Standard vs Premium Fuels

[MY95]

I only ever run my cars on texaco fuels petrol or diesel. I have noticed a difference running the gti6 on texaco 97 however, it just seems to run better and as bizzare as it sounds I'm sure I get slightly better mpg off it too

Don't really see using premium petrol as that much of an expense personally when compared to buying diesel, anything under 1.40 a litre feels like a bargain haha.

On the subject of different countries and what's available on their forecourts, ever seen how much choice French stations have?

I have also noticed after running my rallye on 97 (texaco) for the first time at FCS it starts better, idles sweeter and just generally seemed too pick up better and throttle response seemed alot sharper.

Mine only ever sees the morrisons cheapo fuel with a healthy dose of octane booster. Everything else is texaco.

Nice one, exactly the info I was after! does the octane booster give a noticeable difference?

[RetroPug]

I just run it on 95RON, gti-6s have knock sensors so it doesn't matter whether you use 95-99 in terms of ability to run properly.
You will get higher mpg with higher octances, but then they cost more, and I've never actually sat down and done the maths to work out precisely which is better value for money.

Thanks for the info, where do you get your standard 95 from then? Also have you tried the higher up stuff andnoticed much of a difference?

Typically shell but that's because there is one at the end of my street.

I have run all three of my gti-6s on shell's 95RON stuff, their V-power, tesco's fuels, various independant filling stations while on the road, BP's 95RON, ran on 95RON in Germany for 3 months as well.

Not once did I notice any perceptible difference.

The car has a knock sensor, so it will change the timing dependant on the octane of your fuel, as long as it is between 95 and a certain octane which I can't remember but is probably 99-103. I'm pretty sure the owner's manual says it somewhere. Higher octances within this range will give you more MPG but you've paid more per L and I'm unsure of precisely what the trade-off is. In any case, the car is designed to run on any and all fuels in that range. Maybe the running will be slightly smoother or you'll make +0.1 bhp but I can't say I've noticed.

I would be more concerned with the quality of fuel rather than the octane number, and some people argue that premium fuels are also purer and have better additives. I am sure that 95RON fuel from reputable petrol stations is of perfectly adequate quality though. It is worth bearing in mind that when the gti-6 was first produced fuels with a higher octance than 95 would have been a bit less common than they are now I'd have thought.

[MY95]

I just run it on 95RON, gti-6s have knock sensors so it doesn't matter whether you use 95-99 in terms of ability to run properly.
You will get higher mpg with higher octances, but then they cost more, and I've never actually sat down and done the maths to work out precisely which is better value for money.

Thanks for the info, where do you get your standard 95 from then? Also have you tried the higher up stuff andnoticed much of a difference?

Typically shell but that's because there is one at the end of my street.

I have run all three of my gti-6s on shell's 95RON stuff, their V-power, tesco's fuels, various independant filling stations while on the road, BP's 95RON, ran on 95RON in Germany for 3 months as well.

Not once did I notice any perceptible difference.

The car has a knock sensor, so it will change the timing dependant on the octane of your fuel, as long as it is between 95 and a certain octane which I can't remember but is probably 99-103. I'm pretty sure the owner's manual says it somewhere. Higher octances within this range will give you more MPG but you've paid more per L and I'm unsure of precisely what the trade-off is. In any case, the car is designed to run on any and all fuels in that range. Maybe the running will be slightly smoother or you'll make +0.1 bhp but I can't say I've noticed.

I would be more concerned with the quality of fuel rather than the octane number, and some people argue that premium fuels are also purer and have better additives. I am sure that 95RON fuel from reputable petrol stations is of perfectly adequate quality though. It is worth bearing in mind that when the gti-6 was first produced fuels with a higher octance than 95 would have been a bit less common than they are now I'd have thought.

Thanks for the long reply! Looks like I will just be using normal 95ron then, lots of mixed reviews but doesnt sound as if its worth the extra expense especially as the GTI6/Rallye are meant to run on 95 although can be run on higher ron. Your right about the last part, wouldnt have been many petrol stations at all with premium fuel back in the 90's.

[Ruan]

Eeek!

Higher Octane =/= Greater efficiency!

Higher Octane can be used in engines with higher compression ratios to yield greater efficiency, however we're not changing the CR here. Any increase in MPG is simply from the additives in the fuel, not the octane...

Octane is simply a measurement of how easily the fuel will ignite with compression/heat... Higher the octane, more resistance there is to it detonating, meaning higher CRs and higher temperatures for lairy high compression petrols and heavilly turbocharged engines.

GTi6s really don't need it - I've seen my GTi6 on 95RON on diagnostics on a hot UK day and never seen any evidence of a timing pull by that reckoning 99% of the time, you're going to notice no benefit from the Octane ... Literally, I seriously doubt you could get the ECU to pull timing unless the fuel is REALLY bad or it's a SERIOUSLY hot day and your intake is sucking from infront of the radiator.

If the additives in the fuel are working well, however - fair enough! I've seen additives do good things... But saying switching to 97/98/99 Octane will ensure that you hold both tyres fully lit at the top of 3rd... I think not..

[RetroPug]

Does higher octance not also equal greater calorific content given higher octance fuels are longer chains of hydrocarbons?

[JethroJay]

I seen a MASSIVE change in MPG's in my Vectra 3.2 V6 I had running it on the worst mistake of my life (Tesco's cheap crap) I got 7MPG driving it like a grandad, on Shell V-Power it got a TAD more power, BP Ultimate it got 10MPG and a bit more power that it did with Shell V-Power so ran it on that, then tried to use the Texaco stuff and well yeah needless to say the car run and then cut out again once the Shell had been run through, didn't start with the Texaco stuff in for some reason, it's all dependent on your car and the mods, it's just a case of trying it and seeing if it agrees with it, only way I found out before I detonated my engine for been a prick!

[Ruan]

Does higher octance not also equal greater calorific content given higher octance fuels are longer chains of hydrocarbons?

Higher Octane essentially has nothing to do with H-C chain lengths.

"RON" is simply a measurement of how close to 100% Iso-Octane the fuels' Anti-Knock Resistance is... Petrol H-C chains are generally a set length - it's the additives that control it's Anti-Knock. It doesn't mean it contains Octane - lots of things can be added to fuel to increase its' RON number such as Toulene, MTBE and TEL (Tetraethyllead ala 4-star). Fuel with a pile of TEL in could reach up to 150RON!!

If you're interested in this at all, there's a whole load of different ways of measuring RON, MON and some places have the RM/2 method. 100% on the scale means it's 100% Iso-Octane, 0% n-heptane - a RON of 90 would mean 90% Iso-Octane and 10% n-heptane - it's simply a reference scale. You can however get substances that have a higher knock resistance than Iso-Octane - hence 102RON.

But yes, as said, you simply won't get any improvement in performance if your engine is up to it's mapped in limits - if you're not pulling any timing, then there's no more timing to add once it's not knocking!

[RetroPug]

I am definitely interested, but struggling to remember combustion calculations from engine thermodynamics which would help me make a lot more sense of it all.