Lessons in octane and its requirements

[SIred91]

Lessons in MPG and Octane Requirements. From another source: A Consumer's Guide: Gasoline Octane for Cars from Gasoline Questions & Answers for Your Car API Publication 1580, Sixth edition, January 1996 Q. What is octane? A. Octane is a measure of a gasoline's ability to resist knock or pinging noise from an engine. In older vehicles, knock may be accompanied by engine run-on, or dieseling. Knock is the sharp, metallic-sounding engine noise that results from uncontrolled combustion. Severe knocking over an extended time may damage pistons and other engine parts. If you can hear knocking, you should have your engine checked to make sure it is calibrated correctly and does not have a mechanical or electrical problem, or use a higher octane gasoline. In most vehicles no benefit is gained from using gasoline that has a higher octane number than is needed to prevent knock. However, in some vehicles equipped with a knock sensor (an electronic device installed in many modern engines that allows the engine management system to detect and reduce knock), a higher octane gasoline may improve performance slightly. Q. What determines my car's octane requirements? A. Your car's octane requirements are mainly determined by its basic design. In addition, variations in engines due to manufacturing tolerances can cause cars of the same model to require a different octane of several numbers. Also, as a new car is driven, its octane requirement can increase because of the buildup of combustion chamber deposits. This continues until a stable level is reached, typically after about 15,000 miles. The stabilized octane requirement may be 3-6 numbers higher than when the car was new. Premium or midgrade fuel may be advisable to prevent knock. Other factors also influence your car's knocking characteristics: Temperature - Generally, the hotter the ambient air and engine coolant, the greater the octane requirement. Altitude - The higher the altitude above sea level, the lower the octane requirement. Modern computer-controlled engines adjust spark timing and air-fuel ratio to compensate for changes in barometric pressure, and thus the effect of altitude on octane requirement is smaller in these vehicles. Humidity - The drier the air, the greater the octane requirement. The recommendations that vehicle manufacturers give are for normal- to low-humidity levels. Your engine's spark timing - The octane requirement increases as the spark timing is advanced. Both the basic setting of the spark timing and the operation of the automatic spark advance mechanisms are important in controlling knock. In some computer controlled engines, the spark timing can only be changed by replacing modules in the computer. If they are equipped with knock sensors, these computer controlled engines have the ability to retard the ignition temporarily when a sensor detects knock. This temporarily reduces the octane requirement and may also temporarily reduce vehicle performance. Method of driving - Rapid acceleration and heavy loading, such as pulling a trailer or climbing a hill, may result in a greater octane requirement. Stop-and-go driving and excessive idling can increase octane requirements by causing the buildup of combustion chamber deposits. Malfunctions of emission control systems - An improperly functioning emissions control system can affect the octane requirement by changing the air-fuel mixture or by not providing dilution gases through the exhaust gas recirculation (EGR) system. If a malfunction occurs, your vehicle should be taken to a qualified vehicle service mechanic. Some problems are indicated by warning lights on the driver's instrument panel. Q. How many grades of gasoline are available? A. Most places that sell gasoline offer three octane grades of unleaded gasoline--regular at 87 (R+M)/2, midgrade at 89 (R+M)/2, and premium at 93 (R+M)/2. In high-altitude areas such as the Rocky Mountain Region of the U.S., the (R+M)/2 number may be lower by one or two numbers. After January 1, 1996, no leaded gasoline may be sold for highway use. Q. Which octane grade should I use in my car? A. Use the recommendation in your car owner's manual as a starting point for selecting the proper gasoline. If you notice engine knock over an extended time and your engine is adjusted correctly, try a higher octane gasoline. Also, higher octane may provide a performance benefit (better acceleration) in cars equipped with knock sensors. Many late model and high-performance (turbo-charged and supercharged) cars fall into this category. ------------------------ Fuel octane requirements for gasoline engines vary with the compression ratio of the engine; diesel cetane requirements also vary with the compression ratio. Engine compression ratio is the relative volume of a cylinder from the bottom most position of the piston's stroke to the top most position of the piston's stroke. The higher an engine's compression ratio, the greater the amount of heat generated in the cylinder during the compression stroke. Posted octane numbers on gasoline pumps are a result of testing fuel performance under laboratory and actual operating conditions. The higher the octane rating on fuel the less volatile (evaporative qualities) and the slower the fuel burns. Higher octane fuel contains more POTENTIAL energy but requires the higher heat generated by higher compression ratio engines to properly condition the fuel to RELEASE that higher potential energy. In the refining process, fewer gallons of higher octane fuels are yielded from a barrel of raw crude. If fuel octane is too low for a given compression ratio, the fuel prematurely and spontaneously ignites too early and the fuel charge EXPLODES rather than BURNS resulting in incomplete combustion. The net effect is a loss in power and possible engine damage. The operator hears an audible "knock" or "ping", referred to as detonation. Detonation may vary from a faint noise on light acceleration to a constant, deep hammering noise while driving at a constant speed. Improper timing adjustments, vacuum leaks, or excessively lean fuel mixtures may also cause detonation. Many vehicle owners believe that higher octane fuels are better for their vehicles since they are labeled "PREMIUM." The logic is that since it is a premium fuel it must be better. In reality, the premium label originates from the higher cost to refine and the resultant higher retail cost. Some refiners label their high octane fuels "SUPER." Some owners think that these fuels will make their vehicles more powerful. Only engines with high compression ratios can deliver all the potential energy from higher octane fuels! Always consult the manufacturer's octane recommendation to determine the proper octane requirements for any given vehicle. Generally, engines with compression ratios of 9.3 : 1 or less will safely operate with unleaded 87 octane fuel. Engines with higher compression ratios usually require higher octane fuels. Many owners who operate vehicles designed to operate on 87 octane fuel experience ping and knock. They usually "fix" this problem by purchasing the higher priced, higher octane fuels. Most owner's manuals indicate that some light and intermittent ping is normal but that heavy or sustained ping or knock should be attended to by either purchasing the correct octane fuel or servicing the engine. Most fuel refiners blend fuels for geographic areas and adjust their blends seasonally. These blending techniques compensate for the decrease in oxygen content with an increase in altitude and compensate for volatility during the warmer or cooler seasons. Significant ambient temperature changes (40 degrees Fahrenheit) or altitude changes (4,000 feet) may cause some serious engine detonation. This problem is usually corrected by filling the tank with "local" fuel that has been properly blended for season and altitude. ----------- CULMINATING ACTIVITY: 1. Prepare a short report highlighting the need to determine octane for the selected vehicle. 2. Using your MPG data for each grade of fuel, compare the operating costs for ten years at 10,000 miles per year. Assume fuel costs $1.25 for 87 octane, $1.45 for 89 octane, and $1.65 for 91+ octane. FUEL OCTANE WORK SHEET 1. Octane recommendation (source: owner's manual): _______ 2. Fill tank with 91+ octane fuel; odometer reading: ______ 3. Fill tank with 91+ octane fuel; odometer reading: ______ Gallons purchased: ______ Traffic Mix: ________________________ 4. Fill tank with 89 octane fuel; odometer reading: ______ Gallons purchased: ______ Traffic Mix: ________________________ 5. Fill tank with 89 octane fuel; odometer reading: ______ Gallons purchased:______ Cost per gallon: ______ Traffic Mix:________________________ 6. Fill tank with 87 octane fuel; odometer reading: ______ Gallons purchased: ______ Cost per gallon: ______ Traffic Mix: ________________________ 7. Fill tank with 87 octane fuel; odometer reading: ______ Gallons purchased:______ Cost per gallon:______ Traffic Mix:________________________ 8. Type Fuel Cost/gallon MPG Cost for 10,000 miles 91+ octane 89 octane 87 octane INFORMATION CHECK Place a "T" before the statements that are true and an "F" before the statements that are false. After each false statement, explain why it is false. _______1. 91 octane fuel has more potential power than 87 octane fuel. _______2. To release all the potential power of 91 octane gasoline, it is necessary for an engine to have a compression ratio higher than 9.3 : 1. _______3. Any gasoline engine will efficiently burn any octane fuel. _______4. The engine's compression ratio is the most important factor in selecting gasoline octane. _______5. Posted fuel octane ratings are a result of testing fuel in the laboratory and under actual operating conditions. _______6. Diesel engines require a low octane fuel. _______7. Engine knock or ping is always a result of poor quality fuel. _______8. Gasolines are seasonally and regionally blended. TEACHER'S NOTES This is an important energy saving activity for students. Over the years, our society has somehow gotten the misnoma that a "Premium" grade fuel is better for a vehicle than a "Regular" grade fuel. Most vehicle owners never refer to their Owner's Manual to determine the acceptable octane fuel for their vehicle. Additionally, when a vehicle begins to knock and ping under acceleration, many owners apply a "quick fix" by switching to premium grade fuel. It is true that excessive and continued pinging and knocking is harmful to an engine and this MUST be attended to immediately or serious engine damage will result. Very few recently manufactured vehicles require premium, high octane fuel. To operate these vehicles with lower than required octane fuel could lead to immediate and serious internal engine damage. Most engines that do ping or knock on light acceleration do not need premium fuel; these engines need proper servicing such as timing adjustments, repairing vacuum leaks, or servicing emissions control valves. To operate a properly tuned engine designed for 87 octane with 89 or 91 octane fuel will only increase the cost of operation. No additional power, fuel economy, or durability will result from the higher octaned fuel. ANSWERS TO INFORMATION CHECK: 3. False. To efficiently burn any fuel of any octane, the compression ratio of the engine must be high enough to release all the potential fuel energy. 6. False. Diesel engine fuels are rated in cetane numbers, not in octane numbers. 7. False. Engine knock or ping may be a result of too low an octane rating for that engine's compression ratio; most engine knocking and pinging is a result of an out-of-tune engine or emissions control servicing. RECOMMENDED READING: Ellinger, Herbert E. Automechanics, Fourth Edition. Prentice Hall, Englewood Cliffs, New Jersey. 1988 Modified by SIred91 at 1:53 PM 4/23/2007

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[STREETWERKZ]

what about boost + heat ?

[89efDUSTY]

lowest grade possible will soon be 91

[SIred91]

Originally Posted by STREETWERKZ

what about boost + heat ?

Boost + heat = 105 oct or higher depending on compression and tune.

[STREETWERKZ]

we're talking about 11:1 compression, supercharged at 10 psi.

its not easy to find higher grade gas in my area.

on my old sohc setup, i ran 91, octane booster, colder plugs. at 5-6 psi... it ran okay, but at 10 psi, it would pop at higher rpms.


i also live right near the beach..... so, denser air, high hea from the blower, etc.

[wiZCo]

Great writeup, I'm switching back to 87. Thanks!

[89efDUSTY]

Originally Posted by 89efDUSTY

lowest grade possible will soon be 91

boost

[SIred91]

Originally Posted by STREETWERKZ

we're talking about 11:1 compression, supercharged at 10 psi.

its not easy to find higher grade gas in my area.

on my old sohc setup, i ran 91, octane booster, colder plugs. at 5-6 psi... it ran okay, but at 10 psi, it would pop at higher rpms.


i also live right near the beach..... so, denser air, high hea from the blower, etc.

You will have to tune around the grade of fuel available to you....

[brandoom]

I aways use 91 octane cause i thought it was good for my baby...

[CRX Toad]

Looks like a nice writeup. People still will never search for this. Put it in the FAQ links.

[Malgerus]

I run, 94 octane on an 11.3:1 LS/VTEC.

If you guys want to know if your engine is detonating at all, I suggest you make a detonation sensor. They can be made from plastic air line tubing and copper piping. I'm sure it's been done on here before, so do a search on it.

[janz3n]

i use 91 for better mpg on the highway. i love you SIred91

[janz3n]

oh and your poll is dumb.

[SIred91]

Originally Posted by janz3n

i use 91 for better mpg on the highway. i love you cause im jealous your so cool SIred91

Originally Posted by janz3n

oh and your poll is awesome and you are cool I love you so much.

[janz3n]

something like that

[Skipper75]

hmm..anyone know why i get an occasional small backfire when i let off the throttle?

[SIred91]

It aint got nuttin to do with octane, thats for sure.

Check your timing and do a tune up, check the idle control stuff...etc...

[Skipper75]

k thanks bro

[janz3n]

i get this aswell.. unless your talking about a light rumbling kinda sound thats different.. i get that because my tail pipe fell off.

if you get back fires when you let off the gas is the timing retarded or advanced? but im afraid to mess with timing since i get such good mpg. i also have to timing light so i do it by ear lol.