Exactly so. With no pinging the fuel is 'fine' in that it won't be causing
any damage, but a lower grade of fuel will need the timing to be retarded,
and it is that which causes the loss of performance and increase in running
temperatures.
Because the mixture burns and should not explode in a petrol engine it takes
time for the flame front to move right through the combustion chamber and
convert the energy in the fuel to piston motion. The piston is moving all
the time this is happening, so some of the energy is converted right at the
beginning i.e. ignition and some not until the end i.e., when the piston is
already on its expansion stroke. The more fuel that is converted on the
expansion stroke, and the further down it is converted, the less energy is
converted into piston motion and so must be converted into waste heat.
Therefore with lower grade fuel, less advance, more fuel is converted later
in the stroke so energy is lost as heat.
This isn't just theory, my 73 roadster has always been prone to pinking,
more so with unleaded. Consequently I run it with 98 octane as often as I
can. While touring Scotland a few years ago in the Highlands I could only
get 95 octane and the pinking was so bad on the hills that I had to stop and
retard the timing by about 3 degrees to reduce (but not totally eliminate)
it. The resultant loss of power was immediately apparent, and the increase
on the temp gauge a few miles later.
This is why different engine/distributor combinations have individual timing
settings, to get the best out of them. manufacturers specs are always
conservative to account for manufacturing tolerances etc. giving a greater
or lesser propensity for pinking. Some examples of a given spec of engine
will be able to take more advance without pinking than another, and this
will give greater performance and economy over the standard setting, and I
have experienced this with other BMC engines.
Modern ECU systems detect pinking (knock control) and continually vary the
timing accordingly to get the best out of whatever fuel is being used,
without the driver having to do anything. I've just taken on my son's BMW M
Coupe and reading the handbook for tyre pressures, oil and fuel grades etc.
The fuels section quotes 91 RON as a minimum, but 98 RON 'to achieve its
specified performance and fuel consumption'.
On fixed timing engines designed to run on the lower grade, like the factory
V8, there is nothing to be gained by putting in more expensive, higher
octane unless you can also advance the timing but still stay out of the
pinking range. Not all pinking is audible and the 'silent' type can still
be damaging.
PaulH.
----- Original Message -----
> In Roger William's book he says that using a low grade gasoline (petrol)
> in an engine with high compression will result in extra heat with lesser
> power. I was told that as long as the engine does not 'ping' then the
> fuel is fine...
///
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