GTI fuel rating (95 RON)

[Bill_the_Bear]

No disrespect Bill but having done my own tests over the last 10years regarding this topic I make my judgement on what I find not on science. And the fact that vw make the car in the first place, I am more likely to believe what they are telling me as to regards of what fuel I put in my car. I have always been a firm believer that unless your car asks for 98 then it is ridiculous to put it in your car in the first place. And yes it is just my opinion

I will bow to your tests then as I've not done any myself.  You're probably right about not using it if the car doesn't ask for it.  Its the question of if you do use it does it have any effect that I don't think we've answered.

[Hobojim]

I think this needs to be said again as reading a few posts folks are still referring to their mk 4, 5 + 6 GTIs.

Previous generation golfs will not see any performance differences when increasing the octane of the fuel as they do not have variable valve timing. You cannot base your response to the OPs  question on your experiences with these engines.

You need variable valve timing to take advantage of a higher octane fuel. This allows the ECU to advance the spark, burning the fuel faster/sooner and hence the valves have to keep up.

The new GTI has variable valve timing and so will see an improvement from higher octane fuel. ( the manual above is obviously an arse covering/default bit of writing )

All engines that can vary the timing of the valves and the firing of cylinders will produces more power with a higher octane fuel - up to the limits of how much advance the engine can physically produce (200 octane fuel is probably a waste)

Whether the increase in power is significant is the question. I suspect not in the new gti.  4 cylinders and 2 l only, there is not that much more energy being released. The more cylinders and the greater the volume the greater the increase.

There is no replacement for displacement!

[Hobojim]

I have no idea about mpg effects. I do not know enough about the different types of injection and their effects on economy. Ill get the books out.

[monkeyhanger]

I think this needs to be said again as reading a few posts folks are still referring to their mk 4, 5 + 6 GTIs.

Previous generation golfs will not see any performance differences when increasing the octane of the fuel as they do not have variable valve timing. You cannot base your response to the OPs  question on your experiences with these engines.

You need variable valve timing to take advantage of a higher octane fuel. This allows the ECU to advance the spark, burning the fuel faster/sooner and hence the valves have to keep up.

The new GTI has variable valve timing and so will see an improvement from higher octane fuel. ( the manual above is obviously an arse covering/default bit of writing )

All engines that can vary the timing of the valves and the firing of cylinders will produces more power with a higher octane fuel - up to the limits of how much advance the engine can physically produce (200 octane fuel is probably a waste)

Whether the increase in power is significant is the question. I suspect not in the new gti.  4 cylinders and 2 l only, there is not that much more energy being released. The more cylinders and the greater the volume the greater the increase.

There is no replacement for displacement!

200 octane fuel would undoubtedly shag the engine - it is aviation fuel (Avgas).

Octane rating is a measure of how far you can compress the fuel mix before it ignites without a spark. Compress a low octane fuel too far and the compression ignites it (like diesel ignition) out of sync with the engine, causing your "knocking".

Higher octane fuels are of benefit to engines with a higher compression ratio - compressing the gas further and then igniting it to get a bigger expansion on ignition. Using RON98 in a car optimised for 95 just means that when it is compressed as far as the engine will let it and then you ignite it via modified timing, you will get your combustion. A car running RON 98 when it is optimised with RON95 will be inefficient because the compression ratio will not be as high for the fuel/air mix. Less compression means a less energetic ignition - don't forget it is expansion of ignited gases which powers your car. Less expansion (because the compression ratio was lower prior to ignition) means less power. Try sitting on a balloon to pop it - it makes a bang. Try sitting on a balloon with more air in it (higher compression ratio) - it makes a bigger bang.

Higher octane fuels are all about increased efficiency in higher output engines that are designed to take advantage of the higher resistance to compression ignition.

If VW could show gains of any kind (power or efficiency) in using RON 98/99, they'd be boasting about it and encouraging you to use RON98, but letting you know that if you can only get hold of RON95 then the car will do fine with it.

I do wonder whether anyone's ever looked into sparkless petrol combustion - running the car like a diesel, purely on compression for ignition, like controlled and in-sync "knocking". Maybe petrol is a little more unpredicatable exactly when it is going to ignite under compression.

[Johnc41]

I think this needs to be said again as reading a few posts folks are still referring to their mk 4, 5 + 6 GTIs.

Previous generation golfs will not see any performance differences when increasing the octane of the fuel as they do not have variable valve timing. You cannot base your response to the OPs  question on your experiences with these engines.

You need variable valve timing to take advantage of a higher octane fuel. This allows the ECU to advance the spark, burning the fuel faster/sooner and hence the valves have to keep up.

The new GTI has variable valve timing and so will see an improvement from higher octane fuel. ( the manual above is obviously an arse covering/default bit of writing )

All engines that can vary the timing of the valves and the firing of cylinders will produces more power with a higher octane fuel - up to the limits of how much advance the engine can physically produce (200 octane fuel is probably a waste)

Whether the increase in power is significant is the question. I suspect not in the new gti.  4 cylinders and 2 l only, there is not that much more energy being released. The more cylinders and the greater the volume the greater the increase.

There is no replacement for displacement!

200 octane fuel would undoubtedly shag the engine - it is aviation fuel (Avgas).

Octane rating is a measure of how far you can compress the fuel mix before it ignites without a spark. Compress a low octane fuel too far and the compression ignites it (like diesel ignition) out of sync with the engine, causing your "knocking".

Higher octane fuels are of benefit to engines with a higher compression ratio - compressing the gas further and then igniting it to get a bigger expansion on ignition. Using RON98 in a car optimised for 95 just means that when it is compressed as far as the engine will let it and then you ignite it via modified timing, you will get your combustion. A car running RON 98 when it is optimised with RON95 will be inefficient because the compression ratio will not be as high for the fuel/air mix. Less compression means a less energetic ignition - don't forget it is expansion of ignited gases which powers your car. Less expansion (because the compression ratio was lower prior to ignition) means less power. Try sitting on a balloon to pop it - it makes a bang. Try sitting on a balloon with more air in it (higher compression ratio) - it makes a bigger bang.

Higher octane fuels are all about increased efficiency in higher output engines that are designed to take advantage of the higher resistance to compression ignition.

If VW could show gains of any kind (power or efficiency) in using RON 98/99, they'd be boasting about it and encouraging you to use RON98, but letting you know that if you can only get hold of RON95 then the car will do fine with it.

I do wonder whether anyone's ever looked into sparkless petrol combustion - running the car like a diesel, purely on compression for ignition, like controlled and in-sync "knocking". Maybe petrol is a little more unpredicatable exactly when it is going to ignite under compression.


Totally agree with you, why would vw put 95ron and tell you that it is pointless using 98ron, I am sure they would know. And the reason people are talking about the mark 5 is because vw did recommend 98 for that car. So there is your answer.

[Hobojim]

Johnc41, please have a read about this subject you are obviously missing things still. An engine can be tuned for 95RON or 98RON or 102RON or whatever. If a non VVT engine is tuned for 95RON it will not produce an improvement with 98RON. If an non VVT engine is tuned for 98RON it will not perform better with 102RON but WILL perform WORSE with 95RON.

The mk6 engine is the same as the mk5, but the valve timings were tuned to 95RON but the charge was altered (increase in boost amongst other tweeks) to produce the power increase.

Monkeyhanger, yes i know 200 octane fuel is not for car use it was an exaggeration to the extreme for explanation purposes.

Good explanation of octane rating, to complete it i wanted to add that 95RON fuel in a 98RON tuned engine will potentially ignite early as the compression will be higher than the 95RON fuel is 'designed' for.

They are developing a compression only petrol engine its called homogeneous charge compression ignition HCCI. The problem is the conditions inside the cylinder vary a lot (pressure, temperature etc) and as you said the higher volatility of petroleum makes it difficult to control (you have to actively alter the charge, which is hard to do in real time), and the compression ratios are sky high, and require ridiculously strong blocks, pistons etc.

As to why VW aren't boasting about an increase in power, it might as the increase is so small that it is statistically insignificant. Or that the power produced by engine with 98RON fuel is stepping on the toes of some of the more expensive VAG range.....

[Johnc41]

Johnc41, please have a read about this subject you are obviously missing things still. An engine can be tuned for 95RON or 98RON or 102RON or whatever. If a non VVT engine is tuned for 95RON it will not produce an improvement with 98RON. If an non VVT engine is tuned for 98RON it will not perform better with 102RON but WILL perform WORSE with 95RON.

The mk6 engine is the same as the mk5, but the valve timings were tuned to 95RON but the charge was altered (increase in boost amongst other tweeks) to produce the power increase.

Monkeyhanger, yes i know 200 octane fuel is not for car use it was an exaggeration to the extreme for explanation purposes.

Good explanation of octane rating, to complete it i wanted to add that 95RON fuel in a 98RON tuned engine will potentially ignite early as the compression will be higher than the 95RON fuel is 'designed' for.

They are developing a compression only petrol engine its called homogeneous charge compression ignition HCCI. The problem is the conditions inside the cylinder vary a lot (pressure, temperature etc) and as you said the higher volatility of petroleum makes it difficult to control (you have to actively alter the charge, which is hard to do in real time), and the compression ratios are sky high, and require ridiculously strong blocks, pistons etc.

As to why VW aren't boasting about an increase in power, it might as the increase is so small that it is statistically insignificant. Or that the power produced by engine with 98RON fuel is stepping on the toes of some of the more expensive VAG range.....


I am not missing any point, the point is that vw have said you will get no more power or consumption  from the mk7 gti with 98 Ron. What else do you want to dispute, it is there in black and white.
They make the car so I am sure they have a better idea than all the inconclusive bs tests we have to endure. Sorry but I will just put in my new gti what vw have advised.

[Hobojim]

The dispute is that VW have lots of motives for not declaring that the higher octane fuel increases the power and that engines of the design in the new GTi produce more power with higher octane fuel.

You stick with what the manual says and be happy.

For those of us that are interested, we will just have to rolling road the car with the different fuel grades to find out conclusively. I predict 15bhp maximum power increase from 95ron to 98ron at the same conditions. 230 > 245 bhp maximum.

[Bill_the_Bear]

You stick with what the manual says and be happy.

He is interpreting the comment in the manual to back his opinion and is blind that it might be interpreted in another way.  I think if someone was really in search of the truth they'd consider it more carefully, but if they just want to feel comfortable with a decision already made thats fine...

We still lack conclusive proof of course.

[Johnc41]

You stick with what the manual says and be happy.

He is interpreting the comment in the manual to back his opinion and is blind that it might be interpreted in another way.  I think if someone was really in search of the truth they'd consider it more carefully, but if they just want to feel comfortable with a decision already made thats fine...

We still lack conclusive proof of course.

No Bill sorry as I told you before I don't need any manual to back up my opinion. And the bottom line Bill is if you are so interested in performance and miles per gallon, then get a remap as I have done and for the £300 you will get proper performance and mpg. Even if your wonderful science does work and you did get bit more performance, in the real world the gains are so insignificant that you would be very very hard pushed to notice the difference. One thing you are correct about is that Joe is right,we are looking into it far to deep.