I'm a renowned technological incompetent, but my understanding of 'octane rating' is that high octane fuels can resist greater pressure before self igniting. Pinking is a result of low octane petrol igniting early under pressure (before the spark) in high compression petrol engines (with possible damage to the valves over the long term as a result). Lead used to be added to petrol to increase the octane rating, but modern cars are designed to run on lower octane, lead free, petrol. High octane fuels don't actually generate more power, it's just that high compression engines (which tend to be more powerful, relative to their size) need higher octane fuels to run properly. My old V8 MGB was perfectly happy on any old low octane petrol, because it relied on 'inches' rather than compression for power.

In diesel powered cars, of course, there is no spark plug, and we want the fuel to ignite under pressure alone, and therefore diesel fuel is relatively low octane. Whatever alleged benefits the more expensive diesel fuels (whether from Shell, BP or whoever) have, it's not a higher octane rating surely?

Or am I talking gibberish as usual?

Personally, tried the cheaper diesel and the more expensive stuff and can't see any difference in economy or performance.

I've run all my cars over recent years on the cheapest diesel I can find, usually, my local supermarket, With my previous Yeti I did try a period of about 3 months using only Shell V+ or the BP equivalent if I could not find a Shell garage.

I kept very accurate records before and during my trial and whilst I did see a very marginal improvement in mpg it was not worth the extra cost of the fuel. I could not honestly say that I felt the car was any quieter or performed better. I do think that initially I 'thought' it was better but after the trail period I reckoned that was more likely to be a placebo effect.

I know this subject has been aired before but this is just my experience for what it's worth.

More or less correct Bobdog.

As I understand it ,Pinking is when the petrol explodes (giving the pinking sound),rather than burning .

This can be prevented ,in some modern high performance cars ,by electronically retarding the ignition.

Years ago you could retard the ignition a few degrees by adjusting the distributor on many cars.

This results in less power and more fuel usage.

Early examples of your V8 had very high compression ratios(11:1 I think.)

They were rated at around 155BHP,but had to be run on 5 star 101 Octane fuel.

Later examples ,like yours?,had much lower compression ratios to run on the lower octane fuels.

The power dropped to around 135 claimed Bhp ,partly as a result.

The fundamentals are correct. There is a way to get more power out of higher octane petrol though, and that is because if you use higher RON fuel you can advance the ignition safely, and in this way gain some power. Some cars use dynamic ignition mapping coupled to a knock sensor to be able to constantly run the engine at the most advanced ignition timing without pinking, hence a change of fuel can show an improvement. Additionally, with an older gasoline engine you can get deposits in the combustion chamber which can get hot and act as hot spots and cause pre-ignition; higher RON fuel can overcome this. Finally, the 'super' fuels tend to also have more ignition improves and detergents which can give a cleaner burn and help keep injectors clean and therefore putting out good spray patterns.

Diesel is a different matter. In this case you are more concerned with cetane rating. Higher cetane rating essentially means a better quality 'burn', with reduced delay between injection and ignition of the fuel charge. Premium versions also get the higher detergent and ignition improver dosages for the same benefits as above. Hence you can get an improvement by switching to a 'better' diesel which is not down to suggestion, placebo or wishful thinking.

Some excellent and well reasoned posts above, to which I would only add that where a manufacturer has specified a fuel rating and your petrol engine has not been significantly changed from standard there is no advantage in using a higher octane fuel. On higher performance engines where only a minimum fuel rating is specified there may be some point in using higher octane fuel but only if the engine management is able to adapt-if it is there may well be a dual recommendation.

My only recent experience of additives was on a 2.0Tfsi Seat where injector cleaner threw an engine management light-I doubt their value on modern machinery.

I'm a renowned technological incompetent, but my understanding of 'octane rating' is that high octane fuels can resist greater pressure before self igniting. Pinking is a result of low octane petrol igniting early under pressure (before the spark) in high compression petrol engines (with possible damage to the valves over the long term as a result). Lead used to be added to petrol to increase the octane rating, but modern cars are designed to run on lower octane, lead free, petrol. High octane fuels don't actually generate more power, it's just that high compression engines (which tend to be more powerful, relative to their size) need higher octane fuels to run properly. My old V8 MGB was perfectly happy on any old low octane petrol, because it relied on 'inches' rather than compression for power.

In diesel powered cars, of course, there is no spark plug, and we want the fuel to ignite under pressure alone, and therefore diesel fuel is relatively low octane. Whatever alleged benefits the more expensive diesel fuels (whether from Shell, BP or whoever) have, it's not a higher octane rating surely?

Or am I talking gibberish as usual?

Personally, tried the cheaper diesel and the more expensive stuff and can't see any difference in economy or performance.

For a technological incompetent that's excellent.

Fred

With our old V5 passat I noticed a very real improvement in mpg and smoothness when changing over to V Power. It was fairly dramatic. The better grades contain engine preservatives that work, apart from extra power. I always take V Power or 'nitro' at the pump, and will certainly continue to do so when we get our new Yeti in August. There's nothing 'psychological' involved. I well remember a garage cashier in Holland asking me about it when V Power was newly introduced and reacting with great enthusiasm. It does make a considerable difference and helps considerably to promote the longevity of your engine.

So, is the same true for petrol - will my 1.8TSi benefit from 'premium' unleaded?

Certainly. My passat was a 2.3 litre v5 petrol engine, but it's true, petrol or diesel, for good engine maintenance.

Certainly. My passat was a 2.3 litre v5 petrol engine, but it's true, * petrol or diesel, for good engine maintenance.

*

I would add to your comment the word "allegedly" as no-one, car or fuel manufacturer, has ever said that about any of the "plus" fuels, as they would have to be able to prove it first.

I think the 1.8 Tsi will give more power on 98 octane-will have to read the manual to be sure....

And a diesel test at the end of the it may improve but Im happy with bog standard diesel over more expensive stuff

Wonder how many boy racers forked out for these and ten watched fifth gear lol

So the diesel test produced 6hp more than the standard 155hp 3.9%, WOW!

Still does not do it for me. I'm much more interested it what it cost me per mile and based of my own, personal real world, test the more expensive diesel is not worth buying. Again my view for what it's worth.