BF2S Forums At the gas station

Mitch wrote:

Nope, its a sales tactic. We learned about this in one of our training sessions at work.

"Fill it up with premium?" will get many more people to say yes to paying for the expensive gas, compared to asking "Do you want regular or premium?"

premium is still better and depends on the vehicle if you need it or not.

Mitch wrote:

Nope, its a sales tactic. We learned about this in one of our training sessions at work.

"Fill it up with premium?" will get many more people to say yes to paying for the expensive gas, compared to asking "Do you want regular or premium?"

unsure how quoted post is relevant itt.

Meh, regardless of brand, fill up with the recommended octane in your manual.

In case you don't know: US Octane =/= EU Octane

91 AKI (US) = ~95 RON (EU)

-CARNIFEX-[LOC] wrote:

I've been told that you should simply run the car at the recommended octane (87 for most std. US autos), but if you want to run higher octane (91+) that's fine, but stick with it and don't flip-flop between the two grades.  And, if you think you need to "clean out the system", buy some Sea Foam (or other fuel additive) every once in a while.

Any thoughts as to the validity of this?

Incorrect. The EPA requires all US gas to have the same detergents, regardless of octane.

A different octane rating is just an octane rating. ECU's are designed to adjust to differences in fuel/air pressure/etc. What ever you change is read by the ECU and adjusted accordingly to the known values that have been programmed into it.

depends on the engine.  High compression and FI engines often need higher octane fuel to prevent pre-ignition and detonation.  My s2000 runs like shit if I don't buy premium.  And as dil-x pointed out with his Subaru, the 7% additional cost of premium gets me 15-20% better fuel economy.

i've only been using super for the past few weeks and i think it does make a difference to how my car drives and sounds.

also the petrol station i normally fill up at is only about £1.16 a litre so it's not that much more really.

Its dead even for me, and my car doesn't sound like its running on gravel.

Wait, are we just talking octane ratings?

Believe it or not higher octane gas does not contain any extra power. It is in fact harder to burn.
This is what octane is, a measure of combustibility.

Higher octane gas is typically associated with higher performance because the difference in the engines that are designed to use it.


Spark timing in an engine is a balance between lighting the fuel off as late as possible so that the combustion provides maximum torque on the crank (think of trying to push down on you bike pedal at the top, vs when its horizontal with the ground), and preventing the heat from compressing the air/fuel vapors from igniting the mixture and causing it to explode.
Believe it or not, gas in an engine does not explode under normal circumstances, this is what is referred to as detonation and it is very bad. The fuel should burn so that it gently heats and expands forcing the piston down.

Engines with a later spark, or more often higher compression (and higher temperatures created by this compression) need a less combustible fuel to allow for fuel to be ignited, rather then exploded.

Modern engines can sense this detonation, and will dial spark timing back to run safely at the expense of efficiency/power. So in an engine designed for premium you will get less performance or even less MPG due to the engine being dialed back a bit.

In an engine that is designed for regular, you are doing nothing. If anything you are using a less volatile fuel and producing less power... but this would be pretty damn negligible.


So bottom line, on an engine designed for regular you are wasting your time with better unless you bump up your spark timing.
If your engine IS designed for premium, you may actually be getting less distance for your money with a worse gas... and worse performance along the way.

VicktorVauhn wrote:

Wait, are we just talking octane ratings?

Believe it or not higher octane gas does not contain any extra power. It is in fact harder to burn.
This is what octane is, a measure of combustibility.

Higher octane gas is typically associated with higher performance because the difference in the engines that are designed to use it.


Spark timing in an engine is a balance between lighting the fuel off as late as possible so that the combustion provides maximum torque on the crank (think of trying to push down on you bike pedal at the top, vs when its horizontal with the ground), and preventing the heat from compressing the air/fuel vapors from igniting the mixture and causing it to explode.
Believe it or not, gas in an engine does not explode under normal circumstances, this is what is referred to as detonation and it is very bad. The fuel should burn so that it gently heats and expands forcing the piston down.

Engines with a later spark, or more often higher compression (and higher temperatures created by this compression) need a less combustible fuel to allow for fuel to be ignited, rather then exploded.

Modern engines can sense this detonation, and will dial spark timing back to run safely at the expense of efficiency/power. So in an engine designed for premium you will get less performance or even less MPG due to the engine being dialed back a bit.

In an engine that is designed for regular, you are doing nothing. If anything you are using a less volatile fuel and producing less power... but this would be pretty damn negligible.


So bottom line, on an engine designed for regular you are wasting your time with better unless you bump up your spark timing.
If your engine IS designed for premium, you may actually be getting less distance for your money with a worse gas... and worse performance along the way.

'Combustibility' is a new one. Resistance to pre-ignition, which isn't quite the same, is what we're really interested in.
If you have a low compression ratio engine, using premium and playing with the spark timing will do nothing for you.

Dilbert_X wrote:

'Combustibility' is a new one. Resistance to pre-ignition, which isn't quite the same, is what we're really interested in.
If you have a low compression ratio engine, using premium and playing with the spark timing will do nothing for you.

http://dictionary.reference.com/browse/Combustibility
Resistance to pre-ignition, in other words how capable of combustion it is. It is the same.

And yes, while nothing is ever ALWAYS true, advancing timing will make more power.
No, it will not be like adding a turbo or even likely worth the effort unless your actually racing.

The point isn't that he now needs to go bump up his timing, its that just getting "better" gas alone isn't really better.

Last edited by VicktorVauhn (2010-09-10 00:59:35)

Dilbert_X wrote:

VicktorVauhn wrote:

Dilbert_X wrote:

'Combustibility' is a new one. Resistance to pre-ignition, which isn't quite the same, is what we're really interested in.
If you have a low compression ratio engine, using premium and playing with the spark timing will do nothing for you.

http://dictionary.reference.com/browse/Combustibility
Resistance to pre-ignition, in other words how capable of combustion it is. It is the same.

And yes, while nothing is ever ALWAYS true, advancing timing will make more power.
No, it will not be like adding a turbo or even likely worth the effort unless your actually racing.

The point isn't that he now needs to go bump up his timing, its that just getting "better" gas alone isn't really better.

Sorry, resistance to pre-ignition and combustibility are similar and related, but not the same.

explain the difference?

Dilbert_X wrote:

Pre-ignition is caused by compression and/or hotspots in the engine. Resistance to this is a measurable property.
Whether it detonates or just burns after ignition is also a factor.

Combustibility/Flammability is a slightly different measure which includes the above but also includes tests with a source of ignition, eg a flame or a spark.

So you can have different materials with different resistance to pre-ignition but which will burn about the same once ignited eg by a spark.
Different grades of petrol being an example.

Anecdotally I've heard that low RON unleaded petrol is very hard to extinguish once ignited....

lol, heat from compression is the same as ignition from spark or flame... just localized.

The fact is a temperature is reached, and it triggers a chemical reaction.

Your last bit is irrelevant. Who cares about how well it feeds itself once it burns? That just has to do with how much hotter it burns the temperature needed to ignite.

Um no, thousands of degrees C in a spark or flame are not quite the same as the few hundred from compression or an engine hotspot.
Its not just heat either, the free ions and plasma in a spark or flame have a different effect to pure heat.
And finally, combustion from igntion compared to pre-ignition is very different.

Last edited by Dilbert_X (2010-09-10 18:02:00)