What type of octane I should use???

[SophieSleeps]

Originally Posted by bigsky2

So the burn rate, if you will, is the same across all octanes?

Then why... are there arguments that higher octane or lower octane provide better/worse mileage...

Because they are stupid.

Mileage is determined by how much fuel you use.
How much fuel you use is determined by your air/fuel ratio.
Air/fuel ratio is measured from your primary 02 sensors in your exhaust.
In closed loop operation, your car reads this Air/Fuel ratio and makes adjustments to injector duty cycle.
If your injector duty cycle is higher, you inject more fuel. If lower, less.

Running different octane has no effect on your air/fuel ratio.
Why? Because the ratio is AIR and FUEL.
So naturally it will have no effect on mileage.


If at all, I would think lower octane fuel would be better gas mileage because it is more likely to burn. Then you would have less unburnt fuel exiting via your exhaust. With higher octane fuel, it is less likely to pre-ignite or detonate so you have more a chance of getting unburnt/unused fuel (waste)

[bigsky2]

Originally Posted by SophieSleeps

If at all, I would think lower octane fuel would be better gas mileage because it is more likely to burn. Then you would have less unburnt fuel exiting via your exhaust. With higher octane fuel, it is less likely to pre-ignite or detonate so you have more a chance of getting unburnt/unused fuel (waste)

Understood and noted SophieSleeps; good explanation on the AIR/FUEL issue.

I'm slightly confused about your last point, because again, I was informed that lower octane fuel could (strong emphasis on could) provide for better fuel mileage because it is not burning as fast as a higher octane fuel.

However, you have stated that it burns more easily, and it relates to your statement about the AIR/FUEL mixture.

I'm just getting conflicting information, and I'm trying to make sure I get my information straight

BTW, it's ok if you don't feel like you need to explain any further, I am enough of a go-getter to do some research in the mean time

[SophieSleeps]

Originally Posted by bigsky2

Understood and noted SophieSleeps; good explanation on the AIR/FUEL issue.

I'm slightly confused about your last point, because again, I was informed that lower octane fuel could (strong emphasis on could) provide for better fuel mileage because it is not burning as fast as a higher octane fuel.

However, you have stated that it burns more easily, and it relates to your statement about the AIR/FUEL mixture.

I'm just getting conflicting information, and I'm trying to make sure I get my information straight

BTW, it's ok if you don't feel like you need to explain any further, I am enough of a go-getter to do some research in the mean time

Well...I have not made any reference to burn rates yet...only the likelyness that it will ignite.

Burn rate aside...
Lower octane is more likely to ignite.

Go-getting is good.

If you find more info on octane...let us know.
A word of advice. Be choosy about what you read and take as fact.
A lot of people on forums perpetuate rumors...and they don't provide reasoning.

[Tankota]

Ahhh.. Thank you.. somebody who knows their stuff on gas.. I don't know how many times I've had to try and explain octane to people, and that they only need to run what's spec'd by the manufacture, unless the engine is modified. And I have yet to see one bit of hard proff that running higher octane gas in a lower octane engine provides any power or milage gains (which goes against the logic) Almost all have has some faulty testing or just "well, my uncle Joe says..."

One point I'd like to add - the energy in both normal gas and high octane gas is the same, there is no added energy in high octane gas. thus no added power or milage. It only resists ignition at higher compression rates. Once either gas is ignited they both release the same amount of energy.

Originally Posted by SophieSleeps

Well...I have not made any reference to burn rates yet...only the likelyness that it will ignite.

Burn rate aside...
Lower octane is more likely to ignite.

Go-getting is good.

If you find more info on octane...let us know.
A word of advice. Be choosy about what you read and take as fact.
A lot of people on forums perpetuate rumors...and they don't provide reasoning.

[bigsky2]

Ok, so after visiting two stupid proof sites, I discovered the following:

The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.

From HowStuffWorks http://auto.howstuffworks.com/question90.htm

I then went to Wikipedia, which has a very thorough analysis of "Octane Rating" and I found this:

It might seem odd that fuels with higher octane ratings burn less easily, yet are popularly thought of as more powerful. The misunderstanding is caused by confusing the ability of the fuel to resist compression detonation (pre-ignition = engine knock) as opposed to the ability of the fuel to burn (combustion). However, premium grades of petrol often contain more energy per litre due to the composition of the fuel as well as increased octane.

Complete article at: http://en.wikipedia.org/wiki/Octane_rating

[bigsky2]

Higher octane gives better mileage, in a world where people have no clue.

[silverRS]

yeah im using an Ethanol blend rated at 89 octane wich is just right for the Yaris, the manual at least the Canadian one suggests using Ethanol blends.

[SophieSleeps]

Originally Posted by silverRS

yeah im using an Ethanol blend rated at 89 octane wich is just right for the Yaris, the manual at least the Canadian one suggests using Ethanol blends.

That is actually one thing that may decrease your gas mileage...since ethanol decreases combustability of gasoline...

I think all of Canada is on some sort of Ethanol/gas mix right?

[argylesocks]

i just checked with the toyota chat line... they said for US, they recommend 87 octane for all Yaris.

[TRD_Yaris]

do you guys have 87 or 89?? here in texas we get 93 octane as premium, but i can't remember what the regular unleaded is..

[argylesocks]

Originally Posted by TRD_Yaris

do you guys have 87 or 89?? here in texas we get 93 octane as premium, but i can't remember what the regular unleaded is..

here in new england

regular = 87
midgrade = 89
premium = 93

[07WYarisRS]

Rule of thumb is use only the octane needed to prevent knock (detonation)
Anything higher then that and you lose HP and mpg. BUT
Most of these rumors about high octane fuels do not even apply to modern EFI engines. Carbureted engines only folks...

First off burn rates between high octane and low octane can be and often are the same. The difference is the flash point.
Flash point is the temp the fuel is ignited. With too much heat (compression) the fuel will be ignited before the spark plug ignited the mixture and that causes pre-ignition and detonation.
Most factory cars out there can run on low octane, some recommend higher octane for turbo and supercharged cars. With EFI cars today the ECU uses readings from sensors like the o2 sensors and knock sensors. If the engine starts to knock because of low octane or even high loads using high octane it does two things. #1 it pulls the timing. #2 adds more fuel.
With a carbed engine the engine would just knock until it fell apart...
With EFI systems there is no need to run a higher octane fuel unless you running crazy compression and high boost levels.

WFT is the point of building an econo box that requires the most expensive octane fuel. Give your head a shake people...

FWIW high octane fuels do not have more additives.
They contain fewer additives that promote detonation.
Octane boosters do not add octane. Most only but up the rating 3-7 points witch is less then one octane level. Octane booster do however contain additives the remove or neutralize free radicals in the fuel that promote detonation and pre-ignition.
It’s the additives in fuels that change the burn rates not the octane.

These engine were designed to use 87 octane here in North America, anything over that you are wasting your $$$ and lining the pockets of the oil companies. Not to mention doing exactly the opposite of what the car was designed to do SAVE $$$ and be efficient

When you add high octane to an EFI car right away the car is going to run slightly rich as the higher flash point fuel is not going to burn as efficiently, so the ECU using the reading from all the sensors will adjust the timing and fuel trims to burn at the proper mixture (stoich) Regardless what fuel you use running stioch will produce the same HP and KPG
So doing so you end up with the same HP and MPG but less $$$ in your pocket at the end of each week

[SophieSleeps]

Originally Posted by 07WYarisRS

Rule of thumb is use only the octane needed to prevent knock (detonation)
Anything higher then that and you lose HP and mpg. BUT
Most of these rumors about high octane fuels do not even apply to modern EFI engines. Carbureted engines only folks...

First off burn rates between high octane and low octane can be and often are the same. The difference is the flash point.
Flash point is the temp the fuel is ignited. With too much heat (compression) the fuel will be ignited before the spark plug ignited the mixture and that causes pre-ignition and detonation.
Most factory cars out there can run on low octane, some recommend higher octane for turbo and supercharged cars. With EFI cars today the ECU uses readings from sensors like the o2 sensors and knock sensors. If the engine starts to knock because of low octane or even high loads using high octane it does two things. #1 it pulls the timing. #2 adds more fuel.
With a carbed engine the engine would just knock until it fell apart...
With EFI systems there is no need to run a higher octane fuel unless you running crazy compression and high boost levels.

WFT is the point of building an econo box that requires the most expensive octane fuel. Give your head a shake people...

FWIW high octane fuels do not have more additives.
They contain fewer additives that promote detonation.
Octane boosters do not add octane. Most only but up the rating 3-7 points witch is less then one octane level. Octane booster do however contain additives the remove or neutralize free radicals in the fuel that promote detonation and pre-ignition.
It’s the additives in fuels that change the burn rates not the octane.

These engine were designed to use 87 octane here in North America, anything over that you are wasting your $$$ and lining the pockets of the oil companies. Not to mention doing exactly the opposite of what the car was designed to do SAVE $$$ and be efficient

When you add high octane to an EFI car right away the car is going to run slightly rich as the higher flash point fuel is not going to burn as efficiently, so the ECU using the reading from all the sensors will adjust the timing and fuel trims to burn at the proper mixture (stoich) Regardless what fuel you use running stioch will produce the same HP and KPG
So doing so you end up with the same HP and MPG but less $$$ in your pocket at the end of each week

^ someone who knows his stuff.
I hate it when people use high octane gas in their cars and claim they "feel" a difference. I feel bad for them.

I don't necessarily agree about higher octane = loss of power and fuel efficiency. What is your reasoning behind that?

[07WYarisRS]

Originally Posted by SophieSleeps

^ someone who knows his stuff.
I hate it when people use high octane gas in their cars and claim they "feel" a difference. I feel bad for them.

I don't necessarily agree about higher octane = loss of power and fuel efficiency. What is your reasoning behind that?

Again that's directed more towards NON fuel injected cars/engines.
A fuel that has a high flashpoint will not ignite as quickly and in a engine that can't compensate by adjusting the timming or fuel delivery that's going to cause a slightly rich mixture as it takes longer to ignite. And we all know the leaner your engine runs the more HP it can make. A sign that an engine is getting more fuel then it needs is carbon build up and even black smoke from the exhaust. This unburnt fuel kills the cat convertors quickly and reduces HO even more over time by choking off exhaust ports and build up in ring lands causing premature wear and blocked oil flow.

With an EFI system the car can make the adjustments needed to correct the fuel/air mixture to some extent.
This is why most Etest places recommend NOT using high octane fuel...

I've been building racing ATV/ motorcycle engines for years and have tested several different fuels and oil on the dyno as well as CHT and EGT gauges (cylinder head temp & exhaust gas temp).
If you set up a carbureted engine to run at WOT at say 1250 degrees on 87 octane, then run that same engine on 94 it's going to drop the egt slightly like 1210-1230 and that just means that you are missing out on HP. In most cases I am able to drop down a size or two on the main jet to gain back the HP and still be at safe operating EGTs.

The computer in a efi car will do the same to compensate by adjusting the fuel trims to lean it out and correct the fuel mixture to restore the hp.

Even at cruise speeds when the A/F mixture is cycling back and forth between too rich and too lean you are missing out on some HP.

Toyota's new ECU programming and use of wideband O2 sensors allow for a much more accurate air/fuel mixture. Instead of cycling back and forth between rich and lean like the old systems they are able to maintain a much closer mixture to stoich with out bouncing back and forth from one extreme to the next.

But like I said no matter what octane you use in your car the ECU/injectors etc will deliver a mixture as close as possible to Stoich and regardless what octane is used the engine with a perfect fuel air mixture and correct timing will not knock.

The only time a person needs to worry about knock and may want to think about using a higher octane is if knock is occuring at steady speeds under normal loads. Many times a higher octane is not even needed, you can just switch brands of fuel. If a higher octane is needed go up 2 points. eg 87-89. There is never any need to jump from 87-91-94 unless you have bumped up the compression an extra 50 psi. An engine like ours that is designed to run 87 and is suffering from detonation will not need more then 89 if it does you have a serious problem.

Occasionally our engines will experiance light ping/knock (spark knock/detonation) a bit under heavy loads but this is no cause for concern, it simply means our engines are operating efficiently.

[SophieSleeps]

Originally Posted by 07WYarisRS

Again that's directed more towards NON fuel injected cars/engines.
A fuel that has a high flashpoint will not ignite as quickly and in a engine that can't compensate by adjusting the timming or fuel delivery that's going to cause a slightly rich mixture as it takes longer to ignite. And we all know the leaner your engine runs the more HP it can make. A sign that an engine is getting more fuel then it needs is carbon build up and even black smoke from the exhaust. This unburnt fuel kills the cat convertors quickly and reduces HO even more over time by choking off exhaust ports and build up in ring lands causing premature wear and blocked oil flow.

With an EFI system the car can make the adjustments needed to correct the fuel/air mixture to some extent.
This is why most Etest places recommend NOT using high octane fuel...

I've been building racing ATV/ motorcycle engines for years and have tested several different fuels and oil on the dyno as well as CHT and EGT gauges (cylinder head temp & exhaust gas temp).
If you set up a carbureted engine to run at WOT at say 1250 degrees on 87 octane, then run that same engine on 94 it's going to drop the egt slightly like 1210-1230 and that just means that you are missing out on HP. In most cases I am able to drop down a size or two on the main jet to gain back the HP and still be at safe operating EGTs.

The computer in a efi car will do the same to compensate by adjusting the fuel trims to lean it out and correct the fuel mixture to restore the hp.

Even at cruise speeds when the A/F mixture is cycling back and forth between too rich and too lean you are missing out on some HP.

Toyota's new ECU programming and use of wideband O2 sensors allow for a much more accurate air/fuel mixture. Instead of cycling back and forth between rich and lean like the old systems they are able to maintain a much closer mixture to stoich with out bouncing back and forth from one extreme to the next.

But like I said no matter what octane you use in your car the ECU/injectors etc will deliver a mixture as close as possible to Stoich and regardless what octane is used the engine with a perfect fuel air mixture and correct timing will not knock.

The only time a person needs to worry about knock and may want to think about using a higher octane is if knock is occuring at steady speeds under normal loads. Many times a higher octane is not even needed, you can just switch brands of fuel. If a higher octane is needed go up 2 points. eg 87-89. There is never any need to jump from 87-91-94 unless you have bumped up the compression an extra 50 psi. An engine like ours that is designed to run 87 and is suffering from detonation will not need more then 89 if it does you have a serious problem.

Occasionally our engines will experiance light ping/knock (spark knock/detonation) a bit under heavy loads but this is no cause for concern, it simply means our engines are operating efficiently.

That's a mouthful. I didn't know you were referring to carb'd cars with that comment.

[sroberts]

Here in Denver, CO we have 85,87 and 91 octane. Since we are at a higher altitude it is recommended by the dealer to run the 85. You only need to run higher octane on higher compression engines.

[dbaxter]

Dont think you should be running 100 octane, I only use that when going to the dragstrip in my SS or GTO, just less chances of running on the low octane tables and less chance of knock.

I have been running the it on the lowest regular octane at any of the stations. Truthfully not careing as long as it was cheap. Runs great and getting the advertised 32 mph city. I am sure yanking the spare and putting in the CIA will help more then anything else, header and tuning also.

[riceboy]

Just put in the cheap stuff and drive...