high octane gas or low octane gas

darwin

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I believe in using the manufacturer recommended fuel. Save your money. Only cars I can think of that say to use high octane are cars like Mercedes, BMW, Audi, maybe Vettes etc.. You get the idea. Not all models, just specific engines. I'll have to admit on my m/c's I do usually use midgrade. Why, no reason except I feel better using it and it costs about 20c more per tank. Quality fuel with good cleaners is the answer for me. Over the years I've tried using 91oct in my cars, never ever noticed a difference in normal use. If your engine is pinging something is wrong and needs to be fixed.
 

Karl Snarl

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You have to qualify the type of fuel, and since most of us run on unleaded pump gas. higher octane = slower ignition in the same conditions as lower octane.
You say, "Slower ignition". Hence in other terms, more resistant to ignition. Not slower burning. If it were slower burning, we'd have some problems. Like lack of ability to get to high rpms. If your engine is spinning at 6k rpms burning 87 octane and you introduce a slower burning fuel, you lose rpms. The fuel would not be able to keep up. Think of an Indy car, spinning 18k rpms. If the higher octane fuel they run was slower burning that a normal 87, they would never reach the insane rpms they reach. Octane is just it's resistance to burning, not it's rate of burn. Lead in fuel does not slow the burn rate, it smooths the flame front. Allowing for a more even burn vs and uncontrolled burn, which leads to detonation. The reason why we have high octane fuel, to combat preignition and detonation. Allowing engines to run hotter with a lot higher compression. Which is able to extract more of the heat energy and change it into mechanical motion. Lead absorbs some of the heat, allowing for the burn to be controlled instead of going off like a bomb. Lead also makes an excellent lubricant. Now a days we use other chemicals to fulfill the role of lead. Chemicals that will absorb more heat before they crack and release their energy. Resulting in a smoother flame front and ability to handle the high compression ( higher thermal load on the fuel charge) before igniting. If we want to talk about burn rates, we can also talk about reloading firearm rounds. In those you pick a fuels burn rate, or wind up messing yourself up or destroying your firearm.

Remember at 6k rpms, the engine is firing 6000 times a minute( 60 times a second). If the fuel burnt slower in a high octane engine, high rpms would be unattainable with current designs. We would need huge stroke lengths to take up the slack of a slower burn rate.

p.s. I drew some of this out and made it in easier terms, not for you, I know you understand, but for others who may read this thread. To give them a simpler way of understanding, vs how you and I can go into deep terminology and converse.
 

ImpulseRocket

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You say, "Slower ignition". Hence in other terms, more resistant to ignition. Not slower burning. If it were slower burning, we'd have some problems. Like lack of ability to get to high rpms. If your engine is spinning at 6k rpms burning 87 octane and you introduce a slower burning fuel, you lose rpms. The fuel would not be able to keep up. Think of an Indy car, spinning 18k rpms. If the higher octane fuel they run was slower burning that a normal 87, they would never reach the insane rpms they reach. Octane is just it's resistance to burning, not it's rate of burn. Lead in fuel does not slow the burn rate, it smooths the flame front. Allowing for a more even burn vs and uncontrolled burn, which leads to detonation. The reason why we have high octane fuel, to combat preignition and detonation. Allowing engines to run hotter with a lot higher compression. Which is able to extract more of the heat energy and change it into mechanical motion. Lead absorbs some of the heat, allowing for the burn to be controlled instead of going off like a bomb. Lead also makes an excellent lubricant. Now a days we use other chemicals to fulfill the role of lead. Chemicals that will absorb more heat before they crack and release their energy. Resulting in a smoother flame front and ability to handle the high compression ( higher thermal load on the fuel charge) before igniting. If we want to talk about burn rates, we can also talk about reloading firearm rounds. In those you pick a fuels burn rate, or wind up messing yourself up or destroying your firearm.

Remember at 6k rpms, the engine is firing 6000 times a minute( 60 times a second). If the fuel burnt slower in a high octane engine, high rpms would be unattainable with current designs. We would need huge stroke lengths to take up the slack of a slower burn rate.
As I said, the term slower burning is still somewhat accurate if comparing the two fuels under the same conditions. it's just a laymans term for the average person to understand. To those of us that understand the difference, yes, it's not technically accurate.

You don't need to explain it to me, figured that was obvious from my last post, so you can lay off the encyclopedia explainica.
 

Karl Snarl

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As I said, the term slower burning is still somewhat accurate if comparing the two fuels under the same conditions. it's just a laymans term for the average person to understand. To those of us that understand the difference, yes, it's not technically accurate.

You don't need to explain it to me, figured that was obvious from my last post, so you can lay off the encyclopedia explainica.
As I said, I explained it for others who are reading the post. Please read the whole reply. Thank you.
 

ImpulseRocket

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As I said, I explained it for others who are reading the post. Please read the whole reply. Thank you.
I did, but when you quote me, it's also tagging me and puts me in a position where you are speaking to me directly. So, Expect a reply.

Plus, if we want to get hyper technical, the propagation of the flame front of a higher octane unleaded fuel does actually travel slower in the same atmospheric pressures as a lower octane fuel. That is actually the "slower" burn than the lower octane fuel, which is entirely because of the increased covalent bonds of the longer hydrocarbon chain. So, everything I said still applies about the slower burn with unleaded fuel. Slow is a relative term as we are talking small fractions of a second, so even at insane rpm, it still runs, but just loses efficiency. ;)
 

ImpulseRocket

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For Everybody. You can get some great understanding of burn rate and octane (which are not directly intertwined as I said previously, hence my focus on unleaded pump gas as the metric for everything I have been saying since it is what we all use). Sunoco has a great series of articles in their tech section. Yep, even a manufacturer of fuels acknowledges burn rates change. I like to provide sources, after all.
 

DAMIEN1307

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You say, "Slower ignition". Hence in other terms, more resistant to ignition. Not slower burning. If it were slower burning, we'd have some problems.
From Motor Trend, October 9th 2019.

"Fuel with an 87 octane rating burns more quickly while higher-octane fuels burn more slowly. In engines designed for standard unleaded fuel, efficiency and performance is optimized for 87 octane and could actually perform worse with higher-octane fuel since the burn rate is slower." Oct 9, 2019.

As I said, the term slower burning is still somewhat accurate if comparing the two fuels under the same conditions. it's just a laymans term for the average person to understand. To those of us that understand the difference, yes, it's not technically accurate.

You don't need to explain it to me, figured that was obvious from my last post, so you can lay off the encyclopedia explainica.
CORRECT.

Your splitting hairs again Karl, This is not the rocket fuel your running...lol...Enough said on this topic.
 

darwin

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Gotta say, I'm learning a few things from this back n forth discussion. Carry on....................................
 
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