Pre-ignition is the effect of the fuel air mixture
in the combustion chamber igniting before the optimum point of the piston coming up on the
compression stroke before Top Dead Centre (TDC).Normally the ignition is usually started by the
spark from the spark plug at the correct time allowing for the speed of piston travel and
flame propagation time, the expanding gases should be happening just on TDC to push the
piston down again.
Pre-ignition can be caused by the incorrect timing
of the spark to the plug or some other means of ignition that does not require the spark
from a plug. This means that if ignition is caused by some other means prior to normal
spark, then the gasses will be expanding while the piston is travelling up toward TDC. A
good way to reverse the rotation of the crank!!!
Well maybe not but it is a sure way to loose power!
Considering that fuel and air are the two most important reasons that you can make power
from some metal objects working in various relationships, not forgetting the spark or
ignition that makes it all happen, lets explore this relationship.
FUEL
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Fuel can be categorised into a few types that are
relevant to octane rating and lead content. We know these as STANDARD, SUPER, UNLEADED,
& PREMIUM UNLEADED (PULP). Obviously UNLEADED means no lead added to the fuel for what
was originally to lubricate and cushion valves, and to raise octane ratings.
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For the serious production of power in a turbo
forget UNLEADED, as the octane rating is not high enough to prevent detonation at high
boost. As STANDARD (Leaded Low Octane) is no longer available that I know of, (at least in
Melbourne Australia.) we will disregard it and concentrate on SUPER & PULP.
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SUPER is what us owners of older model turbo cars
run, as it is the only option, and I suspect of various octane ratings dependent on the
Garage (old word now will use SS - Service station from now on) has put in the underground
tank recently. I was under the impression that all SUPER was supposed to adhere to a
government standard on octane rating but real world driving on high boost after filling at
different SS's says otherwise, if detonation has anything to do with it. Has anyone else
noticed this?. After trying SHELL half lead SUPER a few times, I suspect what they say
about its still high octane rating could be true.
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PULP is what the owners of new turbo cars get
stitched up at the SS pumps by a margin of 10 cents a litre over SUPER price.(Think I'll
keep the old STARION.) Disregarding the octane rating in SUPER & PULP and praying its
still over 96RON ( I think ), lets explore the other two factors that govern the control
of detonation where fuel is involved.
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