On 23/08/15 11:27, Dave Plowman (News) wrote:
In article ,
The Natural Philosopher wrote:
You have a car, and you have tow grades of fuel. You adjust it to run at
its best on one grade. It pinks like **** on the other, so you do your
best with the other fuel to get it at least to run without damage. You
achieve this. But now although its not as good as it was, on the high
grade fuel, its still better on that than on the low grade fuel.

You don't 'adjust' an engine to make the best use of a high octane fuel.
You do if you are tuning for performance

It needs/allows things like a higher compression ratio.

And a more advanced spark.

You design the engine for a target octane rating. Knock sensors then alter
the mapping if a lower octane rating fuel is used. There is no point using
a higher octane rating than the engine is designed for.

You contradict yourself.

You design it for a target octane rating. Yes. You lower the mapping for
lower octane. Yes. so its NOW designed for the lower octane fuel!

I(f it has auto mapping it will definitely be better on the higher
octane. If not, its pretty LIKELY but not certain that it will be


What does of course happen is that if its running rich because the fuel
has more carbon in it - typically with aromatics, it will be leaned out
by the ECU so it will do better mpg on higher carbon content fuel.

As I said, its not that simple.


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