The difference between 87 and 91 octane, why it matters for the Si

amirza786

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I was at Costco the other day filling up when one of my co-workers pulls to the empty pump in front of me in his 2013 Toyota Corolla. I notice that he selects the 91 octane button, I did't say anything. When I got to the office I ran into him and I said hey Ed (let's call him Ed), I noticed you filled up with Premium. He smiles and says yeah, I use Premium sometimes to get better performance. I asked him what does the manual say to use, and he says 87, but he sometimes likes to use 91 to periodically "clean out" his engine and it runs a lot better when he uses Premium. He looked really convinced, so I just said OK and went back to my desk.

Like Ed, a lot of people have a huge misunderstanding when it comes to octane ratings, and have preconceived notions that premium gasoline smooths out engines better, gives them more power, and cleans their engine better, although their cars call for 87. So why is it our 1.5L Turbocharged engines do much better on premium than 87? And do even better on E40 or E85 when flex fuel kits are installed and the engine is properly tuned?

The truth is, 91, 93 and even 100 octane gas does not contain more energy than 87. there is not more bang for the buck, so to speak. The difference between the three octane levels is volatility, or how much heat or compression it takes to ignite it, and 87 wins in this category. Gasoline and air are compressed before they are sent to the cylinder, and in higher performance engines or those that are turbocharged or supercharged are compressed even higher, so if gasoline is to volatile, it will ignite to soon (pre-igntion) in the cycle causing knocks and pings and worse, Low Speed Pre-ignition (LSPI).

So why is it that when we put 87 octane in our Si's there is less power? When the ECU detects 87 octane, it wants to prevent knocks and pings and worse, LSPI, so it pulls back on timing, which in turn lowers performance and response. The same energy that is present in 91 or 93 is there, only those octane levels take much hotter conditions to ignite, those same conditions would pre-ignite 87 much sooner, possibly causing engine problems without the intervention of the ECU. With 91 or 93, the engine does not have to pull back performance to prevent knocks and pings, as there is no longer the issue with pre-igniting the fuel/air mixture to early

What about those who's engines that require 87 and they use 91 or 93? In most cases you are just wasting money and are gaining zero benefit, and in some cases you may be harming your engine in the long run as higher octane gas may not fully ignite due to its higher volatility, so you will have unburned gas left over

So to summarize, the difference between 87, 91 and 93 is volatility, with 87 being highly volatile as compared to higher octane gasoline's, which makes it unsuitable for many high performance engines that run hotter, further compress air and gas mixtures, and require higher octane gas. Basically, if your car calls for 87, you are not gaining any benefit from using a higher octane gas, wasting money, and in some cases may even be causing harm to your engine in the long run





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higher octane and more power is not a misunderstanding. You will never see someone gain power by using 87 over 93. Though a low compression, low hp engine wouldnt *really* benefit from 93, you are correct. You could word it differently. In laymens terms and just say higher octane burns slower, therefore takes more time to ping on the same heat.. or you have more crank degrees to play with while tuning.. Reverse engine psychology (just made that up) tells us that if it takes longer to ping on the combustion stroke we can make more power (heat, combustion volume, etc) with timing and still have max cylinder pressure after tdc and continue the combustion cycle. Timing before max pressure will detonate/ping and blow your head gaskets and rod bearings from the combustion pushing the piston down against rotation rather than supplying a good combustion stroke
 
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higher octane and more power is not a misunderstanding.
Eh, I'm siding with the OP. The fuel itself isn't making power so using higher octane fuel in itself won't change anything. The fuel might allow you to make more power by allowing different timing and gives people the opportunity to run higher compression and all that thus increasing power. I'm sure I'm not teaching you anything new but I just wanted to make to clear for those that don't know.
 

turbociv910

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Eh, I'm siding with the OP. The fuel itself isn't making power so using higher octane fuel in itself won't change anything. The fuel might allow you to make more power by allowing different timing and gives people the opportunity to run higher compression and all that thus increasing power. I'm sure I'm not teaching you anything new but I just wanted to make to clear for those that don't know.
No you're exactly right, same everything on a motor that can support 87 octane and not detonate isnt going to make more power simply by a going to 93. I guess i couldnt find my words. Changing fuel composition will, going to e85 on same boost level and you've got achly cooling effects. You're explaining the very thing that happens in our cars.. e40ish makes max power but e85 doesn't make any more.. because it literally is all the air the engine can move at that particular state.

A good test is to dyno in 87 and 93 and have a data log of ignition timing.. I wonder how much our ECU's pull or add timing.
 

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a good test is to dyno in 87 and 93 and have a data log of ignition timing.. I wonder how much our ECU's pull or add timing.
I'd love to see that. I want to know how "bad" it is to run 87 in my Si. Most of my driving is on the highway with the cruise set. I can't imagine it being terrible in that situation.
 

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I'd love to see that. I want to know how "bad" it is to run 87 in my Si. Most of my driving is on the highway with the cruise set. I can't imagine it being terrible in that situation.
Cruising timing is less affected by octane compared to wot timing, 100%. 10.6 is perfectly fine for 87 and cruising.. specially with DI, it takes away a huge knock window.. I would think 87 would only limit WOT power in our situation
 

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In Ed's case. He is just wasting money.

There are many cars out there that the owner has the right to choose which octane to use. The more octane the more power it will make.
For example:
Mustang Ecoboost
Mazda latest 2.5T
And many others have a power rating for regular and another for premium.
 

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There are several out there where the manufacturer won't specify using 91/93, but results show a significant change to performance through empirical observations. A good example is Hyundai's 2.0T.

It really has more to do with the ECU and how it manages ignition in real time. Honda manages timing based on continuous input from the knock sensor. And that can allow for additional gains on the base 1.5T, for example, even through 87 is listed exclusively.

Hondata goes into more depth about it here:
https://www.civicx.com/threads/prem...confirmed-by-honda-rep.828/page-9#post-129781
 
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higher octane and more power is not a misunderstanding. You will never see someone gain power by using 87 over 93. Though a low compression, low hp engine wouldnt *really* benefit from 93, you are correct. You could word it differently. In laymens terms and just say higher octane burns slower, therefore takes more time to ping on the same heat.. or you have more crank degrees to play with while tuning.. Reverse engine psychology (just made that up) tells us that if it takes longer to ping on the combustion stroke we can make more power (heat, combustion volume, etc) with timing and still have max cylinder pressure after tdc and continue the combustion cycle. Timing before max pressure will detonate/ping and blow your head gaskets and rod bearings from the combustion pushing the piston down against rotation rather than supplying a good combustion stroke
What I meant by misunderstanding is that most people assume that premium gas is different from regular and "unlocks" additional power and performance, when in fact it just has a higher ignite temp, which gives automakers the ability to improve efficiency by using higher compression ratios so they can improve spark timing. As we know, higher compression makes more power, but increases pre-ignition.

An interesting side, automakers are pushing for a 95 octane nationwide standard, as this would give them the ability to make engines smaller, more efficient and more powerful, but I think efficiency is the keyword here. As they make smaller engines more powerful and efficient, this introduces LSPI, 95 octane would let manufactures use higher compression with less knock. The additional cost of 95 would be offset by the savings in mpg, or so they say

BTW, I haven't heard from @charleswrivers in awhile, this is probably a discussion right up your alley
 
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What I meant by misunderstanding is that most people assume that premium gas is different from regular and "unlocks" additional power and performance, when in fact it just has a higher ignite temp, which gives automakers the ability to improve efficiency by using higher compression ratios so they can improve spark timing. As we know, higher compression makes more power, but increases pre-ignition.

An interesting side, automakers are pushing for a 95 octane nationwide standard, as this would give them the ability to make engines smaller, more efficient and more powerful, but I think efficiency is the keyword here. As they make smaller engines more powerful and efficient, this introduces LSPI, 95 octane would let manufactures use higher compression with less knock. The additional cost of 95 would be offset by the savings in mpg, or so they say

BTW, I haven't heard from @charleswrivers in awhile, this is probably a discussion right up your alley
I've heard about that push for an higher octane standard as well, but didn't realize they were pushing for 95. I like the idea, but unless the price of premium comes down (and in theory 95 would be even more expensive than 93) due to only needing to produce a single octane for distribution, I think consumers would be up in arms. In my area 93 is typically around 50-60 cents more per gallon than 87, which I highly doubt can be offset in terms of improved MPG.
 
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I've heard about that push for an higher octane standard as well, but didn't realize they were pushing for 95. I like the idea, but unless the price of premium comes down (and in theory 95 would be even more expensive than 93) due to only needing to produce a single octane for distribution, I think consumers would be up in arms. In my area 93 is typically around 50-60 cents more per gallon than 87, which I highly doubt can be offset in terms of improved MPG.
They want to make 95 octane the "Regular" and it would eventually be the only grade fuel eliminating all other grades. GM spokesman Tom Read is quoted as saying:

"This will have customer value if it is done correctly. Don't think of the premium fuel that is available today," Nicholson said. "If it is done in the right framework, it could have a lot of value for customers at a low rate if we pick the right octane level. If you go too high, it'll get expensive. But if you pick the right one, it'll actually work for customers. They can get around 3 percent fuel economy improvement for less than 3 percent"
 

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So he's saying that you'll get a 3% improvement in MPG for a less than 3% increase in cost compared to regular octane? So 93+ octane for ~$3.10 (if regular is ~$3). Seems ambitious to think consumers will be able to get high octane fuel that cheap. But who knows. Obviously one of the reasons higher octanes are so expensive today is due to lower demand. I actually never understood the pricing for the higher grades. 15 years ago you could get mid grade for 10 cents more than regular and premium for 10 cents more than mid. Today, it's more like 30 cents higher for each step. Doesn't make sense to me.
 

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I've heard about that push for an higher octane standard as well, but didn't realize they were pushing for 95. I like the idea, but unless the price of premium comes down (and in theory 95 would be even more expensive than 93) due to only needing to produce a single octane for distribution, I think consumers would be up in arms. In my area 93 is typically around 50-60 cents more per gallon than 87, which I highly doubt can be offset in terms of improved MPG.
There are gas stations in the surrounding areas that charge a dollar more for premium. It's nuts.
 

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I was at Costco the other day filling up when one of my co-workers pulls to the empty pump in front of me in his 2013 Toyota Corolla. I notice that he selects the 91 octane button, I did't say anything. When I got to the office I ran into him and I said hey Ed (let's call him Ed), I noticed you filled up with Premium. He smiles and says yeah, I use Premium sometimes to get better performance. I asked him what does the manual say to use, and he says 87, but he sometimes likes to use 91 to periodically "clean out" his engine and it runs a lot better when he uses Premium. He looked really convinced, so I just said OK and went back to my desk.

Like Ed, a lot of people have a huge misunderstanding when it comes to octane ratings, and have preconceived notions that premium gasoline smooths out engines better, gives them more power, and cleans their engine better, although their cars call for 87. So why is it our 1.5L Turbocharged engines do much better on premium than 87? And do even better on E40 or E85 when flex fuel kits are installed and the engine is properly tuned?

The truth is, 91, 93 and even 100 octane gas does not contain more energy than 87. there is not more bang for the buck, so to speak. The difference between the three octane levels is volatility, or how much heat or compression it takes to ignite it, and 87 wins in this category. Gasoline and air are compressed before they are sent to the cylinder, and in higher performance engines or those that are turbocharged or supercharged are compressed even higher, so if gasoline is to volatile, it will ignite to soon (pre-igntion) in the cycle causing knocks and pings and worse, Low Speed Pre-ignition (LSPI).

So why is it that when we put 87 octane in our Si's there is less power? When the ECU detects 87 octane, it wants to prevent knocks and pings and worse, LSPI, so it pulls back on timing, which in turn lowers performance and response. The same energy that is present in 91 or 93 is there, only those octane levels take much hotter conditions to ignite, those same conditions would pre-ignite 87 much sooner, possibly causing engine problems without the intervention of the ECU. With 91 or 93, the engine does not have to pull back performance to prevent knocks and pings, as there is no longer the issue with pre-igniting the fuel/air mixture to early

What about those who's engines that require 87 and they use 91 or 93? In most cases you are just wasting money and are gaining zero benefit, and in some cases you may be harming your engine in the long run as higher octane gas may not fully ignite due to its higher volatility, so you will have unburned gas left over

So to summarize, the difference between 87, 91 and 93 is volatility, with 87 being highly volatile as compared to higher octane gasoline's, which makes it unsuitable for many high performance engines that run hotter, further compress air and gas mixtures, and require higher octane gas. Basically, if your car calls for 87, you are not gaining any benefit from using a higher octane gas, wasting money, and in some cases may even be causing harm to your engine in the long run
Anyone know if 93 is better in the Si than 91?
 

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