Octane #

[LeGoLaS]

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?

[LeGoLaS]

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?

[SaabJohan]

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!

[Rich]

I think this might help answer some of your questions.

[SaabJohan]

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

[Polygon]

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.

[SaabJohan]

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.

[Sindie]

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/w...D&terms=octane

[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.

[Sindie]

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.

[Polygon]

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.

[texan]

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.

[SaabJohan]

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.

[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.

[texan]

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.