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Old 03-10-2004, 12:46 AM   #1
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Default Octane rating & performance thought

I have always believed that 87 octane was OK to use unless the engine was pinging. Today I was on a LONG trip to a service call (300 miles round trip) and had some thoughts which were mostly bits and snippets of things I had read about late model engine control. So I get to wondering if the engine WAS pinging, the driver would not hear it because the timing would be retarded due to the knock sensor. How would you know if the engine wasnt performing up to par due to low octane?

since I was almost outta gas and eager to test my hypothesis, I stopped and filled up with with 89 octane. About 30 miles later or so..I noticed that the transmission wasnt hunting as much and seemed smoother. Did the extra octane really make a difference or is it all in my head?
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Old 03-10-2004, 02:32 AM   #2
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Default Just some thoughts...

it definitely makes a small difference--if that difference is perceiveable or not, is the question. Octane is simply a rating of how much pressure/heat needs to be applied to your fuel before it ignites without a spark. The higher the octane, the more resistant to pressure/temperature your fuel will be.

Therefore, even though your engine works on 87, there are times when your engine gets really warm and that pressure that builds causes your stage to ignite just before the spark. although it'* maybe not frequent enough to cause an audible knock, it happens, and that decreases your overall efficiency. Just how often that has to happen to make a knock I don't know, but consider youre going down the hiway at 1800 rpm, that means that divided by 6 cylenders, they each fire 300 times. just a fraction of a second ignition before the spark on less than 10 of those times might not be audible, but you won't get as much power.

I've noticed, especially in my supercharged bonnie that if I use 87, there is no get up and go, the car is just a hunk of metal. Although it doesn't knock, there is definitely something going on. So right now, for the past year, I've been trying different octanes and compiling mileage and the sort to come up with substantial proof that a higher octane even might improve your mileage, again, even if you don't hear it knocking audibly at lower octanes.

(I try to use 91 as a rule, and I don't have enough trials to yield a positive result, yet, in my mileage experiment) I live in iowa, and 89 is cheaper than 87, so I put that in all the cars I drive except my bonnie, since it gets premium treatment
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Old 03-10-2004, 02:37 AM   #3
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Almost everything you said is correct. But if the engine is at 1800 rpm, each cylinder will fire 900 times in that minute, being a four stroke engine, 1 power stroke per cylinder every two revolutions.
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Old 03-10-2004, 02:41 AM   #4
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glanghus, don't ever run anything lower than 91 in that SC car. That'* bad news.

To realise the benefit of running higher octane, hook up a scantool and go for a drive. Look at the degrees of timing advance. Then buy premium. After you burn half a tank, go out for a similar drive (same speeds/rpms) and check your timing advance again. It'll be higher (better).
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Old 03-10-2004, 11:22 AM   #5
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I always use 93...I used 89 once because I was extremely low on cash and the difference was unbelievable. There was a significant loss of power, loss of gas mileage, and a shudder that ruined the ride...I will never do that again.

BTW, I would use 91, as my manual reccommends, but it doesn't exist in my neck of the woods...in New England the octanes are 87, 89, and 93, and occasionally 95 (usually at Sunoco).
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Old 03-10-2004, 12:56 PM   #6
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I've always run 89 (midgrade in MN) but on the trip we filled up with 93, a combo of that, a new intake pipe and ratty filter, and misplaced sensors gave me 40 mpg, with plenty of power, and some speed runs.
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Old 03-10-2004, 09:50 PM   #7
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During my journeyman training we watched a video demonstrating the difference between low octane fule and high octane fuel. Basically a small bit of low octane was put into a long glass beaker and swished around and dumped out. Next it was laid on its side and lit at one end with a match. The low octane fuel burned across almost immediately with a big woosh almost a bang. The same demonstration was repeated with a high octane fuel and the fule burned alot slower across like a line of flame from one end to the other. Grant it that the low octane fuel was really low but it was just done to demonstrate the difference between the 2. glanghus is right about how it burns in the cylinder. One common misconception about high octane fuel is that it has more power. In actuality it has less BTUs (British Thermal Units) than lower test.
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Old 03-11-2004, 01:26 AM   #8
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You are exactly right, Canuck. High octane doesn't mean a lot of power. It'* funny listening to people brag about how they put airplane (100+ octane) fuel in their Chevelles and then say how the performance is superior. Once you reach that octane level where your fuel doesn't ignite without a spark (in a chevelle, 91 would be more than sufficient; if the chevelle has original engine, 89), it doesn't matter.

Sorry bout my example with 1800 rpm'*, etc...got ahead of myself with my #'*.

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Old 03-11-2004, 01:36 AM   #9
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But the minimum octane rating for an SSEi of 91 octane is critical. It is a requirement, not an option.
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Old 03-11-2004, 09:24 AM   #10
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Quote:
Originally Posted by willwren
It is a requirement, not an option.
If you want to keep your engine Octane is always optional. I always use 94 when I can however with Gas prices histing 2.09/gallon for 94 I have seriously thought about dropping to 93 or even 91 for a bit. Maybe even go that low and looking into the whole toulene thing. Anything below 91 is a no-no however for Vin 1'*.
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