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rost12
08-23-2003, 01:47 PM
Two questions for the smart people among us:

1) Is higher octane fuel better than low octane for modern cars?
2) What would be the side effects of using 89? (other then less power)


C'mon, aty, tell me it's a repost!

Nick325xiT 5spd
08-23-2003, 01:51 PM
Risk of knock that your ECU won't catch...

And there's this:
http://www.dynospotracing.com/octane.htm

numbersguy
08-23-2003, 01:54 PM
Before an expert sees this I'll give you the layman's version:

High compression motors such as the 3 series BMW, which is greater than 10:1 compression ratio, may cause lesser grades of fuel to detonate from compression instead of waiting for the spark. Diesel engines actually use ultra-high compression to detonate the fuel-air mixture but in gasoline engines this is a bad thing. Gasoline with a high octane rating is more resistant to detonation than lower rated gas, and that's why all high-compression motors, not just bimmers and not just late model cars require premium fuel.

In a motor with a lower compression ratio that is designed to run on regular grade gas it is just a waste of money to burn premium.

2b9m3w
08-23-2003, 01:58 PM
It's safe to use what the manufacturer recommends. Anything above that has been shown by various studies to be a waste i.e. cost doesn't warrant any benefit. See for example:

http://www.mydailydriver.com/mdd_content.cfm?ID=27

Pinecone
08-23-2003, 05:37 PM
It's safe to use what the manufacturer recommends. Anything above that has been shown by various studies to be a waste i.e. cost doesn't warrant any benefit. See for example:

http://www.mydailydriver.com/mdd_content.cfm?ID=27

And many of these are wrong.

Tests on E46 M3s show improvemnts in HP up to about 96 AKI fuel.

Also With a modern engine, there are times when using the recommended grade of fuel may not produce full rated HP. Like high air temps, very cold high pressure days. In these cases, while you won't get more than rated HP, you may get back to rated HP where with the lowest recommended grade of fuel may cause the knock sensor to lower power output.

The idea of never using any higher grade than recommended comes from the days before computer fuel injection and before computer spark advance. Nodays there is no single across the board answer for every car in every circumstance.

2b9m3w
08-23-2003, 09:13 PM
And many of these are wrong.

Tests on E46 M3s show improvemnts in HP up to about 96 AKI fuel.


..Probably tested by an oil company :)

Then again, I could see it being engine dependent. Why don't you try it yourself to see if it makes a difference in your car? I've tried "upgrading" on most of my cars and haven't seen any difference between that and what's recommended. Then again, I probably can't feel an additional 1/10 of a horsepower.

Nick325xiT 5spd
08-23-2003, 09:22 PM
..Probably tested by an oil company :)

Then again, I could see it being engine dependent. Why don't you try it yourself to see if it makes a difference in your car? I've tried "upgrading" on most of my cars and haven't seen any difference between that and what's recommended. Then again, I probably can't feel an additional 1/10 of a horsepower.
I like to run 94 octane during the summer. My car does feel more responsive. During the winter when it's cold, any branded 93 is just fine.

And with all due respect, BMW CANNOT recommend any more than 91 octane in the manual. How do you sell cars in a state when that state does not allow the sale of the recommended octane? Buyers would freak.

2b9m3w
08-23-2003, 09:33 PM
I like to run 94 octane during the summer. My car does feel more responsive. During the winter when it's cold, any branded 93 is just fine.

And with all due respect, BMW CANNOT recommend any more than 91 octane in the manual. How do you sell cars in a state when that state does not allow the sale of the recommended octane? Buyers would freak.

That's true. I wonder what they recommend in Germany or other countries where higher octane is available everywhere?

Pinecone
08-24-2003, 04:18 AM
..Probably tested by an oil company :)

Then again, I could see it being engine dependent. Why don't you try it yourself to see if it makes a difference in your car? I've tried "upgrading" on most of my cars and haven't seen any difference between that and what's recommended. Then again, I probably can't feel an additional 1/10 of a horsepower.

Nope, for one Car & Driver magazine did a test.

Yes, it is engine dependent. But as I said, even running the uggested octane, you may not get full rated HP in all conditions.

And if you have never found the throttle stop in your car, there will be no difference at all. The recommended octane rating (or technically AKI in the US), is for full throttle, max power concerns.

Pinecone
08-24-2003, 04:29 AM
That's true. I wonder what they recommend in Germany or other countries where higher octane is available everywhere?

Apples and oranges.

There are two OCTANE rating systems. RON and MON. Back in the old days the US used RON on the pumps. Except RON is not indicative of the performance of the fuel under all conditions. But then neither is MON. So the US adopted the AKI (Anti Knock Index) which is the average of the RON and MON (RON+MON/2). It turns out this is the best overall predictor of how the fuel will work in street driven cars.

The difference between the two tests has to do with operating conditions of he test engine. MON uses higher temps for intake air, oil temp, etc.

Europe and many onther places still use RON on the. And since there is no specific relationship between RON and MON the 98 RON gas n Germany may be higher OR LOWER AKI than US 93 AKI gasoline.

In fact BMW found that they were getting a number of engine failures from long autobahn runs. The tracked it down to a too low of MON for the fuel, even though the RON was fine. They found if the MON was lower than 10 points below the RON the engines could blow in sustained high RPM, moderate to high load conditions.

Funny, 98 RON fuel with the MON at 10 points below, works out to be 93 AKI. :) So our 93 AKI fuel is safer than Euro 98 RON with unknown MON fuel.

This also points out another potential problem with low AKI fuel, long high speed runs. There is something going on that the knock sensor is not catching that can cause engine damage.

Interesting, if you go to a specialty racing fuel company like VP Fuels, they publish MON or RON for various fuels depending on the use. The various uses makes the difference in what is more important.

And before someone asks, aviation fuel is not rated the same as auto fuel, so the tow sets of numbers are not directly comparable.

My personal opinion is, if you can't afford the right gas to run the car, you are probably over extended on the car itself.

rost12
08-24-2003, 08:48 AM
Low and high octane gasolines have the same amount of energy per unit volume, period.

Okay. So what IS the difference? Ease of "combustability"? :)

Nick325xiT 5spd
08-24-2003, 08:55 AM
Okay. So what IS the difference? Ease of "combustability"? :)
Yes. Lower octane fuel combusts at lower temperatures.

xspeedy
08-24-2003, 10:15 AM
Yes, it is better if your car requires it or the mfgr recommends it. Otherewise, it is not any better. The higher octane fuels don't contain more detergent - they are just higher octane.

Bottom line is if your car doesn't need it, you are throwing your money away by purchasing premium fuel.

Pinecone
08-24-2003, 10:48 AM
Yes. Lower octane fuel combusts at lower temperatures.

Nope, it is simply a measure of the resistance to detonation. Which is NOT the same thing as compression pre-ignition.

Detonation occurs AFTER the spark event, not before. It is a very complexprocess having to do with the heat and pressure in the combustion chamber during the burn time.

TerryY
08-24-2003, 11:04 AM
I believe that the total package of the gasoline may be as important to engine performance as the octane rating.

I have two 4 liter v-8's. Both are chipped with "Octane Neutral" Euro programmed chips. Both require 89 aki per the Owners Manual.

If I fill up alternating 89 and 92 at the local "cut rate" convenience store (and sometimes at a Texaco) I can save at most a penny or two a gallon. This then requires a bottle of injector cleaner every two (cheap cleaner) or three (good cleaner) tanks.

If I fill up at a Shell station with just 92 (yes another variable) I do not need to use injector cleaner at all.

Mileage with the cheap gas 11.9 to 12.4 mpg in town. With the Shell and constantly clean injectors it rose to 14.4 to 14.6 mpg. I have not done a long highway run yet but the "Before" mileage was a constant 24 and small change between Portland and Eugene. Short runs now I have seen 26+ but need a long run to be certain of the difference.

The 740 was the prime vehicle for this information. It was chipped for a while before switching gasoline’s. A Huge performance difference was noted with the chip change.

Performance increase after the swap to all Shell 92 was noticeable but probably due to clean injectors.

I am not advocating a change to Shell gasoline, just to a brand with a very good cleaning package. Chevron (Techroline) should have a very good package-their injector cleaners were the best that I found on the shelf but stations are rather few and far between here.

If I fill up at the normal level I buy about 14 to 15 gallons. So at 10 cents per gallon difference, $1.50 times 3 tanks is $4.50 and the good injector cleaner was about $5 a bottle.

WOW I'm saving money with 92!! 8+% better gas mileage and no injector cleaner!!

I might be able to save more running straight 89 octane from Shell but with the chips it would probably be beyond borderline on the octane. So I won't.

F1Crazy
08-24-2003, 03:06 PM
One of the benefits of running higher octane fuel is lower temperatures of cylinder head and thus less problems with the valvetrain and less strain on the oil.

Pinecone
08-24-2003, 06:10 PM
One of the benefits of running higher octane fuel is lower temperatures of cylinder head and thus less problems with the valvetrain and less strain on the oil.

Octane has nothing to do with burn temp. It ONLY relates to resistance to detonation.

Pinecone
08-24-2003, 06:15 PM
I believe that the total package of the gasoline may be as important to engine performance as the octane rating.

I have two 4 liter v-8's. Both are chipped with "Octane Neutral" Euro programmed chips. Both require 89 aki per the Owners Manual.

If I fill up alternating 89 and 92 at the local "cut rate" convenience store (and sometimes at a Texaco) I can save at most a penny or two a gallon. This then requires a bottle of injector cleaner every two (cheap cleaner) or three (good cleaner) tanks.

If I fill up at a Shell station with just 92 (yes another variable) I do not need to use injector cleaner at all.

Mileage with the cheap gas 11.9 to 12.4 mpg in town. With the Shell and constantly clean injectors it rose to 14.4 to 14.6 mpg. I have not done a long highway run yet but the "Before" mileage was a constant 24 and small change between Portland and Eugene. Short runs now I have seen 26+ but need a long run to be certain of the difference.

The 740 was the prime vehicle for this information. It was chipped for a while before switching gasoline?s. A Huge performance difference was noted with the chip change.

Performance increase after the swap to all Shell 92 was noticeable but probably due to clean injectors.

I am not advocating a change to Shell gasoline, just to a brand with a very good cleaning package. Chevron (Techroline) should have a very good package-their injector cleaners were the best that I found on the shelf but stations are rather few and far between here.

If I fill up at the normal level I buy about 14 to 15 gallons. So at 10 cents per gallon difference, $1.50 times 3 tanks is $4.50 and the good injector cleaner was about $5 a bottle.

WOW I'm saving money with 92!! 8+% better gas mileage and no injector cleaner!!

I might be able to save more running straight 89 octane from Shell but with the chips it would probably be beyond borderline on the octane. So I won't.

Two different things. But a good point none the less. Typically higher AKI fuels have a better additive pacakge than lower AKI (cheaper fuels) at the same station. Brand name (SHell, Texaco, Mobil, Chevron, etc) have better additive packages than off brand or spot market stores.

Even in a vehicle that only required 87 AKI, I have found that every so often a tank or two of Mobil Super Premium would increase the gas mileage by cleaning the injectors. Or I could by my own fuel system cleaner and add it to the cheap gas. :)

Difference gas mileage between two brands can also be due to formulation differences in some areas. So cheaper fuel may have more alcohols and other stuff to cut the expensive stuff. Engines burn fuel by the pound, but we buy it by the gallon, so a less dense fuel will require more volume per mile than a higher density fuel. And then there is the BTU per pond, which may also be lower, leading to using even more fuel.

The Roadstergal
08-24-2003, 06:40 PM
Nope, it is simply a measure of the resistance to detonation. Which is NOT the same thing as compression pre-ignition.

What is the difference between the two? If detonation is the combustion that occurs after spark, why do you want the gas to resist it instead of combusting completely?

CascadeTelcom
08-24-2003, 07:34 PM
I believe that the total package of the gasoline may be as important to engine performance as the octane rating.

I have two 4 liter v-8's. Both are chipped with "Octane Neutral" Euro programmed chips. Both require 89 aki per the Owners Manual.

If I fill up alternating 89 and 92 at the local "cut rate" convenience store (and sometimes at a Texaco) I can save at most a penny or two a gallon. This then requires a bottle of injector cleaner every two (cheap cleaner) or three (good cleaner) tanks.

If I fill up at a Shell station with just 92 (yes another variable) I do not need to use injector cleaner at all.

Mileage with the cheap gas 11.9 to 12.4 mpg in town. With the Shell and constantly clean injectors it rose to 14.4 to 14.6 mpg. I have not done a long highway run yet but the "Before" mileage was a constant 24 and small change between Portland and Eugene. Short runs now I have seen 26+ but need a long run to be certain of the difference.

I use Chevron "Techron" in all my vehicles old and new, especially those that call for 87 Octane fuel.

The 740 was the prime vehicle for this information. It was chipped for a while before switching gasoline’s. A Huge performance difference was noted with the chip change.

Performance increase after the swap to all Shell 92 was noticeable but probably due to clean injectors.

I am not advocating a change to Shell gasoline, just to a brand with a very good cleaning package. Chevron (Techroline) should have a very good package-their injector cleaners were the best that I found on the shelf but stations are rather few and far between here.

If I fill up at the normal level I buy about 14 to 15 gallons. So at 10 cents per gallon difference, $1.50 times 3 tanks is $4.50 and the good injector cleaner was about $5 a bottle.

WOW I'm saving money with 92!! 8+% better gas mileage and no injector cleaner!!

I might be able to save more running straight 89 octane from Shell but with the chips it would probably be beyond borderline on the octane. So I won't.

I use Chevron "Techron" in all my vehicles old and new. Especially those that call for lower 87 octane fuels.

bimNaround
08-24-2003, 07:40 PM
What is the difference between the two? If detonation is the combustion that occurs after spark, why do you want the gas to resist it instead of combusting completely?

Here is an explanation that I found. It correlates what I was told about the difference between detonation and pre-ignition from my pilot training days. Abnormal ignition in an aircraft engine is much worse of a problem than in a car in terms of the safety aspect.
-------------

The octane quality of a gasoline is its ability to resist detonation, a form of abnormal combustion. Detonation occurs when the air-fuel mixture reaches a temperature and/or pressure at which it can no longer keep from self igniting. Two types of abnormal combustion are common: the first is detonation as previously mentioned and the other is preignition.

Detonation occurs after the spark plug has ignited the air-fuel mixture and the flame front is moving smoothly across the combustion chamber. If, during this burning process, the unburned air-fuel mixture reaches a temperature and/or pressure at which it is no longer stable, it burns very rapidly causing a new flame front to collide with the one that originated at the spark plug. Maximum pressure in the cylinder occurs before the piston reaches top dead center (TDC) and that pressure try's to push the piston down before it is ready to go down. Piston burning and rod bearing damage are the result.

Preignition is the other bad actor and is usually started by a hot spot in the combustion chamber which causes the mixture to ignite before the plug fires. Under wide open throttle conditions, preignition will destroy pistons in seconds.

Pinecone
08-25-2003, 04:36 AM
Here is an explanation that I found. It correlates what I was told about the difference between detonation and pre-ignition from my pilot training days. Abnormal ignition in an aircraft engine is much worse of a problem than in a car in terms of the safety aspect.
-------------

The octane quality of a gasoline is its ability to resist detonation, a form of abnormal combustion. Detonation occurs when the air-fuel mixture reaches a temperature and/or pressure at which it can no longer keep from self igniting. Two types of abnormal combustion are common: the first is detonation as previously mentioned and the other is preignition.

Detonation occurs after the spark plug has ignited the air-fuel mixture and the flame front is moving smoothly across the combustion chamber. If, during this burning process, the unburned air-fuel mixture reaches a temperature and/or pressure at which it is no longer stable, it burns very rapidly causing a new flame front to collide with the one that originated at the spark plug. Maximum pressure in the cylinder occurs before the piston reaches top dead center (TDC) and that pressure try's to push the piston down before it is ready to go down. Piston burning and rod bearing damage are the result.

Preignition is the other bad actor and is usually started by a hot spot in the combustion chamber which causes the mixture to ignite before the plug fires. Under wide open throttle conditions, preignition will destroy pistons in seconds.

Nice, simple, accurate description.

When seen on a cylinder pressure graph, detonation is very not nice loooking. Lots of sharp peaks at a very high frequency. It sort of hammers the piston top. The combustion temp goes way up also.

This leads to the classic detonation failure, a semi melted hole pounded through the top of the piston.

Pinecone
08-25-2003, 04:38 AM
What is the difference between the two? If detonation is the combustion that occurs after spark, why do you want the gas to resist it instead of combusting completely?

There is combustion and there is detonation. Combustion is a smooth progression of burning across the combustion chamber.

Detonation is a very harse multiple spikes of high presure with extra high temperatures.

Combustion is a smooth push.

Detonation is like heating the piston higher than normal and hammering on the top wiht a hammer.

2b9m3w
08-25-2003, 02:50 PM
Ok, I now believe that there are conditions where a higher than recommended AKI gas can be advantageous.

From PV=nRT I can understand that higher ambient air temp would facilitate knocking, or cause the ECU to reduce engine output to avoid knocking. But what about ambient pressure? Does atmospheric pressure affect what happens inside the combusion chamber enough to cause a difference? Actually, come to think about it, maybe it's an air density thing. More air molecules = more reactants?

Pinecone
08-27-2003, 04:03 AM
Ok, I now believe that there are conditions where a higher than recommended AKI gas can be advantageous.

From PV=nRT I can understand that higher ambient air temp would facilitate knocking, or cause the ECU to reduce engine output to avoid knocking. But what about ambient pressure? Does atmospheric pressure affect what happens inside the combusion chamber enough to cause a difference? Actually, come to think about it, maybe it's an air density thing. More air molecules = more reactants?

Yeap, higher pressure = more air molecules. Only a player at WOT, as at lower opening, you would modulate the opening for the performance you want. But at WOT the engine will get more air molecules, requiring more fuel, and leading to higher cylinder pressures and best of all MORE POWER, but only if you don't get into detonation range and have the ECU reduce power.