STOPPING THE SHOW
Anything that contributes to lowering the temperature that the endgas reaches
will make detonation less likely. Anything that slows the process of conversion
from normal gasoline into a sensitive explosive, will make detonation less
likely. Anything that speeds up combustion, so that is it completed before the
conditions needed for detonation can develop fully, will make deto less likely.
Therefore the following will work;
(1) Lower intake temperature
(2) Lower throttle position, lower volumetric efficiency, or reduced turbo boost
the less mixture that enters the cylinder, the less it is heated by compression.
(3) Lower intake pipe, crankcase, and/or cylinder, piston, or head temperatures.
This year's Yamaha 250cc road race engine, for instance, has a copper cylinder
head insert to conduct combustion heat away faster, resulting in a lower
combustion chamber surface temperature.
(4) Lower compression ratio. The less you squeeze it, the less it is heated.
(5) A more breakdown resistant fuel, such as toluene or isooctane. If straight
chain molecules are not present, the fuel will not be broken down so rapidly by
preflame reactions.
(6) A negative catalyst something that will either pin down active radicals or
convert them into something harmless. Tetraethyl lead, MMT, or other antiknock
compounds are the medicine.
(7) Retarded timing shortens the time during which proknock reactions can take
place.
(
Incylinder turbulence or anything else that will speed up combustion (faster
burning fuel such as benzene). This works by completing combustion before the
time bomb of preflame reactions cooks long enough to cause autoignition.
(9) Higher engine rpm This simply shortens the time during which the mixture is
held at high temp. In Honda experiments in the 1960's, they found that an
engine's octane requirements began to decrease steadily over 12,000 rpm, and
were under 60 octane up near 20,000. In a more accessible example, note that
engines knock when they are "lugged" run at low rpm, wide open throttle and
stop knocking promptly when you shift down a gear and let the engine rev up
more. This stops deto by not allowing enough time for the reactions that cause
it.
(10) Redesigning troublesome exhaust pipes. Some pipes give great numbers on the
dyno, but can't be used because they cause seizures. They either simply
overcharge the engine in some narrow rpm band (pushing it into detonation just
as too much turbo boost would do), or back pump mixture from the header pipe
that has picked up too much heat (this is why nobody heat wraps header pipes
anymore).
(11) Avoiding excessive backpressure. Exhaust pipes always create back pressure,
but the more there is, the higher the fraction of hot exhaust gas that will be
unable to leave the cylinder during exhaust. Its heat, added to the fresh charge
that next enters the cylinder, may push the engine over the line into
detonation. Sometimes a one or two millimeter reduction in tailpipe ID will get
you a couple of extra horsepower, but it may also push enough extra heat into
the charge to make the engine detonate after a few seconds.
The number of ways of playing footsie with detonation is endless, but nothing
works every time. This guarantees that we will never be bored, and will never
run out of seized pistons.
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