Rough idle after WOT

StarGrinder said:
Its not rocket science really, higher octane rating = bigger bang = less fuel compared to 91 octane to create same bang = better fuel economy.. Ive noticed around 50kms more per tank compared to 91 octane.
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thats actually not quite right sladey, but you're on the right track

Higher octane doesnt make a bigger bang, it makes sure the bang occurs at the right time. fuel combusts under a specific set of circumstances, what upping the octane does is make sure that that combustion doesn't happen too early. You are essentially upping the precision of the circumstances required to cause combustion in the cylinder. in cars that can run 91, it means that 98 will usually improve the response (as mentioned by jamente) and economy as mentioned by yourself.

but yes, I am considering making the switch. for the sake of another $50 or even $100 a year on petrol, seems like it could be a worthwhile investment..... just gotta make sure that if my dad borrows her and decides to do me a favour, doesn't end up watering down my tank :p
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In addition to this, higher octane fuel is harder to ignite, and lower octane fuel will burn quicker and more easily
 
lilandonaki said:
In addition to this, higher octane fuel is harder to ignite, and lower octane fuel will burn quicker and more easily
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are you sure about that? because e85 is around 110 octane i think but it burns a bit quicker than a normal tank of fuel as it combusts more easily.

i think its the percentage of ethanol, more ethanol the quicker it will burn.
 
7R3N7 said:
lilandonaki said:
In addition to this, higher octane fuel is harder to ignite, and lower octane fuel will burn quicker and more easily
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are you sure about that? because e85 is around 110 octane i think but it burns a bit quicker than a normal tank of fuel as it combusts more easily.

i think its the percentage of ethanol, more ethanol the quicker it will burn.
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Im pretty sure - higher octane is used in higher compression cars to prevent pinging/pre-detonation in the cylinder, whereas lower octane fuels burnt easily and quickly before we had all this Prem 98 etc etc.

You are right though, ethanol does have a higher octane rating, but doesnt give off as much energy per unit as gas does. Thats why ethanol blended fuels (not e85, but e10 and such) give less km/tank then gas

Edit: You know, it might actually be that higher octane fuels are harder to ignite, but burn the same, when compared to lower octane.
 
If they were harder to ignite, wouldn't that just extend the detonation problem? I always saw higher octane to mean easier ignition, more explosive energy, this would also mean less is needed to keep it making the same power (more economy)..
 
The octane numbers (91, 95, 98 etc) used at the fuel bowsers, do not relate to how much energy is in the fuel, or how many "octanes" it has, it merely compares the combustion characteristics of the fuel at the 91 pump, to a mixture of octane and heptane that has the proportions of 91% octane to 9% heptane.

They are RONs (research octane numbers) and are based of a test that is performed on the fuel, which measures its resistance to self-ignition (ie, ignition without the use of a pilot light, or spark, but due to the raised internal energy of the fuel itself)

The work done by the combustion process, is mainly due to efficiency of the process (compression ratio of the engine), the temperature of combustion (actual cylinder pressure), and the heating value of the fuel.

The heating value of the fuel varies with each batch the refinery make, even the change between winter and summer will produce a difference in the heating value, the heating value is not representative of the octane number.

Adding ethanol, which has a much lower heating value than regular petrol (roughly 47 KJ/g to 30KJ/g) will effect the energy that is produced.


Also, keep in mind that this is the properties of combustion, not how fast your car goes.
 
Yup, basically the higher the octane number, the higher compression ratio, (mad NA pistons) or higher boost (turbros) you can run without the fuel igniting before the spark plug gets a chance to do its job.

Homebrew old school racers used to add Toluene to there fuel to up the octane, its like 120 or something. However its kinda harmful to your health, dunno if its done much these days. E85 is up around 105 or something.
 
Im pretty sure - higher octane is used in higher compression cars to prevent pinging/pre-detonation in the cylinder, whereas lower octane fuels burnt easily and quickly before we had all this Prem 98 etc etc.
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thats basically it, the higher octane fuels are designed primarily to prevent engine knock in engines that are designed with tighter timing tolerances regarding when combustion happens. engine knock is when the fuel combusts too early in the stroke.

Also, keep in mind that this is the properties of combustion, not how fast your car goes.
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couldnt have said it better myself.
 
-_- no its harder to be ignited without the use of a spark. why else do you thing timing has to be delayed in low octane environments.
 
Of course it won't ignite without spark, i'm not even going to continue arguing -_-
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ok.... so you know how diesel engines dont have a spark plug because they ignite under pressure rather than from a spark... well, petrol can explode under pressure too. when the fuel is compressed too much it can explode prematurely. so, in high compression engines that are prone to this, we use higher RON rated fuel to make sure it doesnt explode early under pressure and only explodes when we tell it to via the spark plug.

no need to be snippy, we're just trying to help you understand :)
 
sladey121 said:
Of course it won't ignite without spark, i'm not even going to continue arguing -_-
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Oh, but what about diesel? That is usually "ignited" without spark in trucks and cars? Same line of principle with gas lol

Edit: StarGrinder beat me to it :p
 
Diesel's use glowplugs and compression to ignite the fuel. They have a compression of upwards of 16:1.. Most petrol cars have no more than 11:1 and turbo petrols have around 8.5:1 (just an average can be lower and higher depending on application).. I have asked every single mechanic at my work and they have all said the same. 98 ignites the same as 91, but 98 burns more efficiently without the waste that you get from 91.
 
Lol not that i really even care. I use 98 no matter what and i get good fuel economy from it compared to 91, end of story.
 
sladey121 said:
Lol not that i really even care. I use 98 no matter what and i get good fuel economy from it compared to 91, end of story.
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This, i think we can all agree on.
 
The Octane rating you see at the pump is simply a standard measure for the performance of the fuel. The higher the octane number, the more compression the fuel can withstand before detonating. Fuels with a higher octane rating are used in high-compression engines that generally have higher performance. High-performance cars tend to make their extra power from generating higher pressures in the combustion chambers of the engine, either by having high compression ratios or by using turbo or superchargers.

The compression ratio is directly related to power and proportionally, thermodynamic efficiency of an internal combustion engine. During the compression stroke of an internal combustion engine, as the air/fuels mix is compressed its temperature rises. A fuel with a higher octane rating is less prone to auto-ignition and can withstand a greater rise in temperature during the compression stroke of the engine without auto-igniting, allowing more power to be extracted from the Cycle.

If during the compression stroke the air/fuel mix reaches a temperature greater than the auto-ignition temperature of the fuel, the fuel auto-ignites. When auto-ignition occurs (before the piston reaches the top of its travel) the up-rising piston is then attempting to squeeze the rapidly heating fuel charge. This will usually destroy an engine quickly if allowed to continue.

Contrary to common belief. Paying for higher octane fuel greater then that specified for your vehicle does not improve performance, though it wont hurt. The engine is already able to perform at its highest efficiency on the recommended fuel. The extra octane rating has no effect. It is generally the 'cleanliness' of the more premium fuels that help with your efficiency.

Most modern cars have an engine management system with a knock sensor that monitors if knock is being produced by the fuel being used. If so, the ignition timing will be automatically altered by the fuel management system to reduce the pre-ignition occurrence.

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