worse fuel economy on super unleaded

[Jasikasisback]

anyone know why this could be?or is esso watering down the petrol?

[itavaltalainen]

97 ron has less than specific heating value (calorific value) than 95 ron as the gain in octane rating is achieved by adding mtbe/etbe to the petrol, which has a FAR lower calorific value than other petrol components.

for engines designed to run 95 ron the benefits from higher octane rating are outweighed by the lower calorific value.

only engines with high compression that are specifically designed for 97 ron or higher will be able to take advantage of the higher anti knock index and adjust the ignition timing accordingly which will increase torque (with the same amount of petrol being injected compared to 95). to get the same torque from such an engine on 95 the ignition timing needs to be changed closer to UDC to avoid knocking which results in a reduced efficiency compared to 97/98 hence they need to inject more of 95 petrol to get the same power - i.e. it needs more petrol for the same amount of torque compared to 98.
so they are less efficient on 95, which reduces the benefit of the higher calorific value whereas on 98 they can take advantage of the higher aki which counteracts the lower calorific value (to some extent).
 

[dom]

Or in other words, a MK3 golf with a standard engine was designed to be ran on 95Ron and won't benefit from super. Stop wasting your money on it.

[tweed]

But a 2stroke 115cc engine will produce more power using shell nitro super than bp normal unleaded. Same amount of fuel used as its carb and never changed timing 

[Jasikasisback]

I was told that it is a good idea to run a tank of super every once in a blue moon...guess it isn't true..
will go back to regular unleaded..two quid thrown out the window...

[madsb]

I was told that it is a good idea to run a tank of super every once in a blue moon...guess it isn't true..
will go back to regular unleaded..two quid thrown out the window...

This could be true. Take Shell's vpower 98 for example. They add a great deal of products to it that supposedly help clean your injectors etc. I guess a lot of other petrol companies add this too, though.. Also, I've actually found that I run the longest on vpower. Not sure if it's due to their "fuelsave" technology or w/e.

[itavaltalainen]

You can get the effect of "fuel system" or "injector" cleaners by adding anhydrous isopropyl alcohol (this is the main component). you can buy 5 liters for twice as much on ebay as a can of the "cleaners" in halfords....

I add 250ml every 3rd/4th fill up.

[Jasikasisback]

is this safe for the seals?

[madsb]

Should be safe to use, just make sure it's anhydrous.. Also, chances are that the fuel you use already has around 5 % ethanol, which has similar effects (except that it in theory contains water). EU-directives demand that fuel companies use 10% biofuel in their fuels by 2020 or something. Biofuel = ethanol