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Increased fuel economy with 97RON over 95RON
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Author:  Sirius [ Mon Nov 17, 2008 1:49 pm ]
Post subject:  Increased fuel economy with 97RON over 95RON
Can someone explain to me the theory (if there is any) behind the higher octane fuels yielding a higher fuel economy.

Many of the petrol companies claim this - 50 extra miles per tank etc but is it actually true?

Also talking to a friend at the weekend they said that thier diesel van returned 60 miles per tank more when using BP Ultimate Diesel over the normal stuff.

Thoughts anyone? :)
Author:  Diceman [ Mon Nov 17, 2008 2:45 pm ]
Post subject: 
I believe you are correct that all grades of fuel have the same calorific value and hence you should be able to obtain the same energy/mpg from them.
As i understand it, ( a fluffy answer coming) high octane fuels are more resistant to explosion and hence burn slower and in a more controlled manner. This allows the engine to harness the power better as the rotating assembly cannot repond quick enough to a short burst of higher energy. (Typically engien builders try to design stuff so that the combustion occurs over as much of the power stroke as possible)

Lots of modern engines with advanced electronics will vary the timing & fuelling to make the most of the grade of fuel you have just filled up with.

Older engines that can't compensate will run better and more efficiently on the fuel they have been designed to run on.

there is also an element of marketing hype in there :-)

my 1 pence worth anyway - I hope someone will come along with a proper answer :-)
Author:  two_OH_five [ Mon Nov 17, 2008 3:09 pm ]
Post subject: 
I think it's entirely true

if I fill the Mondeo with Tesco's finest ullage it gets about 320 miles where an equivalent fill of Full fat Optimax will return somewhere near 360.

In my head it's entirely down to the flame speed
Slower burn means that while the overall bang is the same (same calorific fuel quantity) it's spread out over a longer duration giving slightly more torque. In effect you are making the power stroke marginally longer
Makes sense to me anyway but for sure I do see the effect
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