Yes,i also noticed 98 petrol has better fuel economy.

I think it's cuz of the additives,which smoothen the engine, rather than the octane rating itself.

If you do a search online, you can find the answer. The higher octane rating actually "retards" the combustion process or they like to call it "slow burn".

Let's say your car's engine has the typical 10: 1 compression ratio (this means the air/fuel mixture is compressed to 1/10th of its original volume) and uses the recommended 95 RON. And let's say if you use 92 RON you start to experience pre-ignition, meaning before the air/fuel mixture can be compressed to 1/10th of its original volume it became unstable and self ignited making the car jerky and loss of power. Using 95 RON the air/fuel mixture is stable enough to hold itself until the spark is delivered and the mixture burn away fast enough to give out the energy needed to push the piston head down. What 98 RON does is that it burns slower dan 95 RON, it's stable under high compression, does not self ignite, and as it burns slower it burns the air/fuel mixture more uniformly giving out that "slightly more energy feel" that you feel during acceleration and some ppl claim better fuel economy.

To summarise, 92 RON is what I would call most "volatile" when it comes to combustion, this fuel has little or no additives added and it's in its most original state. The higher the RON rating more additives are added to make the petrol less "volatile" and more stable under high compression conditions.

So to me the premium grade fuel is actually "contaminated" with additives and it's not better in anyway as quality is concerned. So now you know the misconception that most ppl have on premium grade petrol.

watwheels, ruykava is right, this is due to the additives not so much the RON.

RON only matters when you have FI or High comp NA

watwheels is right. imagine before the pistons move to the end, there are 'minor combustions' that retard the piston's direction.

with higher RON, the petrol can sustain a higher level of compression before combustion occurs. this is very critical for engines with high compression ratio.

Here's a good read that quite niftily sums up the causality of higher octanne fuel to power. Saves everyone a need to type out long explanations

In fact, do go thru the whole site. It's really quite informative.

http://www.carbibles.com/fuel_engine_bible...tml#octanepower