Lower MPG om high octane fuel?

[Johan007]

I have the 2002 model 100% standard.

I get 42+ MPG with standard fuel (95ron) driving a mixture of dual carriage ways, traffic jams and blasting it once per journey.

Strangely high octane fuel reduces my MPG to 38 on the same mundane journey to work and back!

I check my MPG on a fuel calculator and it is accurate to half a decimal place (as long as miles recorded on the trip computer is accurate), so why does high octane fuel reduce efficiency by so much in this engine when it should get better or at worse same MPG? I would have thought being a "sporty" car it would be tuned for high octane feul.

[127.0.0.1]

higher octane means less energy per gram of fuel

it only makes more power if:

you have higher compression engine that can make use of the
(obviously) bigger injectors, higher air/fuel compression pre-spark and pack
more air in (bigger intake, forced air induction)

or

if you have a normal motor, but it is running crappy or has deposits
on the cylinder, or for whatever reason the knock sensor is always
retarding the ignition...then high octane might make the knock sensor
quiet down and you'd get better overall performance. But this
is treating a symptom not fixing the root cause


High octane fuel in an engine that doesn't need it is a waste of money
and you get less mpg


all high octane does is, it has less tendency to detonate at the higher compressions,
[so you can squeeze the crap out of it and then light it]. Now you get more power and
speed, and perhaps the same MPG. but without high comp pistons, and special
intake mods (turbo-blower) and an ecu designed for it...you will only end up with less
power and less mpg with high octane.

[woof]

I've never owned a four-cylinder that actually like 87 octane (USA's lowest). I always end up using 91/93 and always get better power and mileage everytime.

When you drive a manual, you seem to really feel the smaller differences sense you are more attuned to the car's pulse.

[tcpty]

Originally Posted by 127.0.0.1

higher octane means less energy per gram of fuel

it only makes more power if:

you have higher compression engine that can make use of the
(obviously) bigger injectors, higher air/fuel compression pre-spark and pack
more air in (bigger intake, forced air induction)

or

if you have a normal motor, but it is running crappy or has deposits
on the cylinder, or for whatever reason the knock sensor is always
retarding the ignition...then high octane might make the knock sensor
quiet down and you'd get better overall performance. But this
is treating a symptom not fixing the root cause


High octane fuel in an engine that doesn't need it is a waste of money
and you get less mpg


all high octane does is, it has less tendency to detonate at the higher compressions,
[so you can squeeze the crap out of it and then light it]. Now you get more power and
speed, and perhaps the same MPG. but without high comp pistons, and special
intake mods (turbo-blower) and an ecu designed for it...you will only end up with less
power and less mpg with high octane.

RIGHT ON THE MONEY!! EXCELLENT EXPLANATION !! KUDOS!!

[127.0.0.1]

Originally Posted by woof

I've never owned a four-cylinder that actually like 87 octane (USA's lowest). I always end up using 91/93 and always get better power and mileage everytime.

When you drive a manual, you seem to really feel the smaller differences sense you are more attuned to the car's pulse.

I would love to dyno this comment...if you are talking about a motor that will
ordinarily use low octane.

in a motor designed for 87 and above, (the Toyota VVT-i Yaris motor)
and if it is has no maintenance problems, what you state is not possible
except with bad gas...which can be a real issue depending on where
you get your gas and the tanks they store the bulk in

you simply cannot go faster and get better mpg with a hydrocarbon
that has less energy content -unless - you boost compression and air

[Yaris Hilton]

Originally Posted by 127.0.0.1

higher octane means less energy per gram of fuel

it only makes more power if:

you have higher compression engine that can make use of the
(obviously) bigger injectors, higher air/fuel compression pre-spark and pack
more air in (bigger intake, forced air induction)

or

if you have a normal motor, but it is running crappy or has deposits
on the cylinder, or for whatever reason the knock sensor is always
retarding the ignition...then high octane might make the knock sensor
quiet down and you'd get better overall performance. But this
is treating a symptom not fixing the root cause


High octane fuel in an engine that doesn't need it is a waste of money
and you get less mpg


all high octane does is, it has less tendency to detonate at the higher compressions,
[so you can squeeze the crap out of it and then light it]. Now you get more power and
speed, and perhaps the same MPG. but without high comp pistons, and special
intake mods (turbo-blower) and an ecu designed for it...you will only end up with less
power and less mpg with high octane.

All of that's true except the first and last sentences. Higher octane fuel doesn't inherently have less energy per gram, and running higher octane than necessary won't hurt your mileage or performance. You just won't gain anything, and will have wasted your money. Now, if the higher octane is gained by adding more alcohol or other "oxygenates," then the first and last sentences become true. But it's because of the lower energy density of "oxygenates," not because it's higher octane. That is a common way for different octane levels to be produced today.

[frownonfun]

the sheer fact that you track your MPG should tell you not to buy higher octane. it's more expensive which defeats the whole purpose of trying to get good mileage don't you think.

[40oz]

Im just lurking because I want to buy a yaris in the next 1-2 years but... I heard higher octane on cars that are recommended for the low one is bad. Higher octane fuel burns slower and if you use higher octane in a car that is tuned for a lower octane then things can be bad. Well thats what I heard.

[ozmdd]

I ran 93 octane for a couple of months to see the effect, and my mileage was slightly lower during that period. Obviously anecdotal, since I didn't control all of the variables, but it was a long enough test to be fairly sure that I wasn't getting any improvement out of it. Switched back, and mileage went back up by a couple mpg.

[HANZO_187]

As far as I know the lowest octane we should use for sure is 87 and in the manual it sais to research octane 91 but it just sais.87 octane fuel or higher. Any where passed 93 is just too much IMO.

Regardless, I'm gonna see about one step higher of an octance right above 87 Octane.
These vehicles might just be able to take one octance higher. It just can't be below 87 Octane. Remember that!

[47_MasoN_47]

Ol Johan hasn't been online in nearly 4 years, lol. Good ol zombie threads.

[YarisSedan]

The higher the octane the lower the burn rate which is why you need it for high compression motors. Putting high octane in a low compression motor is going to result in incomplete combustion so that means you are not utilizing all the fuel that is getting sprayed into the combustion chamber which results in partially burnt fuel which will result in more carbon build up on the top of the pistons and valves as well as poorer fuel economy. Putting high octane gas in our cars is simply a waste of money.

[HANZO_187]

I understand but if you were to add a turbo charger to the car the engine stays the same how does that work? Does the engine become a high compression engine?

I wanna know cause also in the manual for our cars its 87 octanenor higger up to 91 octane.

@47_Mason_47 right? I usually read but ignore the dates, lol I'm like the Yaris world reviver of dead threads. CLEAR! BUZZZZZZZZZZZZZZZZZZ

[cali yaris]

^ Sort of. Since there is more than one atmosphere of pressure in the cylinder, higher octane will burn more completely without causing detonation.

Higher octane in a stock motor will have no effect at all on power output.

[HANZO_187]

Well, thats good to know. Getting this car has been one hell of a learning experience.

So mileage stays the same also. Unless its a higher compression engine which the stock VVT-i Engine in the Yaris is not one, mileage and power are not affected and we should use the the minimum of 87 Octane for our cars. Correct?