Pining, twin plugging, etc.

[Robert Harris]

According to Heywood "Knock ... when essentially spontaneous ignition of a
portion of the end-gas - the fuel, air, residual gas, mixture ahead of the
propagating flame - occurs."

The faster the combustion process, within limits, the better. What is
desired is the controlled combustion of chemical energy and then conversion
of this heat into controlled pressure on the piston which converts the
pressure to rotational energy.

Spark advance starts the conversion process of fuel to pressure BEFORE
Piston Top Dead Center. All of the pressure generated BEFORE TDC effectively
opposes the rotation of the crankshaft - making negative power.  It is
necessary simply because the speed of combustion is such that the process
must be started early to reach optimum pressure at around 15 degrees after
TDC. This point generates Maximum Best Torque (MBT) by transferring the most
work from the combustion process's to the piston.  All timing does is start
the fire in such a fashion as to maximize MBT. Increasing the speed of
combustion reduces the advance needed to reach MBT. This is why higher
compression, turbo boost etc. require LESS advance.  The resulting mixture
burns quicker and therefore, MBT is reached by reducing the lead time of
ignition.  They also waste less power OPPOSING the rotation (Ignition
Advance).

Small fast burn chambers - such as the early closed chamber heads on V-8's
make significantly more power and LESS pollutants that the later "Open
Chamber" heads.  This is because the flame front moves significantly faster
in the small chambers.  They can run higher compression on the same octane
because the much faster flame front consumes the end-gas BEFORE it can
detonate.  The squish/quench area forces a controlled jet just before
ignition that introduces swirl and turbulence to the mixture, enhancing its
burning speed.  The quench area cools the temperature of the end-gas's below
the detonation point and allows them to be re-introduced to the combustion
process in a controlled manner.

Side effects of ignition.  A small fast burn chamber is significantly less
sensitive to ignition.  The mega-volt mega multi spark Dr Weird Science
system tied directly to Hoover Dam will generally show little or any
improvement on a fast burn chamber - but will greatly improve a larger open
chamber. As the mixture moves significantly leaner and approaching lean
misfire, however, even the fast burn chamber shows improvement.

The fast burn chamber also significantly reduces pollution. The faster, more
robust combustion allows significantly more EGR to be introduced without
degradation of the combustion process. Heywood also states simply that the
compression ration has little to do with the formation of NOx products.  The
burn time and temperature have much more to do with it.  Raising the ratio
reduces the burn time by speeding the flame front and increasing the
tolerance to EGR so if properly done has no effect on NOx.  The faster
combustion also reduces CO products.

The least desirable chamber from a power, pollution or economy point of view
is a flat disk with the ignition at the side - precisely what many engines
have. From an all around view point the high squish, small chamber wedge
with a bowl in piston is probably the best - however the less efficient
hemi/canted valve pent roof (but much better than the open disk) give's
better breathing so quantity once again supersedes quality.

The advantage of multiple plugs is highly dependent on the chamber design -
with the most improvement being shown in the poorest chamber or crappiest
fuel (nitromethane for example). Aircraft run twin plugs primarily for
reliability.  Breakdowns at 20,000 feet are a real bitch. The location
swamps the increase in efficiency.  Ideally, the first consideration in
location is to burn the mixture under the exhaust valve(s) first as this is
the hottest, most likely to detonate part of the mixture, followed by a
central location to get the largest, quickest flame front possible.
Multiple central plugs on a hemi/pent roof with 3,4 or 5 valves meet both
requirements quite nicely and will probably show the most modest improvement
because of that.  Since nitromethane is doggie doo doo from all fuel
considerations except sheer power, a nitro engine shows remarkable
improvement from multiple plugs.  The effort to put multiple plugs in a
chamber for gasoline would probably not return much on your investment.  Of
course, if self inflicted gunshot wounds to the foot appeal to you, it might
be worth a try.  The worst that will happen is that you'll ruin a perfectly
good set of heads and maybe the engine under it.