Octane #

I was wondering if there could be a problem when you use to high of octane for your engine, or does the higher the octane number only help your engine?

Say for example I put Octane 93 in my 2.3l car, and i dont run any turbo's or turbo chargers. Is this just a waste of money or will i get better mileage?

It depends on how good your engines ECU will use the more knock resistant fuel.

It also depends how hot it's outside. Maybe you can save fuel on the summer but not in the winter.

Try and see!

I think this (http://www.automotiveforums.com/vbulletin/t7649.html) might help answer some of your questions.

There is two test methods used for octane rating, RON and MON.
RON, Reasearch Octane Number and MON, Motoring Octane Number.
RON usually gives a higher rating than MON.
(RON + MON)/2 = antiknock index

I for one run a turbo and can't put anything lower than 91 octane in my car, otherwise I get detonation. I can also SAFELY put 120 octane in my car. For a regular engine it would probably ruin it if you ran 120 octane. No, the higher the number doesn't mean better. The higher the octane the more explosive it is.

Higher octane doesn't mean more explosive. It mean that the fuel is harder to ignite (also to self ignite).
To run 120 octane in a normal engine is no problem, at the local airport they run the lown mower on airplane fuel which is around 120 octane.
But to get something from by using higher octane the ignition timing must be set a little earlier, and/or with a turbo a little more boost when charging.

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 have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting. The compression ratio of your engine determines the octane rating of the gas you must use in the car.

Too high will lead to unburn gas escaping out your tail pipe and will ruin your CAT.

Try reading: http://chemistry.about.com/library/weekly/aabyb100401.htm?iam=howstuffworks_SKD&terms=octane

A good modern ECU will adapt for new running conditions. If a higher octane fuel is used the ECU will automaticly compensate for this.

Originally posted by SaabJohan
A good modern ECU will adapt for new running conditions. If a higher octane fuel is used the ECU will automaticly compensate for this.

It can only compensate so much. If there is a lack of compression compared to the octane level you will have unburnt fuel.

Alright, go fill up a Corolla with 120 octane and drive it around town and see what happens to the engine. It is not good for a regular engine.

Originally posted by Sindie


It can only compensate so much. If there is a lack of compression compared to the octane level you will have unburnt fuel.

I'll go ahead and argue with that one.

Why is this true? Are you saying that higher octane gasolines have slower burn rates than lower octane gasolines? Because burn rate and octane are two seperate fuel properties, and should be discussed as such. Just because a specific fuel has a high resistance to heat based ignition doesn't mean it burns slower than another, often times it's quite the contrary for racing fuels which are known to be used at higher than normal engine speeds.

Most racing fuels can be used i standard engines, MON rating over 100 is common. But to get the extra horsepower some new maps will do the trick.

Like texan says, it doesn't mean that a higher octane fuel will burn slower. That is true, and it's only high octane not nitromethane we are talking about. If you use a fuel that burn slower, it will burn when it leave the combustion chamber.

The combustion in an engine (gas or diesel) are done very fast, it is done before the crankshaft have rotated 90 degrees. In engines that are using nitromethane (which burn slow and is hard to ignite) like top fuels the fuel is burning while leaving the cylinder. (but in these engines the exhaust valves also opens very early)

I know that there is a limit to what the ECU can control, let take the lambda sensor for an example, it can control around 10% of the fuel flow. If a turbocharged engine is used it can control much more. An example: The compression ratio is 9, maximum boost pressure is 1,4 bars. The total compression ratio can now go from under 9:1 up to 21.6:1.
If knock occurs, this is done:
1, More fuel to the engine
2, Ignition timing set a little later
3, decrease boost pressure
If no knocking occurs it will do the opposite.
In a racing engine the compression can be 6:1 and maximum boost pressure can be 4 bar, this mean that the total compression ratio can be 6:1-30:1. These engines use high octane, but they must also run well during no boost.

Engines today are built so the can run lambda 1 (with gasoline SAFR 14,7). This can only be done up to around 66% of the total rpm range and it can't be done during heavy load. This is because otherwise knocking will occur (fuel is a good cooling).

If unburnt fuel leave the cylinder it will damage the cat, but I have never heard anyone who had ruined it by using too high octane. This is mostly done because of misfires or if the engine gets to much fuel.

I have been looking into this problem a little more now, and this is what I've found:

If you are already using the proper octane fuel, you will not obtain more
power from higher octane fuels. The engine will be already operating at
optimum settings, and a higher octane should have no effect on the management system. Your driveability and fuel economy will remain the same. The higher octane fuel costs more, so you are just throwing money away. If you are already using a fuel with an octane rating slightly below the optimum, then using a higher octane fuel will cause the engine management system to move to the optimum settings, possibly resulting in both increased power and improved fuel economy. You may be able to change octanes between seasons ( reduce octane in winter ) to obtain the most cost-effective fuel without loss of driveability.

Can I increase fuel octane?
It is preferable to purchase a higher octane fuel such as racing fuel, aviation gasolines, or methanol.

Originally posted by SaabJohan
I have been looking into this problem a little more now, and this is what I've found:

If you are already using the proper octane fuel, you will not obtain more
power from higher octane fuels. The engine will be already operating at
optimum settings, and a higher octane should have no effect on the management system. Your driveability and fuel economy will remain the same. The higher octane fuel costs more, so you are just throwing money away. If you are already using a fuel with an octane rating slightly below the optimum, then using a higher octane fuel will cause the engine management system to move to the optimum settings, possibly resulting in both increased power and improved fuel economy. You may be able to change octanes between seasons ( reduce octane in winter ) to obtain the most cost-effective fuel without loss of driveability.

Can I increase fuel octane?
It is preferable to purchase a higher octane fuel such as racing fuel, aviation gasolines, or methanol.

I completely agree.

That last sentence you have to be careful with though, most avgas is still leaded and methanol will quickly destroy many parts of the fuel system if used.

This is HUGE - but it will answer all your questions (and more) about gasoline:

http://www.faqs.org/faqs/autos/gasoline-faq/part1/

(Parts 6 and 7 talk about octane ratings)
:cool:

Originally posted by fritz_269
This is HUGE - but it will answer all your questions (and more) about gasoline:

http://www.faqs.org/faqs/autos/gasoline-faq/part1/

(Parts 6 and 7 talk about octane ratings)
:cool:

That's what I refered from in my previus post.

I really recommend you to read that faq.

DAM...

I would of never thought i would have gotten this much input back, thanks. So if i heard this right, If i am driving say my 91 topaz 2.3l 4cyl.
which runs fine at 87 octane, and i put say 92 octane in, this will not increase my fuel economy.

You can try. In worst case you have lost a few bucks in the more expensive fuel.

How can an octane rating go over 100? I thought that the octane rating was the percentage of C8H18 in the gas, with most of the remainder heptane.

Yes, technically you can't go over 100 on that scale in terms of real values. Octane is 100, heptane is 0, but what we really mean by 104 octane gas is that this fuel is 4% more resistant to compression based ignition than pure iso-octane in MON and/or RON testing.

yep...

I think it's important to keep in mind that the octane number on your fuel from the gas station doesn't really tell you anything about the composition of the fuel, except that whatever is in there has anti-knock properties similar to a particular mixture of octane/heptane. In other words, 87 octane gas from the station isn't likely to be 87% octane, 13% heptane, it just knocks as though it was.

Cool. Thanks. :)

Heres my little spiel (didn't read thread, so forgive any dumb stuff)

Higher Octane=Lower power
Higher Octane required for: High compression engines, high-strung engines, super/turbo charged engines, engines with a high volumetric efficiency.

Now that your all screaming, heres why. Octane is the resistance to combustion. The higher the number, the more energy required to ignite it. The scale is based on Octanal (C3H8), but is actually made of many different things, that is why octane can go over 100 (%100)
So the higher the octane rating, the more heat is required to ignite it, for the same amount of power (not possible, but more on that later.) So if you took 50joules to ignite 50 octane, and 100 joules to ignite 100 octane. Both produce 10000 joules of energy. So the 50 octane produces 50 more joules than the 100 octane.
So why dont we all run super-low octane (like ethlyene, the most reactive gas around, which is C2H2)?? Well, then the gas would start burning before it should. Like then the piston is only half up. This would blow your engine up. Because when the piston rises up, it creates heat from the compression. And its not like the block itself is cold. So this is energy, and if its enough to ignite the gas, you have a problem. You want the gas to stay unlit until the spark plug lights it. (note: diesel engines work by letting the gas ignite from the heat created by compression, and there can be no screwing with the diesel octane, because that would cause problems that you now understand). Having a turbo on the car, or high V.E., that means there is more air and gas in the cylinder, and more heat is created from compression, requiring a higher octane to keep premature ignition down.

If the gas does ignite early, it can cause damage. If all you hear is a ping, put in a higher octane to stop it. If you hear a knocking, stop immediatly and put in lots of octane booster, and hope you dont kill the engine.

Now to the fact that gas is very different from one octane to the other. Different substances release different amounts of heat (ethylene might make 10 joules per ml, octane might make 8 joules) So the trick is trying to put in the chemicals that create the most amount of heat, but have the highest resistance to detonation. This happened in the 80's with F1, the teams making super-expensive fuels that had really high octane ratings, but also produced alot of energy. This was stopped due to the amazing cost.

Any questions?

First off, octanal is NOT the high point sample fuel used in MON and RON testing. It's iso-octane as earlier stated, which is C8H18. Octanal is CH3(CH2)6CHO, and is used in artifical scent and flavor production (it has a strong fruity odor).

Secondly, octane rating is not resistance to combustion. It is resistance to heat based combustion, and there's a huge difference there. Your example of lower octane gasoline having a net increase in total power efficiency assumes that we are using heat based ignition, which we aren't. This is an Otto cycle engine we are speaking of with spark based ignition, and one of the basic tenants in such an engine design states that an increase in static compression, assuming there is no onset of auto-ignition, will ALWAYS increase the thermal efficiency of said engine. The positive effects of increased static compression on engine power output, low RPM throttle response and cylinder filling are well documented in the annals of engine performance.

There are other points to be made in reference to your post, but I suggest you first read the earlier mentioned gasoline FAQ (http://www.faqs.org/faqs/autos/gasoline-faq/part3/) before moving on (so you can see where some of us have gotten most of our information from). Specifically, notice why one fuel is more resistant to heat based ignition than another (HINT: the trick is in end gas decomposition).

Um, I forgot to mention that keeping the gas from igniting meant you could use more compression or a higher volumetric efficiency, which would greatly offset any drop in potential power from the fuel itself.