HJ, have you tried a tankful of Tesco 99 and compared it to Shell V-power ?
Be interested to hear what your findings turn out to be.

Thorney Motorsport tested an Astra VXR with Tesco 95 and Tesco 99 petrol. They used 2 different cars and the results showed a 9% improvement in power and 10% improvement in fuel economy, that's pretty impressive.

a900ss, your findings support those of Thorney Motorsport. They tested using 2 standard tuned Astra VXR's and then retested using ECU upgrades, both sets of tests produced significant improvements in power and fuel economy.

I just don't buy the 'engine works better at low speeds' argument. For mild acceleration and certainly cruising, even a modest engine should be operating at a moderate BMEP and shouldn't require any spark retard from the ECU. I just don't see how the octane could be beneficial.

It be of course that the fuel's calorific value is slightly higher, or that the burn rate is faster. Fast burn is very important to efficiency. Sadly people get very attached to numbers (like Ocyane rating) because it's easy to make comparisons. but there's a hell of a lot more to fuel that facile numbers.

I remember that Honda had problems with "high octane " petrol when used in the 1960's on their 5 cylinder 125's which were reving above 20,000rpm. The high octane (probably 110 ish) burnt too slowly and restrcted the revs, so they had to switch to lower octane fuel with a faster burn.

I believe that the octane rating is far too crude a measure to determine the quality of a fuel and there could be additional useful information to help choice(apart from the usual BS rating).

Perhaps the fuel providers wouldn't be so keen ?

Well actually Vauxhall states in the Astra Owner manual that for 2.0 VXR they recommend 95 RON, they also state "Use of 95 RON fuel reduces power and torque. Slight increase in fuel consumption."

Steve, the Vauxhall quoted power/torque figures for the Astra VXR of 240PS and 320NM are based on the use of 95 RON. If higher RON fuel is used then both power and torque are increased as well as a reduction in fuel consumption, see link below further details.

http://www.thorneymotorsport.co.uk/tuning/October_2008_Fuel_Test_Report.shtml

I would be interested to hear about cars that also show similar power/torque increases and fuel consumption reduction when changing from 95 RON to a higher RON.

Apart from octane rating, does anyone know any other ingredient difference between Tesco 95 and 99?

Additionally, does anyone know the ingredient difference between the other premium products?

I'm not talking about advertising hype, but actual chemicals and their effect?

I would expect their concentrations to be secret, but I'm sure someone must have done chemical finger-printing on the various brews and published the data somewhere, though I've had no luck with the usual search engines. I probably need a specialist one(?)

Back in 2000 the A4 1.8T avant quattro i had was designed to run on 98 octane, that gave 180 bhp along with 171 lb/ft.

However it could be run with a slight power loss on 95, which i used 90% of the time, i could never find out what the power loss was, but the performance gain was only occasionally noticeable, no difference was evident with economy, 25mpg round town, 32mpg on the motorway.

If you have a VAG car its worth checking the hand book, just to see what octane is recommended.

I only used the 98 when towing, but at 5 pence per litre extra at the time, for me the cost was prohibitive.

It was quick on 95 anyway.