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Old 06-11-2009, 04:03 AM   #1
kuksul08
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100 Octane Gas

There's some 100 octane gas at a 76 station near my friend's house. At $9 a gallon he put a few gallons in and said it was noticeably faster (e36 M3).


Will my car benefit at all from it or not? It would just be to try a little...
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Old 06-11-2009, 04:18 AM   #2
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Probably not. He has a high compression ratio so he can benefit from higher octane gas, unless you have a FI'ed M52tu/M54, 91-93 should be the most you put in. It's like if you have a car made to run on 87 and you put 93 in nothing changes.

It might make your car run a little better i.e. whatever gas you have in now might not be fully reaching the cars potential but thats a trade off you make for the increased price and 91 is more then reasonable. Even if you did put it in, the car would only run like it was on the top usable fuel (for example, 95) so you'd be wasting a lot of money. From memory we have a 10.2:1 compression ratio and the E36 M3 has a 11.2:1, so it can harness a higher octane.

Last edited by Acarder; 06-11-2009 at 04:24 AM.
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Old 06-11-2009, 07:16 AM   #3
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From my experience I found it will make a slight difference. When running a higher octane like 100 really what happens is that the fuel combust more rapidly and hotter creating a slight bit of HP. My father and I have a oval race car that runs 112.

One day at the track I figured I'd put a splash of 105 Sunoco race fuel in my car and I must say she was running a bit more SPIRITED!!!!!

You have to be careful running high.....I'm mean really high octane because you can combust the fuel to HOT
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Old 06-11-2009, 07:27 AM   #4
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Originally Posted by TX_Bullet View Post
From my experience I found it will make a slight difference. When running a higher octane like 100 really what happens is that the fuel combust more rapidly and hotter creating a slight bit of HP. My father and I have a oval race car that runs 112.

One day at the track I figured I'd put a splash of 105 Sunoco race fuel in my car and I must say she was running a bit more SPIRITED!!!!!

You have to be careful running high.....I'm mean really high octane because you can combust the fuel to HOT
Wrong. Completely wrong. High octane burns SLOWER and as a result COLDER than low octane.

OP, it won't do a thing. Unless it's leaded, then it'll just plug your cats in a couple of minutes.
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Old 06-11-2009, 07:34 AM   #5
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Higher octane fuel just takes more energy to detonate, so when you have a high compression engine (ie, S52/S54) using a higher octane means that the fuel wont prematurely detonate due to the higher compression and cause the engine to "knock" while the piston is going down.

Last edited by Acarder; 06-11-2009 at 07:39 AM.
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Old 06-11-2009, 07:37 AM   #6
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From wat i remember about high octane is the higher the engine compression more octane it needs, so it doesn't pre-detonate, like turbos, chargers and high compression race or sport cars, just my 2 cent
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Old 06-11-2009, 07:39 AM   #7
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My 325 ran better on Shell V-Power Racing 100 RON but no longer available. 98 RON the best available now unfortunately.
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Old 06-11-2009, 07:50 AM   #8
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you can def feel the difference in the performance of the car
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Old 06-11-2009, 08:24 AM   #9
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If you get a shark injector or some kind of performance software it states it allows you to benefit from changes in octane 87-93 pump fuels... I don't see why this software wouldn't help you benefit from race fuel...
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Old 06-11-2009, 10:27 AM   #10
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using a higher octane means that the fuel wont prematurely detonate due to the higher compression and cause the engine to "knock" while the piston is going down.
Engine knock is premature ignition on the UP stroke of the engine otherwise known as the compression stroke...
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Old 06-11-2009, 10:30 AM   #11
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If you get a shark injector or some kind of performance software it states it allows you to benefit from changes in octane 87-93 pump fuels... I don't see why this software wouldn't help you benefit from race fuel...
unless you are increasing the compression ratio of the engine, i.e. milling the head, new pistons, supercharger, or turbo charger, etc. there is absolutely NOOO benefit to using higher octane fuels... you'd be wasting money. Software does not increase compression.
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Old 06-11-2009, 10:37 AM   #12
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unless you are increasing the compression ratio of the engine, i.e. milling the head, new pistons, supercharger, or turbo charger, etc. there is absolutely NOOO benefit to using higher octane fuels... you'd be wasting money. Software does not increase compression.
To my understanding, changing the valve timing - which can be accomplished through the VANOS, which is controlled by software - CAN change the compression. Same way the compression is backed off a bit when the anti-knock sensors start to detect knock.

Can't believe I'm replying to an octane thread.
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Old 06-11-2009, 10:44 AM   #13
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To my understanding, changing the valve timing - which can be accomplished through the VANOS, which is controlled by software - CAN change the compression. Same way the compression is backed off a bit when the anti-knock sensors start to detect knock.

Can't believe I'm replying to an octane thread.
THAT I didn't know - most engines that I'm accustomed to working with use Ignition Timing to control knock, not compression ... with that said, I REALLY doubt software could change valve timing so much that you'd need anything higher than 91-92 octance ... okay I'm done ...
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Old 06-11-2009, 10:46 AM   #14
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Originally Posted by nathancarter View Post
To my understanding, changing the valve timing - which can be accomplished through the VANOS, which is controlled by software - CAN change the compression. Same way the compression is backed off a bit when the anti-knock sensors start to detect knock.

Can't believe I'm replying to an octane thread.
LOL. Valve timing has nothing to do with compression. Knock sensors retard ignition timing, they cannot and do not change compression.
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Old 06-11-2009, 10:47 AM   #15
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Originally Posted by nathancarter View Post
To my understanding, changing the valve timing - which can be accomplished through the VANOS, which is controlled by software - CAN change the compression. Same way the compression is backed off a bit when the anti-knock sensors start to detect knock.

Can't believe I'm replying to an octane thread.
Valve timing is varied with RPM to assist with cylinder filling (and therefore cylinder pressures) and broaden the peak of the torque curve, as cylinder heads have different airflow characteristics as RPM (and therefore air velocity) changes. It has nothing to do with compression ratio.

Similarly, the knock sensors are used to retard ignition timing (and therefore the timing of the increase in cylinder pressures due to combustion) and have nothing to do with compression ratio.

Matterhorn's point remains that unless you are doing something to either a) increase cylinder pressures or b) advance the timing of said cylinder pressures beyond what BMW has designed, you will not benefit from using fuel that is ultimately less prone to ignition than 91 octane.
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Old 06-11-2009, 11:02 AM   #16
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Thanks for the clarification about the ignition timing, though it doesn't really make sense. If "knocking" happens when the fuel detonates before the spark, how does retarding the spark prevent knocking? Can't find a clear answer with searching; everything that I find just says that the software "changes the running conditions" to prevent knock - how vague.

Also, though the compression ratio doesn't change, the "cylinder pressure" can change if you change the valve timing - pardon my semantics in the previous post, perhaps I should have used the words "cylinder pressure" instead of the word "compression" which I didn't mean to be the same as "compression ratio." So, then, how does the Shark injector - widely regarded to be good and reliable software - claim to improve power and fuel economy with the use of higher-octane fuel? It changes the valve timing to increase cylinder pressure, correct?

Last edited by nathancarter; 06-11-2009 at 11:05 AM.
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Old 06-11-2009, 11:17 AM   #17
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Haha, yes I understand how octane works, and that our cars adapt to the type of fuel we put in using sensors. You're supposed to use 91 or higher, but 87 works and it just changes the timing. I thought maybe it would be able to advance the timing (more so than with 91) and maybe put out a little more power.

Probably little to no difference practically though...
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Old 06-11-2009, 11:52 AM   #18
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Originally Posted by nathancarter View Post
If "knocking" happens when the fuel detonates before the spark, how does retarding the spark prevent knocking? ........

So, then, how does the Shark injector - widely regarded to be good and reliable software - claim to improve power and fuel economy with the use of higher-octane fuel? It changes the valve timing to increase cylinder pressure, correct?
"Knocking" happens when you have too much ignition advance and the spark plug ignites the a/f mixture before the piston has reached TDC and begins traveling downwards. "Pre-detonation", on the other hand, occurs when the mixture auto-ignites (ie, without a purposeful ignition source) due to hot spots on the cylinder head, lean a/f mixtures, or the heat generated by compression.

I'm not familiar with the shark injector, but it's much more likely that it advances ignition timing instead of cam timing.
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Old 06-11-2009, 12:16 PM   #19
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Retarding ignition timing stops detonation becasue of when it ignites the fuel. As gasoline is compressed it becomes more easily ignitable and under enough pressure it will ignite on it's own (this is becasue of the heat caused when it is compressed). Because of the velocity that the piston moves at, your timing is ALWAYS advanced of TDC. The spark is fired well before (compared to the speed of the piston) the piston ever reaches the top. As the combustability of the fuel increases it takes less spark and less time to ignite it, therefore the spark needs to be released at a later time so that it will iginte at TDC.


Increasing your Octane past it's recomended level, without any modifications to the ignition system, can actually decrease performance. If the combustability of the fuel is to low it will ignite after TDC thus wasting power.

Considering your engine runs only about a 15-25% percent efficientcy, I wouldn't figit with it if you don't know what your doing. (A.K.A. adding unrecomended fuel)
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Old 06-11-2009, 12:20 PM   #20
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One of the strokes is the compression stroke, where the engine compresses a cylinder-full of air and gas into a much smaller volume before igniting it with a spark plug. The amount of compression is called the compression ratio of the engine. A typical engine might have a compression ratio of 8-to-1. (See How Car Engines Work for details.)

The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.

The compression ratio of your engine determines the octane rating of the gas you must use in the car. One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. So a "high-performance engine" has a higher compression ratio and requires higher-octane fuel. The advantage of a high compression ratio is that it gives your engine a higher horsepower rating for a given engine weight -- that is what makes the engine "high performance." The disadvantage is that the gasoline for your engine costs more.


http://science.howstuffworks.com/gasoline4.htm
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