5.0 idles better on 89 octane than 93? - Page 2 - Mustang Evolution

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Old 03-16-2015, 07:37 AM   #36
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Originally Posted by scurfie View Post
89 octane is more explosive than 91 or 93. Therefore it can't be compressed as highly as 93 with out detonating.
Its not more explosive, that is the wrong term. Chemicals are added to higher octane petroleum to keep detonation waves from occurring in higher compressions environments with early timing.

Correct Burning produces more power than detonation. Once Deflagration turns into detention, you lose power and destroy things.
Deflagration - Wikipedia, the free encyclopedia

In engineering applications, deflagrations are easier to control than detonations. Consequently, they are better suited when the goal is to move an object (a bullet in a gun, or a piston in an internal combustion engine) with the force of the expanding gas.
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Old 03-16-2015, 08:08 AM   #37
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I think you may be right with this. I think I have the same problem
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Old 03-16-2015, 08:12 AM   #38
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I think I am pretty convinced the idle problem with higher Octane is gas that is bad from sitting too long at bargain stations [ FYI Kroger and Tom Thumb use tier 1 gas] don't sell much of it. I had a bad idle once in my past V6 and it was from gas at a bargain station in the boonies far from home.
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Old 03-16-2015, 08:41 AM   #39
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I always use sunoco, and always used 93. I never noticed the Odell until I byaccidently put 89 in. And it ran really nice. I haven't tryed my bama sct x3 yet. But I think I will this weekend. Do I need to save my stock tune to my laptop before doing it?
SCT tuners will "save" the stock tune on the tuner every time you use a custom tune.
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Old 03-16-2015, 10:08 AM   #40
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Just to set the record straight. The only difference between 87, 91, and 93 is the fuels ability to resist pre-ignition. The lower the number the less compression/heat the fuel will take before it ignites. It's all about compression ratio...........mostly.
So, motors with VCT will ****** the ignition when using lower grade fuels.
In my drag car I had 15to1CR and used 114/116............and the difference in power from one to the other was less than 2%.
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Old 03-16-2015, 10:41 AM   #41
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Just to set the record straight. The only difference between 87, 91, and 93 is the fuels ability to resist pre-ignition. The lower the number the less compression/heat the fuel will take before it ignites. It's all about compression ratio...........mostly.
So, motors with VCT will ****** the ignition when using lower grade fuels.
In my drag car I had 15to1CR and used 114/116............and the difference in power from one to the other was less than 2%.
It is my understanding the cam shafts can be electronically adjusted on the 5.0 to allow more overlap between the intake and exhaust lift (lobe center) to decrease the pressure within the cylinder as well, effectively lowering the actual amount of air compression, making it less susceptible to pre-detonation. Maybe one of our in house tuners can verify that.
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Old 03-16-2015, 11:12 AM   #42
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And just so we have terminology straight, Pre-Ignition causes Detonation; circular references. A fuel-air charge that does not pre-ignite deflagrates instead when an even flame front and no detonation wave.
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Old 03-16-2015, 11:15 AM   #43
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Idle re-learn (see your manual)

this is in addition to the bit about the insert.
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Old 03-16-2015, 11:42 AM   #44
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And just so we have terminology straight, Pre-Ignition causes Detonation; circular references. A fuel-air charge that does not pre-ignite deflagrates instead when an even flame front and no detonation wave.
I'm impressed!
So, the higher the compression the slower do deflagrate.
Everybody that builds race motors is always hoping for even flame front in the combustion chamber...........easy to say, just a little harder to produce.
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Old 03-16-2015, 11:54 AM   #45
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I'm impressed!
So, the higher the compression the slower do deflagrate.
Everybody that builds race motors is always hoping for even flame front in the combustion chamber...........easy to say, just a little harder to produce.
Almost, Hi compression and timing advance increase any fuel/air mixture to detonate. Adding Chemicals to increase octane keep the mixture from detonating sooner by slowing the reaction down to keep a even flame front.
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Old 03-16-2015, 12:07 PM   #46
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Almost, Hi compression and timing advance increase any fuel/mixture to detonate. Adding Chemicals to increase octane keep the fuel from detonating sooner by slowing the reaction down to keep a even flame front.
I concur.
The flame front is mechanically controlled by combustion chamber/piston design and is a great word to talk about, just a little harder to get right in a real motor.
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Old 03-19-2015, 12:11 PM   #47
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Originally Posted by olerodder View Post
Just to set the record straight. The only difference between 87, 91, and 93 is the fuels ability to resist pre-ignition. The lower the number the less compression/heat the fuel will take before it ignites. It's all about compression ratio...........mostly.
So, motors with VCT will ****** the ignition when using lower grade fuels.
In my drag car I had 15to1CR and used 114/116............and the difference in power from one to the other was less than 2%.
Yep. Though lower octane has more potential energy. It takes less effort to combust it, so you could make more power from it. Although as you stated we want to avoid detonation and so make fuel harder to combust. And the overall compromise is a negligible amount of power made/lost.

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Old 03-19-2015, 12:22 PM   #48
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I would think that 93 would burn cleaner
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Old 03-19-2015, 12:37 PM   #49
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I would think that 93 would burn cleaner
What do you mean by cleaner?

You'll get the most efficiency from which ever fuel burns the most completely. Higher octane resist combustion and therefore takes more effort to combust. Which can result in less efficiency compared to lower octane fuel.

Cleanliness is not a concern across fuel grades.

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Old 03-19-2015, 01:00 PM   #50
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I think fuel and fuel rating is totally misunderstood by a of people.
Octane is no more than just a rating and has nothing to do about burning cleaner or making more HP.............unless the motor is modified. It's like taking the stock tune and putting in a modified tune. My SCT tuner has three tunes, one for 87, one for 91 and one for 93. Although I can't use the 93 because I don't have access to 93.............but trying to run 91 with the 93 tune could cause an issue that may turn the motor into a boat anchor.
Fuel is pretty complex and because fuels respond differently under varying engine loads, a gas may get a different octane rating on a free running engine vs. one under load.
There were/are two different ways Uncle Sam comes up with fuel rating..............."motor method rating" (basically where a motor is run under "load") and "research method rating" (where a motor is running free...without a "load") and to get the number you see on the gas pump they take an average of the two rating systems and come up with that number.
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Old 03-19-2015, 01:09 PM   #51
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In the good old days the "High Test" fuel was "cleaner". That fuel usually had more detergents too. Today? Not so much. Each brand takes a stock gas and adds their own dye and additives. There is only one pipe used to pump fuels from the refinery. When diesel is pumped, there is residue in the pipeline. The first couple of hundred gallons of gas pumped down that pipe is separated and sold at discount to the no-name gas companies. Stick with name brand fuel.
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Old 03-19-2015, 01:43 PM   #52
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In the good old days the "High Test" fuel was "cleaner". That fuel usually had more detergents too. Today? Not so much. Each brand takes a stock gas and adds their own dye and additives. There is only one pipe used to pump fuels from the refinery. When diesel is pumped, there is residue in the pipeline. The first couple of hundred gallons of gas pumped down that pipe is separated and sold at discount to the no-name gas companies. Stick with name brand fuel.
This is correct.
My grandfather was in the fuel delivery business for many years.
Those tanker trucks that you see delivering fuel at the gas station are the same trucks that they use to deliver diesel fuel, aviation fuel, and every other type of fuel that you can think of. At times, there may be a tiny bit of diesel mixed in with the gasoline, or vice versa.

But like my old Grampa says: "Just be happy that you're driving and not riding in a horse and buggy, like I used to have to do".
Blah, blah, blah...
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Old 03-19-2015, 03:40 PM   #53
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Maybe the 93 was not 93.
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Old 03-19-2015, 03:43 PM   #54
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Octane does not make fuel harder to combust, but resist detonation and stay at a even deflagration burn rate without a shock wave under more compression and added timing advance.
Chemicals are added to fireworks, rocket, and gun powders for the same reasons. Think rifle bullets vs pistol bullets. Rifle powder burns slower to get more net power out of a charge of powder because the barrel is longer and can burn longer before the bullet leaves. But that same charge in a pistol and you get a huge flash of wasted power out the front. Pistol powders have to burn quicker, but only up to the point they don't detonate. While this is not a direct comparison to gas octane, but to demonstrate how chemical modifiers and catalysts can control burning rates of fuels; powder or liquid fuels. It does not make the Rifle powder harder to combust, it modifies total burn time.

I agree on the pipeline post on discount brands vs top Tier. Today's top tier have an established standard for their additives and detergents that go beyond gov specs. Ford also Recomends top Tier gasoline.
Top Tier Gasoline

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Old 03-19-2015, 03:47 PM   #55
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Octane does not make fuel harder to combust, but resist detonation and stay at a even deflagration burn rate without a shock wave under more compression and added timing advance.
Chemicals are added to fireworks, rocket, and gun powders for the same reasons. Think rifle bullets vs pistol bullets. Rifle powder burns slower to get more net power out of a charge of powder because the barrel is longer and can burn longer before the bullet leaves. But that same charge in a pistol and you get a huge flash of wasted power out the front. Pistol powders have to burn quicker, but only up to the point they don't detonate. While this is not a comparison to gas octane, it is to educate on chemical modifiers and catalysts to control burning rates of fules, powder or liquid fuels.

I agree on the pipeline post on discount brands vs top Tier. Today's top tier have an established standard for their additives and detergents that go beyond gov specs. Ford also Recomends top Tier gasoline.
Top Tier Gasoline

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That does not make sense. The octane rating is the fuel's resistance to ignition/combustion under compression.

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Old 03-19-2015, 04:02 PM   #56
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That does not make sense. The octane rating is the fuel's resistance to ignition/combustion under compression.

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Oh and your ammo analogy is off too. Rifle powder and pistol powder can both have fast or slow burn rates. You select the best burn rate based on the weight of the bullet AND to repeat the same pressure curve every time you shoot. So if you want to talk about flame front propagation that would make more sense. Though simply as I said octane resist ignition, it does not modify burn rate / pressure waves. That's where the compression and the physical geometry of the cylinder come into play.

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Old 03-19-2015, 04:06 PM   #57
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What ever I do understand ammo very well and it seems you do as well. I do understand what you are saying is also correct. Still its is about burn rate modifiers and my example was to demonstrate how they are used.
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Old 03-25-2015, 08:49 AM   #58
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The gas quality is just not what it used to be, like the AM guy said. I used to fill my boat with pump gas all the time. Can't anymore, damn ethanol just turns to sludge crap and ruins gas tanks after it sits for a few weeks.
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Old 03-28-2015, 07:29 PM   #59
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2011 GT Premium...21K...always use Costco 91 octane. Purrs like a kitten...a horse kitten.
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