure of a fuel's resistance to detonation.
Higher octane gas literally burns slower (and has less power potential) than some 87 unleaded.
What the club is trying to police is super oxygenated fuels. These are fuels blended with various oxygen bearing compounds (like forms of nitro) that do indeed have a power enhancing capability.
These are expensive and maybe harmful to the grid workers so the club sees the need to ban them. The problem is that certain regular pump gas formulations can trigger the test meters like the banned fuels, so we have confusion in the rules.
Regards.
I've had this same question... thanks for asking...
You are right in that octane in and of itself does not enhance performance... BUT... since timing is unlimited... this can be a major advantage if you tune to the limits and run 100+ octane.
I will tune to run the highest octane legal I believe... so 106 appears to be within the rules!?!
You are right in that octane in and of itself does not enhance performance... BUT... since timing is unlimited... this can be a major advantage if you tune to the limits and run 100+ octane.
I will tune to run the highest octane legal I believe... so 106 appears to be within the rules!?!
Super high octane is NOT going to allow you to "tune" much for greater performance.
IT Racing is a "naturally aspirated" racing class with compression limits (OEM + .5).
If you could run turbos (artificial compression raising) or a way lot more static compression, you will need the octane.
The only thing you gain with more octane in a modern car is a computer contolled timing advance--that is not going to smoke the dyno.
IT Racing is a "naturally aspirated" racing class with compression limits (OEM + .5).
If you could run turbos (artificial compression raising) or a way lot more static compression, you will need the octane.
The only thing you gain with more octane in a modern car is a computer contolled timing advance--that is not going to smoke the dyno.
tom_sprecher
Super Moderator
joeg is right. Increased octane is only necessary to prevent detonation in increased compression engines. Running it in a lesser engine is a waste of $ and possibly performance. The best octane is the one high enough to prevent pinging w/the maximum advance your engine should be tuned to.
It only gives more HP up to a point, after that it goes back down. It does so by optimizing the charge ignition timing so that the "explosion" occurs at precisely the right moment for the engine to extract the maximum power from it.
Regardless of the engine / octane rating, there is a point after which it is firing too much before TDC and causing a kind of kick-back. Most 10.0:1 engines - even with no knock sensor - will optimize before they exceed the anti-knock abilities of 93 octane.
Regardless of the engine / octane rating, there is a point after which it is firing too much before TDC and causing a kind of kick-back. Most 10.0:1 engines - even with no knock sensor - will optimize before they exceed the anti-knock abilities of 93 octane.
Norm has put it correctly.
Simple example--My naturally aspirated performance street car will retard the ignition (thereby slightly reducing HP)if it does not get its 91 octane. It does not increase it much if you go over 91 (e.g. 93) and you are throwing away money if you were to put 100 unleaded in there.
At most, we are probably talking a 2-5 hp spread between highest and lowest.
This would be much more dramatic with a forced induction (e.g., Turbo) set-up.
Simple example--My naturally aspirated performance street car will retard the ignition (thereby slightly reducing HP)if it does not get its 91 octane. It does not increase it much if you go over 91 (e.g. 93) and you are throwing away money if you were to put 100 unleaded in there.
At most, we are probably talking a 2-5 hp spread between highest and lowest.
This would be much more dramatic with a forced induction (e.g., Turbo) set-up.
handfulz28
New member
Heh, you guys care about octane & ignition timing?
I love the smell of racing gas in the morning!!!
Seriously though, I hear some people like a "racing" fuel because it doesn't have a lot of additives that "street-legal" fuel may have.
YMMV
Michael
I love the smell of racing gas in the morning!!!
Seriously though, I hear some people like a "racing" fuel because it doesn't have a lot of additives that "street-legal" fuel may have.
YMMV
Michael
I do not think this subject has been explained properly yet. The higher the octane of fuel, the slower it burns. That also means given the same amount of fuel the higher octane will burn longer. That being said If you could ignite it sooner by advancing the timing, it would allow the flame front to push against the piston longer. This will increase power if your engine can take advantage of it. Every Motor is different and the only way to know if it is an advantage is to test on a dyno.
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Peter Linssen
ITE Volvo 740 Turbo
ITB Opel Manta
Oregon Region
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Peter Linssen
ITE Volvo 740 Turbo
ITB Opel Manta
Oregon Region
RacerBill
New member
Posted by ITANorm: 'Lower octane fuel has more "punch" than higher octane fuel.'
SHHHHHHHHH! Don't let the oil companies know. They'll charge more for it!
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Bill Stevens
Mbr 103106
BnS Racing
83 ITA Shelby Dodge Charger
www.motorpride.com/BnSRacing
SHHHHHHHHH! Don't let the oil companies know. They'll charge more for it!
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Bill Stevens
Mbr 103106
BnS Racing
83 ITA Shelby Dodge Charger
www.motorpride.com/BnSRacing
Combustion is not an explosion at all, but a controllable burning process. For the first 10% of combustion in the time domain, there are many things an engineer can do to malipulate the burn rate.
Once the first 10% of the combustion process occurs, the burning rate is a function of physics based on the speed of sound in the medium, the number of flame kernals, and the shape of the squish zone.
Concerning cylinder pressure, gasoline engines cannot compare to tthe BMEP numbers diesels put up (220 bar)... i.e. gasoline engines are not mechanically limited by peak cylinder pressure.
Now, while you can Joeg may not get a performance improvement from putting 100 octane in his daily driver... it is not because of the combustion recipe limitations... it is becuase the ECU cannot add fuel and spark in closed-loop mode... because it is not programmed to do it.
To read about this in great detail, see Taylor, Charles, The Internal Combustion Engine in Theory and Practice, MIT, 1997, pg.11-52
Regards,
Matt
Once the first 10% of the combustion process occurs, the burning rate is a function of physics based on the speed of sound in the medium, the number of flame kernals, and the shape of the squish zone.
Concerning cylinder pressure, gasoline engines cannot compare to tthe BMEP numbers diesels put up (220 bar)... i.e. gasoline engines are not mechanically limited by peak cylinder pressure.
Now, while you can Joeg may not get a performance improvement from putting 100 octane in his daily driver... it is not because of the combustion recipe limitations... it is becuase the ECU cannot add fuel and spark in closed-loop mode... because it is not programmed to do it.
To read about this in great detail, see Taylor, Charles, The Internal Combustion Engine in Theory and Practice, MIT, 1997, pg.11-52
Regards,
Matt
Which is why I put it in quotes . . ..
Gasoline engines are not limited by peak cylinder pressure, no - but if you fire the mixture off at a point such that the first expansion in the chamber occurs prior to the point at which it can provide propulsion in the proper direction, it won't be a benefit. It will be a detriment. And if your chamber design and compression ratio are such that the flame front does not create preignition on 93 octane fuel, at that ignition timing, there is no reason to run higher.
Read the books - then build the car - then set your ignition timing on a dyno. Play with different fuels if you like. Let me know what you find. I'm just relaying my particular findings - which are also supported in engineering journals.
dyoungre
New member
ITANorm is on the money, and ITManta is close, but has a bit of a misunderstanding of how burn rate will effect the net 'area under the curve'.
Combustion has (3) key phases, the first two with which we are most concerned: Ignition delay, and combustion. 3rd phase, quench, is important for emissions purposes.
The key to 'Octane' rating is the ignition delay - the time it takes for the atoms to dissociate. Now understand, we are talking at a true "molecule to molecule" level - I'm not talking about the flame front, which mckeaaan explained well. By increasing the delay at a molecular level, the flame front then stays at a more controlled rate, reducing the chance for uncontrolled combustion - knock.
Now, on an in-cylinder pressure standpoint, peak power will result when the peak pressure occurs at 5 deg after top dead center (Plenty of emperical data out there). No earlier, no later; the spark timing should be adjusted to get that timing - timing that is called MBT, or Mean Best Torque. NOW - if you look at area under the curve comparing burn rates of different octane fuels (each adjusted for that 5 degree after TDC peak pressure location), you will find that the FASTER the burn rate, the higher the peak pressure, AND the higher the area under the curve - the IMEP. Slower burn, even adjusted for the best spark advance, will give you less torque, because the peak pressure is much much lower. You want a fast, controlled flame, to give you the most time at a high pressure, each cycle.
When it comes to fuel, the devil is in the details, because non-hydrocarbon additives end up determining the octane rating; from a hydrocarbon to hydrocarbon standpoint, though, Norm is right that the heating value is low for trimethylpentane (C8H18, or isooctane). RULE OF THUMB is that lower octane pump gas provides more power; some of the racing fuels from Sunoco, VP, etc have more power with higher octane because of the exotic chemicals that are included.
If you are using pump gas, use the lowest octane rated gas you can get away with, and adjust your spark timing, either advanced or retarded from your current setting, to get that magical 'MBT'. If not knocking, slightly over advanced will be closer to best torque than slightly under.
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Dave Youngren
NER ITA RX7 #61
Combustion has (3) key phases, the first two with which we are most concerned: Ignition delay, and combustion. 3rd phase, quench, is important for emissions purposes.
The key to 'Octane' rating is the ignition delay - the time it takes for the atoms to dissociate. Now understand, we are talking at a true "molecule to molecule" level - I'm not talking about the flame front, which mckeaaan explained well. By increasing the delay at a molecular level, the flame front then stays at a more controlled rate, reducing the chance for uncontrolled combustion - knock.
Now, on an in-cylinder pressure standpoint, peak power will result when the peak pressure occurs at 5 deg after top dead center (Plenty of emperical data out there). No earlier, no later; the spark timing should be adjusted to get that timing - timing that is called MBT, or Mean Best Torque. NOW - if you look at area under the curve comparing burn rates of different octane fuels (each adjusted for that 5 degree after TDC peak pressure location), you will find that the FASTER the burn rate, the higher the peak pressure, AND the higher the area under the curve - the IMEP. Slower burn, even adjusted for the best spark advance, will give you less torque, because the peak pressure is much much lower. You want a fast, controlled flame, to give you the most time at a high pressure, each cycle.
When it comes to fuel, the devil is in the details, because non-hydrocarbon additives end up determining the octane rating; from a hydrocarbon to hydrocarbon standpoint, though, Norm is right that the heating value is low for trimethylpentane (C8H18, or isooctane). RULE OF THUMB is that lower octane pump gas provides more power; some of the racing fuels from Sunoco, VP, etc have more power with higher octane because of the exotic chemicals that are included.
If you are using pump gas, use the lowest octane rated gas you can get away with, and adjust your spark timing, either advanced or retarded from your current setting, to get that magical 'MBT'. If not knocking, slightly over advanced will be closer to best torque than slightly under.
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Dave Youngren
NER ITA RX7 #61
I have worked in single cylinder development applications where more area under the curve can be achieved by maintaining the IMEP.
The faster the fuel will burn, the greater push you get at MBT... but the more likely you are to develop knock at the limit.
Knock is a phenomena where the burn rate is so quick that it creates a tremendous pressure spike in the area(s) of fuel/air that have not been burned yet. This pressure creates enough molecular friction to autoignite fuel/air in the outer regions. Once this occurs, you have two high-rate flame fronts hitting each other at the speed of sound... this in turn resonates the cylinder liner. Your ear interprets this as knocking or pinging.
So... as dyoundre eluded to... the "area under the curve", or IMEP (indicated mean effective pressure) can be raised with less MBT but more pressure from 5 to perhaps 35 degrees... this works best with under-square engines, because the piston tends to dwell at the top longer (which I have in the 83 Supra).
The other issue is that knock is a persistent phenomena; that is, once you start knock, you must lower the gas pressure well below a generally acceptable condition to get it to stop. So, if you are tuning to the limit with fast burning fuel, you will be more prone to inducing knock somewhere in your duty cycle around the track...that in turn will cause you to back of the timing so you don't burn or scuff a piston.
On the other hand, you can turn the timing up on a 100 octane fuel and get better imep numbers in the process.
One other side benefit is that you can run a touch leaner because the temperature gradient is less severe if the burn is more controlled.
This is a good discussion. Thanks for pointing out the books ITANorm, I have a good library of them. If you go to the SAE Conference next year in Detroit, perhaps you can see a presentation I'm planning dealing with comustion recipe development.
p.s. If you could get 10+ spark plugs in each cylinder bowl (within the rules and packaging constraints - (of course you can't)... that would produce MBT worth talking about!)
Regards,
Matt
The faster the fuel will burn, the greater push you get at MBT... but the more likely you are to develop knock at the limit.
Knock is a phenomena where the burn rate is so quick that it creates a tremendous pressure spike in the area(s) of fuel/air that have not been burned yet. This pressure creates enough molecular friction to autoignite fuel/air in the outer regions. Once this occurs, you have two high-rate flame fronts hitting each other at the speed of sound... this in turn resonates the cylinder liner. Your ear interprets this as knocking or pinging.
So... as dyoundre eluded to... the "area under the curve", or IMEP (indicated mean effective pressure) can be raised with less MBT but more pressure from 5 to perhaps 35 degrees... this works best with under-square engines, because the piston tends to dwell at the top longer (which I have in the 83 Supra).
The other issue is that knock is a persistent phenomena; that is, once you start knock, you must lower the gas pressure well below a generally acceptable condition to get it to stop. So, if you are tuning to the limit with fast burning fuel, you will be more prone to inducing knock somewhere in your duty cycle around the track...that in turn will cause you to back of the timing so you don't burn or scuff a piston.
On the other hand, you can turn the timing up on a 100 octane fuel and get better imep numbers in the process.
One other side benefit is that you can run a touch leaner because the temperature gradient is less severe if the burn is more controlled.
This is a good discussion. Thanks for pointing out the books ITANorm, I have a good library of them. If you go to the SAE Conference next year in Detroit, perhaps you can see a presentation I'm planning dealing with comustion recipe development.
p.s. If you could get 10+ spark plugs in each cylinder bowl (within the rules and packaging constraints - (of course you can't)... that would produce MBT worth talking about!)
Regards,
Matt
... the "area under the curve", or IMEP (indicated mean effective pressure) can be raised with less MBT but more pressure from 5 to perhaps 35 degrees... this works best with under-square engines, because the piston tends to dwell at the top longer... Which is the reason why Rotaries love regular.
Good racing.
Bill
Good racing.
Bill
I'm impressed with this discussion from all the Chemist and Chem E grads (tongue well placed in cheek). I run 93 octane in my car 'cause it allows me to screw with my jets and timing which I feel gives me better performance. If I run a course with extended flat out running, then I'll run Cam 2 or similar racing unleaded with a higher octane to cool the fire below with out losing any boogie. Saves the pistons and the head gaskets! Key word to above is "feel".
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Chris Harris
ITC Honda Civic
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Chris Harris
ITC Honda Civic