The end-all be-all octane thread
10-20-2021, 02:28 PM
#1
The end-all be-all octane thread
I work as a specialist in octane rating. I know it inside and out. I see endless misinformation spread around every time I read a thread on octane, power, or mpgs. Some posters hit on the truth, but it is often drowned out in the noise. I'm hoping to write down some of what I know to help clarify what octane ratings mean and what they don't mean. I'm going to get a little technical, so feel free to ask for clarification if I get confusing.
What is engine knock
Engine knock occurs when fuel pre-detonates. This means it ignites independent of the spark plug firing. Pre-detonation is entirely dependent on heat and your fuel's octane rating. During the compression stroke, if the fuel/air mixture gets hot enough, it will auto-ignite. This creates a "flame front" that moves through the combustion chamber. When the spark plug fires, it creates a second flame front. The 2 flame fronts collide, creating a large spike in pressure and a loud noise that you usually hear as a "ping." That pressure spike is the reason knocking is bad. It can do bad things to engine components. Here's a decent illustration of the colliding flame fronts: https://2.bp.blogspot.com/_MI2abPmEr...e-knocking.gif
What is octane and how is it measured?
Ever wonder this? Did you know octane ratings are different in the US and Europe?
Octane rating is a completely arbitrary scale that was developed in the late-1920s to help standardize fuel quality so that the budding car industry could design its engines around consistent, high quality fuels. 100 years later and we still use the same scale and the same test methods. The developers of the octane scale defined it based on the knock characteristics of two pure fuels: Iso-octane and n-heptane. Iso-octane resists knock very well and was called 100 octane. n-heptane resists knock very poorly and was called 0 octane. Their knock characteristics blend together fairly linearly, and so the scale of 0-100 is based on volumetric blends of the two fuels (e.g. 85 octane reference fuel = 85% iso-octane and 15% n-heptane). With a known scale, every gasoline can be tested against prepared blends of iso-octane and n-heptane and given an octane rating.
The testing is done in the lab on a single-cylinder internal-combustion engine with a variable compression ratio. A knock sensor is inserted into the combustion chamber. A sample fuel is tested in comparison to reference fuels of known octane number. A gasoline sample rating is a calculation of how hard it knocks relative to the reference fuels. Easy peasy, lemon squeezy.
It gets a little more complicated. There are 2 types of test engines, and they simulate different conditions. One simulates driving down a flat highway (low load) and is called a RON engine (Research Octane Engine). The other simulates heavy acceleration up a hill in the summer (high load) and is called a MON engine (Motor Octane Engine). As you might guess, the octane rating from the MON is always lower than the RON (sometimes by as much as 10 octanes). In the US, a rating from both engines is taken and averaged. Next time you're at the pump, notice it'll say something like (R+M)/2 next to the fuel octane. In Europe, only the RON is tested. This is why Europe sells 93/95/98 octane fuels and the US sells 87/89/93. They're virtually the same fuels.
This is all octane ratings are. It's a measurement of how easily a fuel knocks. It isn't a test for energy density, tendency for carbon buildup, or level of particulates.
The role of heat
Every fuel has an autoignition temp, making heat a crucial component to whether you get knocking from your engine. You are much more likely to hear knock when hauling a heavy load up a steep hill in the middle of summer than coasting to a stoplight. Everything heats up under load.
Yet octane recommendations are often tied to the engine's compression ratio. Why? When you compress a gas, it heats up, and the more you compress it, the hotter it gets. A high compression ratio means the fuel/air mixture is compressed to a higher psi than their counterparts, which means the mixture gets hotter before the spark plug fires. High compression ratio engines need fuels with high octane ratings strictly for this reason.
The role of altitude on engine knock
Which octane should I use at altitude? This is one of the more common questions on here.
If you were to do a compression check on your truck at sea level and then drive to 10000 feet and do it again, what would you expect in your readings? Will your psi be higher, lower, or the same at 10000 feet?
The answer is lower. The relevant formula is PSI = (Compression Ratio)*(Atmospheric Pressure). The compression ratio in your engine is set and never changes. If atmospheric pressure is lower, cylinder PSI is lower. Less compression = less heat from compression = less tendency to knock. This is why high altitude locations sell 85 octane as regular. They perform similarly to an 87 octane fuel at sea level. Your engine has no way to compensate for the loss of atmosphere, unless it is turbocharged. That last bit is kind of an important thing for late-model F150 owners to know, yeah?
Turbocharging is where my knowledge base ends in relation to octane. I believe turbocharging compensates for lower atmospheric pressure, but I'm not 100% sure. I believe this is why the owner's manuals for late-model F150s all say to use 87 regardless of elevation.
I am positive it is okay to use 85 octane fuel in the naturally aspirated engines at altitude. I'm so certain about it, it's all I use in my 5.4L up here in the Rockies.
How modern engines compensate for knock
Modern engines have knock sensors that tell the computer to retard timing when pinging is detected. It's a really cool solution to protect owners from making costly mistakes, but it isn't fool proof. Retarding theknock spark does nothing to prevent pre-detonation, but firing the spark plug a moment later changes the timing of the collision of the flame fronts and reduces the severity of the pressure spike. However, later spark results also results in less force exerted on the crankshaft, so you get less HP. It's also possible to run a gasoline with such low octane that it will overwhelm the computer's ability to compensate.
Will running higher octane fuel improve my HP or MPGs?
You can probably answer this yourself at this point. The answer is no. Run the fuel your engine is designed for. It is true that the chemistry is quite different between premium and regular fuels. But premium is not cleaner or cleaner burning or more powerful than low octane fuel. Premium will not reduce carbon buildup or anything else. It is just better at resisting knock. There may be small differences in energy density, but they are negligible.
The misconception comes from high performance engines that require high octane fuel. Some think, "A Ferrari runs best on premium fuel, so it must be good for my engine too." The need for premium fuel is due to higher compression ratios and nothing else. You gain nothing by running 100 octane fuel in an engine designed for 87. If your engine is knocking with the fuel it's designed for, something else is going on and you should get it fixed!
What is engine knock
Engine knock occurs when fuel pre-detonates. This means it ignites independent of the spark plug firing. Pre-detonation is entirely dependent on heat and your fuel's octane rating. During the compression stroke, if the fuel/air mixture gets hot enough, it will auto-ignite. This creates a "flame front" that moves through the combustion chamber. When the spark plug fires, it creates a second flame front. The 2 flame fronts collide, creating a large spike in pressure and a loud noise that you usually hear as a "ping." That pressure spike is the reason knocking is bad. It can do bad things to engine components. Here's a decent illustration of the colliding flame fronts: https://2.bp.blogspot.com/_MI2abPmEr...e-knocking.gif
What is octane and how is it measured?
Ever wonder this? Did you know octane ratings are different in the US and Europe?
Octane rating is a completely arbitrary scale that was developed in the late-1920s to help standardize fuel quality so that the budding car industry could design its engines around consistent, high quality fuels. 100 years later and we still use the same scale and the same test methods. The developers of the octane scale defined it based on the knock characteristics of two pure fuels: Iso-octane and n-heptane. Iso-octane resists knock very well and was called 100 octane. n-heptane resists knock very poorly and was called 0 octane. Their knock characteristics blend together fairly linearly, and so the scale of 0-100 is based on volumetric blends of the two fuels (e.g. 85 octane reference fuel = 85% iso-octane and 15% n-heptane). With a known scale, every gasoline can be tested against prepared blends of iso-octane and n-heptane and given an octane rating.
The testing is done in the lab on a single-cylinder internal-combustion engine with a variable compression ratio. A knock sensor is inserted into the combustion chamber. A sample fuel is tested in comparison to reference fuels of known octane number. A gasoline sample rating is a calculation of how hard it knocks relative to the reference fuels. Easy peasy, lemon squeezy.
It gets a little more complicated. There are 2 types of test engines, and they simulate different conditions. One simulates driving down a flat highway (low load) and is called a RON engine (Research Octane Engine). The other simulates heavy acceleration up a hill in the summer (high load) and is called a MON engine (Motor Octane Engine). As you might guess, the octane rating from the MON is always lower than the RON (sometimes by as much as 10 octanes). In the US, a rating from both engines is taken and averaged. Next time you're at the pump, notice it'll say something like (R+M)/2 next to the fuel octane. In Europe, only the RON is tested. This is why Europe sells 93/95/98 octane fuels and the US sells 87/89/93. They're virtually the same fuels.
This is all octane ratings are. It's a measurement of how easily a fuel knocks. It isn't a test for energy density, tendency for carbon buildup, or level of particulates.
The role of heat
Every fuel has an autoignition temp, making heat a crucial component to whether you get knocking from your engine. You are much more likely to hear knock when hauling a heavy load up a steep hill in the middle of summer than coasting to a stoplight. Everything heats up under load.
Yet octane recommendations are often tied to the engine's compression ratio. Why? When you compress a gas, it heats up, and the more you compress it, the hotter it gets. A high compression ratio means the fuel/air mixture is compressed to a higher psi than their counterparts, which means the mixture gets hotter before the spark plug fires. High compression ratio engines need fuels with high octane ratings strictly for this reason.
The role of altitude on engine knock
Which octane should I use at altitude? This is one of the more common questions on here.
If you were to do a compression check on your truck at sea level and then drive to 10000 feet and do it again, what would you expect in your readings? Will your psi be higher, lower, or the same at 10000 feet?
The answer is lower. The relevant formula is PSI = (Compression Ratio)*(Atmospheric Pressure). The compression ratio in your engine is set and never changes. If atmospheric pressure is lower, cylinder PSI is lower. Less compression = less heat from compression = less tendency to knock. This is why high altitude locations sell 85 octane as regular. They perform similarly to an 87 octane fuel at sea level. Your engine has no way to compensate for the loss of atmosphere, unless it is turbocharged. That last bit is kind of an important thing for late-model F150 owners to know, yeah?
Turbocharging is where my knowledge base ends in relation to octane. I believe turbocharging compensates for lower atmospheric pressure, but I'm not 100% sure. I believe this is why the owner's manuals for late-model F150s all say to use 87 regardless of elevation.
I am positive it is okay to use 85 octane fuel in the naturally aspirated engines at altitude. I'm so certain about it, it's all I use in my 5.4L up here in the Rockies.
How modern engines compensate for knock
Modern engines have knock sensors that tell the computer to retard timing when pinging is detected. It's a really cool solution to protect owners from making costly mistakes, but it isn't fool proof. Retarding the
Will running higher octane fuel improve my HP or MPGs?
You can probably answer this yourself at this point. The answer is no. Run the fuel your engine is designed for. It is true that the chemistry is quite different between premium and regular fuels. But premium is not cleaner or cleaner burning or more powerful than low octane fuel. Premium will not reduce carbon buildup or anything else. It is just better at resisting knock. There may be small differences in energy density, but they are negligible.
The misconception comes from high performance engines that require high octane fuel. Some think, "A Ferrari runs best on premium fuel, so it must be good for my engine too." The need for premium fuel is due to higher compression ratios and nothing else. You gain nothing by running 100 octane fuel in an engine designed for 87. If your engine is knocking with the fuel it's designed for, something else is going on and you should get it fixed!
Last edited by jewbacca; 10-20-2021 at 02:58 PM.
The following 14 users liked this post by jewbacca:
angelo7 (10-20-2021),
Bahamaroot (10-21-2021),
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and 9 others liked this post.
10-20-2021, 02:38 PM
#2
Senior Member
10-20-2021, 02:39 PM
#3
Senior Member
Spot on....you summed it up at the end. It really is that simple.
Now...since I've never looked at my owners manual ....what is a 2.7L eco supposed to run?
Now...since I've never looked at my owners manual ....what is a 2.7L eco supposed to run?
10-20-2021, 02:42 PM
#5
Senior Member
10-20-2021, 02:53 PM
#6
10-20-2021, 02:55 PM
#7
Senior Member
I'm not seeing end-all be-all here. Just another endless opinion thread.
Have fun with it.
Have fun with it.
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10-20-2021, 03:06 PM
#8
The following 2 users liked this post by jewbacca:
Bahamaroot (10-21-2021),
dsg2003mach1 (10-21-2021)
10-20-2021, 04:01 PM
#9
Senior Member
Citations are needed. Very common in the scientific and engineering world. The words you wrote are your own interpretations of things that you feel that you understand. You seem to realize that, you used the word "believe" for a statement or two. You haven't disclosed your credentials so you're just a guy who might be reading Wikipedia and decided to start an endless opinion thread. If you are a trained engineer or scientist then you will easily be able to find published studies that support your opinions. Without those, you're just a guy writing stuff on the internet. Like me. Why should anyone believe either of us?
I believe turbocharging compensates for lower atmospheric pressure, but I'm not 100% sure. I believe this is why the owner's manuals for late-model F150s all say to use 87 regardless of elevation.
I am positive it is okay to use 85 octane fuel in the naturally aspirated engines at altitude. I'm so certain about it, it's all I use in my 5.4L up here in the Rockies.
Will running higher octane fuel improve my HP or MPGs?
You can probably answer this yourself at this point. The answer is no.
I am positive it is okay to use 85 octane fuel in the naturally aspirated engines at altitude. I'm so certain about it, it's all I use in my 5.4L up here in the Rockies.
Will running higher octane fuel improve my HP or MPGs?
You can probably answer this yourself at this point. The answer is no.
10-20-2021, 05:35 PM
#10
in the house
Gawd...ya have to love it when a self-proclaimed 'expert' just HAS to offer his...not OPINION, but FACTS! And all 19 posts into things...
One way to build post counts I suppose...but the biggest issue here is...someone REALLY thinks this is the FIRST time information like this has been posted? What next...THE definitive "why dino oil is better than synthetic" thread?
One way to build post counts I suppose...but the biggest issue here is...someone REALLY thinks this is the FIRST time information like this has been posted? What next...THE definitive "why dino oil is better than synthetic" thread?