| Author | Message | ||
| "The Don" (Mlemus)
Advanced Member Username: Mlemus Post Number: 4706 Registered: 8-2002 |
Peter, For our carb cars, 89 works just fine. 87 is fine for winter but with the heat of summer, 89 should be fine. 92 is a waste. Matt | ||
| Andrew A. Illes (Andyilles)
New member Username: Andyilles Post Number: 34 Registered: 10-2002 |
James - I think it has to do with the Veeblefitzer Equation. If you're really into the combustion temp thing, you might find researching water injection in WW II bomber engines interesting. As to "Why would higher octane provide opportunity to advance timing?", basically, the slower the fuel-mix burns, the earlier you can light it off to get max expansion and combustion pressure when the piston passes TDC. Hope that helps. Andy | ||
| James Selevan (Jselevan)
Member Username: Jselevan Post Number: 551 Registered: 6-2002 |
As I suggested, additives increase octane. Any questions? (How many on this Chat room, other than Professor Germane, have their Nobel?) Jim S. | ||
| G. J. Germane (Germane)
New member Username: Germane Post Number: 24 Registered: 7-2002 |
As combustion proceeds in a spark-ignition engine, the flame front compresses the unburned, pre-mixed charge ahead of the advancing reaction zone. The unburned charge increases in temperature and is thus more susceptible to spontaneous combustion (often detonation) that gives rise to additional flame fronts and pressure spikes (knock). These potentially damaging reactions generally first occur in the end gas. The end gas is a small portion of the original charge. Good combustion chamber design includes squish areas to help cool the unburned charge to reduce the tendency for combustion knock. Combustion knock in the end gas leads to hot spots in the cylinder that serve as potential pre-ignition sites. Pre-ignition can create dramatically greater combustion chamber pressures, leading to charge heating and spontaneous combustion in multiple locations ahead of the advancing flame front. The process, unchecked, leads to thermal and mechanical overload of engine components. The tendency to knock can be mitigated in a number of ways that do not directly affect ignition temperature of a fuel air mixture. Two approaches, one fuel structure-related, and one additive-based, are briefly presented. Highly branched hydrocarbons (alkylates, such as 2,2,4 tri-methyl pentane (iso-octane)), and ring hydrocarbons (aromatics, such as toluene and xylene), are inherently knock resistant because of chemical structure. The combustion reactions that break bonds, consume carbon, hydrogen and oxygen, and produce intermediate species in the reaction, cannot proceed as rapidly when branches are encountered compared with straight carbon chains, which are consumed more rapidly. The effect is realized in reducing the tendency of the end gas to spontaneously combust. Tetra-ethyl lead (TEL), one of the single most effective octane improving additives yet discovered, is thought to affect end gas combustion chemistry. As combustion proceeds, various forms of lead oxide are created. Free radicals formed in the combustion process that are thought to promote knock are "blanketed" or otherwise rendered ineffectual by lead oxide (one form seems to be more effective than the others). The reactions in the end gas slow because of the altered chemistry, thereby reducing or eliminating spontaneous combustion ahead of the flame front. Discoveries continue to be made in understanding the complex reactions and species that occur during pre-mixed, high pressure, hydrocarbon/air combustion. The engineering explanations given above do not do justice to the detailed mechanistic theories that have been developed over the years. However, I intend and hope them to be consistent with the present understanding of the engine and fuel research and development community. | ||
| James Selevan (Jselevan)
Member Username: Jselevan Post Number: 550 Registered: 6-2002 |
Andrew - with Germane's expert help, let's examine this issue of "...higher-octane makes the gas burn slower..." I suspect that the mechanism of this "slow down" in burn rate is related to the temperature at which the fuel ignites. I cannot think of another mechanism by which adding a chemical to the gas would slow down ignition. If the neighboring molecules must now achieve a higher temperature to ignite, then a few nanoseconds will be required to achieve this higher temperature, and when multiplied by the cascade of 10 to the 26 molecules (Avogadro's number), it may take a few microseconds longer. Thus, until Professor Germane explains how an additive will slow down combustion (other than raising ignition temperature), I stand by my comment of higher-octane fuel having a higher ignition temperature. The net effect, as you and Professor Germane have expertly elucidated, is a slow down in combustion. This now is clearly linked to avoidance of "knocking" with the use of higher-octane fuel, as it allows the piston to achieve TDC before the pressure wave smashes against the crown. Thank you. Jim S. | ||
| G. J. Germane (Germane)
New member Username: Germane Post Number: 21 Registered: 7-2002 |
This thread has been well developed with technical information that is mostly accurate. I'm impressed that this thread is generally devoid of the misinformation and folklore that so often accompanies a discussion of gasoline octane characteristics and internal combustion engine octane requirements. This area has been the topic of much of my past university research and teaching. The important consideration is probably obvious: the fuel anti-knock properties must match the engine octane requirement. There are many factors that influence the anti-knock characteristics of gasoline, and the octane requirement of an engine. For a detailed discussion of fuels and octane, I would refer those interested to a paper that I authored, and published by the Society of Automotive Engineers: "A Technical Review of Automotive Racing Fuels," Society of Automotive Engineers Transactions (1985), Society of Automotive Engineers Fuels and Lubricants Journal, 1, 876-878, 1985. The paper can be accessed in hard-copy or on-line in university or public libraries. The influence of selected engine design and operating characteristics can be summarized as follows: Octane requirement generally increases with ignition spark advance, charge density (from super- or turbo-charging, throttle opening, or higher barometric pressure), decrease in humidity, increased inlet air temperature, higher coolant temperature, utilization of ethylene glycol antifreeze coolant, and increased engine load. Engine octane recquirement generally decreases with retarded ignition timing, higher altitude or smaller throttle opening operation, increased humidity, decreased inlet temperature, lean or rich fuel-air ratios compared with maximum knock mixture, exhaust gas recirculation operation at part throttle, reduced engine load, and higher engine speeds. The engineering reasons for the above listed trends can be reviewed in a good internal combustion engine textbook and probably on-line at some appropriate website. By the way, just a small correction to one of the posts: a four-stroke cycle engine operating at 8000 rpm experiences 4000 ignition events per cylinder per minute. I hope that this information provides some technical basis for many of the points articulated by others. | ||
| Andrew A. Illes (Andyilles)
New member Username: Andyilles Post Number: 29 Registered: 10-2002 |
James - As has been noted, octane simply means how rapidly fuel burns, and the way ignition timing relates is this.... The standard timing reference is "TDC", which means Top Dead Center, when the piston is at the very top of it's compression stroke. It takes time for gasoline to burn and combustion gasses to expand to drive the piston down on the power stroke, after TDC. Keep in mind that at, say, 8000 RPM, this is happening 2,000 times a minute in a 4-stroke engine. If you "advance" the timing, meaning fire the spark plug before the piston gets to TDC, the fuel has time to burn and start expanding by the time it can do some good after TDC, when the piston is on the way down and pushing on the crankshaft to rotate it. "Pinging", "detonation", etc means the fuel has been ignited too soon, and is burning too fast and expanding too soon, which starts pushing the piston down BEFORE it's at the top of it's stroke. That's the noise you hear. So, hypothetically, at any given advance timing, a low octane fuel might burn too rapidly and start pushing on the piston before it "gets over the hump" = "ping" or "detonatin"... while a higher octane fuel will burn slower, and not expand enough until the piston is ready to start back down. Ideally, you want maximum combustion pressure to occur just after TDC. Hope that makes sense. Andy | ||
| Dave Helms (Davehelms)
New member Username: Davehelms Post Number: 16 Registered: 5-2003 |
John, what Steve said is spot on as to the octane requirements. All I would add is at this altitude, finding fuel with no alcohol is a major advantage. Alcohol has a much lower boiling point than "pure" gas, and in a carb car that can cause major problems on a hot day. When every other car needs to go through emissions testing crackling hot, the carb cars need to go through stone cold, so the fuel in the carbs is not percolating. Once the fuel starts to boil, the emissions go way out of spec. One problem I ran into this spring was a car that was taken out and driven this winter on one of the 75 deg. days, filled with gas and put away until just recently. It is incredible that the winter blend fuel here even burns on a cold day, but how it reacts to a hot day is unjust! The winter blend fuel was tripping the knock sensors at idle! There are still a few gas stations around town that sell "non alcohol" fuel, that is regular 91 octane and priced well under $2/gal. | ||
| arthur chambers (Art355)
Intermediate Member Username: Art355 Post Number: 1619 Registered: 6-2001 |
James: Pre-ignition is one problem. That can be solved with the lead on the timing. Detonation is something else. People confuse them. Detonation is usually caused by the fuel burning too fast, and you get detonation. A higher octane can help with that because a higher octane burns slower. In fact, when Honda started racing, their engines used high octane gas. They turned so many RPMs that they actually had to reduce the octane to get adequate performance. Sounds backwards, but true. Notice when you lug an engine under strain, you sometimes get a knock? That's an example of detonation. Having said that, the 308 doesn't turn enough rpm to have that make any difference. Increasing the octane will allow a higher compression, and a little more ignition lead. It will make a difference, but because you are changing the lead on the timing, but only to the point of pre-ignition, you will gain at a higher rpm because of the advance in the ignition. The factory will set the lead at the lowest level because they are unsure about the quality of the fuel. If you have engine management, the computer will set the lead to the greatest number within the spec on the car, which the manufacturer sets for the highest octane available (it wouldn't make an difference if that were unavailable, because the management system would just retard the spark if knocking occurred.) Art | ||
| James Selevan (Jselevan)
Member Username: Jselevan Post Number: 545 Registered: 6-2002 |
Andrew - "Electronic management CAN take advantage of slightly higher octane by advancing ignition timing, a little." Thus you restate what Arthur has said. My question is, notwithstanding Drew's observation, the lower limit on octane is "pre-ordained" by the compression ratio of the cylinder. Why would higher octane provide opportunity to advance timing? The fuel/air mixture will either ignite, or it won't ignite, before the spark. Moving the timing of the spark simply provides more or less compression of the fuel/air mixture. It is this compression, along with the octane, that determines whether pre-ignition will occur. I am not sure that I am articulating my thoughts well. Think about this issue for a moment, and hopefully you will sense the conundrum. Jim S. | ||
| Frank Parker (Parkerfe)
Intermediate Member Username: Parkerfe Post Number: 2303 Registered: 9-2001 |
When I was a kid leaded gas was cheaper than unleaded Amoco, which was called white gas and the only unleaded gas sold in the 1960s that I am aware of, because the gas companies claimed it cost more to remove the lead. Now, leaded gas is more expensive because they claim you have to add the lead. It seems we have had the wool pulled over our eyes yet again. | ||
| Andrew A. Illes (Andyilles)
New member Username: Andyilles Post Number: 28 Registered: 10-2002 |
Hi all. Though all the comments are pretty much on-target, but it's getting esoteric. The bottom line on the octane question is.. does the engine "ping" or "knock" (preignition) on the fuel you're using? If it's not, the fuel is fine and you won't gain a thing from a higher octane fuel. Electronic managament CAN take advantage of slightly higher octane by advancing ignition timing, a little. You can do the same thing on a non-electronic car by advancing the timing manually, but it usually doesn't produce enough of a gain to make it worthwhile. The primary determinant of octane requirement is an engine's compression ratio (notice "primary"). The RPM range it runs in is mainly immaterial | ||
| Drew Altemara (Drewa)
Junior Member Username: Drewa Post Number: 139 Registered: 2-2002 |
Hi James and Arthur, A technical comment here. Raising octane does not raise the flash temp of the fuel. The molecules that give you octane are in about the mid-point of the boiling range. If you want to raise flash you either cut the front end to a higher boiling point or more often than not do not add as much butanes which is the component that has the lowest flash. The reason octane is expensive is that to get it you must take straight chain molecules and make them into cyclic compounds. In doing that you undergo volumetric loss. Since in the United States we sell fuel on a volumetric basis you have less fuel to sell but at a higher price. Hope this helps a little. | ||
| James Selevan (Jselevan)
Member Username: Jselevan Post Number: 544 Registered: 6-2002 |
Arthur - question for you regarding your post. Raising the octane simply raises the "flash temperature" of the fuel. Thus, for an existing engine with fixed compression ratio that does not "pre-ignite", using higher-octane fuel is an appreciated contribution to the oil companies, but a waste of money for the driver. Now, you suggest that with ECU and the ability to advance and retard timing based on "knock sensors", using higher octane fuel may improve performance, but "at the high end of the power band." Absent a spark, a knocking cylinder is Dieseling. I'm not sure I understand why advancing or retarding the spark timing will change the propensity to pre-ignite. If the spark ignites the fuel/air mixture early, then this is, by definition, not pre-ignition. There seems to be some circular logic spinning around in my brain. Thanks for your insight. Jim S. | ||
| arthur chambers (Art355)
Intermediate Member Username: Art355 Post Number: 1613 Registered: 6-2001 |
Unless you have access to a dyno, and someone to tune your car, an absolute waste of money. The combination of ignition leed, compression, blow by, etc. and the abiliity to read the plugs, you are wasting your time. If you do tune the engine for maximum power, then whenever the bariometric pressure goes up, you need to either re-time or rejet. You'll only notice the difference at the high end of the powerband, so why bother. On the other hand, those with automatic timing (read: electronic ignition, adjustable timing etc, modern engine management) will benefit, but almost all of those gasolines have lead which in turn destroys the cat. If you have test pipes, that will work. There are gases that are 116 octane (the purple stuff) but it costs 8 - 9/ gallon and sells in 55 gallons drums. Expensive to use on the street. Art | ||
| Steve Magnusson (91tr)
Intermediate Member Username: 91tr Post Number: 1787 Registered: 1-2001 |
Increased altitude lessens the fuel octane requirement. (Increased temperature raises the fuel octane requirement.) Do an internet search on "RON, MON, Octane" -- you'll get enough reading for the next century  | ||
| John Pray (Juanito308)
New member Username: Juanito308 Post Number: 19 Registered: 3-2003 |
Stock 308 carb here. What about high altitude? How does it (if at all) fit into the mix? I am a mile high in Denver. | ||
| Rob Schermerhorn (Rexrcr)
Member Username: Rexrcr Post Number: 582 Registered: 11-2002 |
Ditto what Ulf and Jack stated. Wanna go racing, then let's talk. "High octane" fuel equates to higher knock resistance or tolerance, allowing higher combustion chamber PRESSURES (BMEP). Compression ratio can be argued as irrelevant compared to BMEP, a dynamic value, not static like CR. | ||
| Jack Habits (Ferraristuff)
Member Username: Ferraristuff Post Number: 573 Registered: 4-2003 |
If you don't need the higher octane, yes.... The 308 has no sophisticated motor management system so it can not "detect" that it can advance the ignition further without any knocking / pinging. Basically, if your engine is stock CR, all that stuff does no good at all. Only if you use high CR pistons / skimmed heads or are tinkering with ignition settings does it do any good. Otherwise, waste of money IMHO. Jack | ||
| PeterS (Peters)
Member Username: Peters Post Number: 435 Registered: 1-2003 |
Then what about additives? A waste of money also? | ||
| Jack Habits (Ferraristuff)
Member Username: Ferraristuff Post Number: 563 Registered: 4-2003 |
PeterS, High Octonane fuel allows for higher compression and earlier ignition thanks to a higher knock resistance. Simply pouring it into a stock engine is a waste of money. High octane fuel will NOT give more power by itself as it doesn't have a higher caloric value than normal fuel. Jack | ||
| Ulf Modig (Ulf308qv)
Junior Member Username: Ulf308qv Post Number: 69 Registered: 3-2001 |
Unless you have a high compression engine, with modern engine management (knock sensors) system or a modified engine, there is no benefit. You might actually lose power due to the "slower burning" high octane fuel. | ||
| PeterS (Peters)
Member Username: Peters Post Number: 433 Registered: 1-2003 |
I have purchased high-octane fuel for my 308 and could never tell the difference between that and regular unleaded gasoline (Which is all I use now). I have heard people state that for the higher running RPM engines, there is a difference. Whats the straight scoop here? |