Posted on 02/24/2003 10:59:35 PM PST by oceanperch
WFTR
Bill
LOL. True, and funny. Thanks for the chuckle.
In the past a high octane rating allowed you to run higher compression ratio's and advance your ignotion timing, resulting in more power, improved efficency and better throttle response.
That was when the timing curve was fixed and controlled with centrifugal force and vacuum canisters however.
As it stands now, the computer in your car will detect engine "knock" (pre-ignition) and will alter the ignition timing to compensate, (sometimes even on a per-cylinder basis) as "knock" is harmful if it continues for extended periods (read, REALLY, REALLY, insanely LONG periods.) of time.
It's no big deal, run whatever you want in it for your trips to the grocery store.
The first manual, entitled Changes in Gasoline & the Automobile Service Technician, was originally published in 1987. Over a four year period it was periodically updated to focus on fuel related areas of greatest interest to automobile service technicians. The first version of the manual achieved a circulation of 345,000 copies. Specifications&Standards In order to understand fuel quality standards and how they affect the automobile, it is important to have a basic understanding of gasoline, how and why quality standards are set, and what significance they have on the driveability, performance and durability of an automobile engine and related systems.
Gasoline is not a single substance. It is a complex mixture of components which vary widely in their physical and chemical properties. There is no such thing as pure gasoline. Gasoline must cover a wide range of operating conditions, such as variations in fuel systems, engine temperatures, fuel pumps and fuel pressure. It must also cover a variety of climates, altitudes, and driving patterns. The properties of gasoline must be balanced to give satisfactory engine performance over an extremely wide range of circumstances. In some respects, the prevailing quality standards represent compromises, so that all the numerous performance requirements may be satisfied.
By properly controlling specifications and properties, it is possible to satisfy the requirements of the hundreds of millions of spark ignition engines in the marketplace with just a few grades of gasoline.
The most commonly used gasoline quality guidelines are established by the American Society for Testing and Materials (ASTM). ASTM specifications are established by consensus based on the broad experience and close cooperation of producers of motor gasoline, manufacturers of automotive equipment, users of both commodities, and other interested parties such as state fuel quality regulators. ASTM Standards are voluntary compliance standards. However, the United States Environmental Protection Agency (EPA) and some states have passed regulations and laws which, in some cases, require gasoline to meet all, or a portion of, the ASTM gasoline guidelines. Currently, ASTM D 4814 is the standard specification for automotive spark ignition engine fuel. There are several test methods encompassed in the D 4814 specification. It should also be noted that in addition to ASTM standards, some petroleum companies and pipeline operators may have specifications which go beyond the ASTM guidelines. For instance, some refiners may specify a higher minimum motor octane or use of a specific deposit control additive. Recently more attention has been focused on the environmental requirements that gasoline must meet. However even with adjustments in composition to comply with environmental standards, gasoline should still meet the performance standards established by ASTM.
This chapter addresses ASTM specifications and other fuel quality parameters and their importance. OctaneQualityandVehicleOctaneRequirement Gasolines are most commonly rated based on their Antiknock Index (AKI), a measure of octane quality. The AKI is a measure of a fuel’s ability to resist engine knock (ping).
The AKI of a motor fuel is the average of the Research Octane Number (RON) and Motor Octane Number (MON) or (R+M)/2. This is also the number displayed on the black and yellow octane decal posted on the gasoline pump. Optimum performance and fuel economy is achieved when the AKI of a fuel is adequate for the engine in which it is combusted. There is no advantage in using gasoline of a higher AKI than the engine requires to operate knock-free. The RON and MON of fuels are measured by recognized laboratory engine test methods. Results of these tests may generally be translated into approximate field performance.
In general, the RON affects low to medium speed knock and engine run-on or dieseling. If the Research Octane Number is too low, the driver could experience low speed knock and engine run-on after the engine is shut off. The MON affects high speed and part-throttle knock. If the Motor Octane Number is too low, the driver could experience engine knock during periods of power acceleration such as passing vehicles or climbing hills.
The antiknock performance of a fuel, in some vehicles, may be best represented by the RON, while in others it may relate best to the MON. Extensive studies indicate that, on balance, gasoline antiknock performance is best related to the average of the Research and Motor Octane Numbers, or (R+M)/2. This formula is continuously reviewed for its accuracy in predicting gasoline performance in new automobiles.
The RON of a fuel is typically 8 to 10 numbers higher than the MON. For instance, an 87 octane gasoline typically has a MON of 82 and a RON of 92. Most vehicles give satisfactory performance on the recommended octane-rated fuel. But in some cases, using the fuel specified will not guarantee that a vehicle will operate knock-free, even when properly tuned. There can be signifibustion temperatures are also a factor with higher combustion temperatures increasing ONR. Therefore, intake manifold heat input, inlet air temperature, and coolant temperature have an indirect affect on octane requirement. Additionally the Exhaust Gas Recirculation (EGR) rate can affect ONR. Combustion chamber design affects octane requirements. However the effect of various designs is difficult to predict. In general, high swirl (high turbulence) combustion chambers reduce ONR, thus permitting the use of higher compression ratios. The compression ratio itself is one of the key determinants of octane requirement. As compression ratio increases, so does the need for greater octane levels (Figure 1-3).
Excessive combustion chamber deposits can increase the octane requirement of an engine due to increased heat retention and increased compression ratio. There are also atmospheric and climatic factors which influence ONR. Increases in barometric pressure or temperature increase octane requirement. Increases in humidity will lower octane requirements. Octane requirements decrease at higher altitudes due to decreases in barometric pressure. Many of the variables related to octane and octane requirement can be totally or partially compensated for by the engine control systems in most late model vehicles. For instance, vehicles equipped with knock sensor devices allow the engine control system to advance or retard the ignition timing in response to engine knock. Other vehicles with electronic engine controls employ the use of a barometric (baro) sensor to compensate spark timing and air/fuel mixture in response to barometric changes. The effect of altitude on octane requirement in these late model vehicles is about onethird that of engines not so equipped.
A number of myths about octane have grown over the years. There is a widespread perception that the greater the octane the better the performance. However, once enough octane is supplied to prevent engine knock, there is little, if any, performance improvement. One exception to this would be in vehicles equipped with knock sensors. In these vehicles, if octane is insufficient, the computer will retard the timing to limit engine knock. If the vehicle is operating in the “knock limiting” mode (retarded timing), using a higher octane fuel will allow timing to be advanced, resulting in some level of performance increase. However, even in these vehicles, tests have shown that there is no perceptible performance improvement from using a fuel of higher octane than that recommended by the vehicle manufacturer.
Another myth is that using a higher octane fuel will result in improved fuel economy (increased miles per gallon). Octane is nothing more than a measure of anti-knock quality. Fuel economy is determined by a number of variables including the energy content of the fuel. Some premium grades of fuel may contain components which increase energy content. In those cases, fuel economy may improve slightly as a result of higher energy content, but not as a result of the higher octane. Two fuels of identical octane could have different energy content due to compositional differences. Consumers need only use a gasoline meeting the vehicle manufacturer’s recommended octane levels. If engine knocking occurs on such fuels and mechanical causes have been eliminated, then the consumer should purchase the next highest octane gasoline (above the manufacturer's recommendation in the owners manual) that will provide knock-free operation.
Also has all you ever wanted know about additives, oxygenates (ethanol, MTBE, etc.)
Full pdf here: http://www.ethanolrfa.org/pdf/Gasoline.pdf
Most vehicles give satisfactory performance on the recommended octane-rated fuel. But in some cases, using the fuel specified will not guarantee that a vehicle will operate knock-free, even when properly tuned. There can be signifi-cant differences among engines, even of the same make and model, due to normal production variations.
The actual loss of power and damage to an automobile engine, due to knocking, is generally not significant unless the intensity becomes severe. Heavy and prolonged knocking, however, may cause damage to the engine.
Whether or not an engine knocks is dependent upon the octane quality of the fuel and the Octane Number Requirement (ONR) of the engine. The ONR is affected by various engine design factors and in-use conditions. (See Table 1-1) Engines experience increased octane number requirement when the ignition timing is advanced.
The air/fuel ratio also effects ONR with maximum octane requirement occurring at an air/fuel ratio of about 14.7:1. Enriching or enleaning from this ratio generally reduces octane requirement. Combustion temperatures are also a factor with higher combustion temperatures increasing ONR. Therefore, intake manifold heat input, inlet air temperature, and coolant temperature have an indirect affect on octane requirement. Additionally the Exhaust Gas Recirculation (EGR) rate can affect ONR.
Carolyn
So, after using premium (93 octane) in my Firebird since the day I bought it brand new, I decided to step down to the lowest grade (87 octane). Within 3 weeks, I had to take the car in to have the fuel injectors cleaned out - something I never had to do before through other tune-ups. (these cleanings are not cheap,either)
Needless to say, I went back to premium gas, no matter the price.
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