On the subject of "indexing" spark plugs:
I can't see how this indexing does any good, unless the idea is to
"douse" the electrodes with the mixture. After all, the intake valve
has long since closed by the time the plug fires, making the orientation
of the side and center electrodes irrelevant. Can anyone elaborate on
the supposed benefits of indexing the spark plugs?
First off, indexing the spark plugs, while a legitimate practice, falls
under the category of giving angels smaller shoes so that more of them
can stand on the head of a pin. The effects are real but tiny. As Carroll
Smith says, a lap time difference of 0.02 seconds over a hundred laps equals
a two-second margin of victory by the time the checker falls. Tuning is
looking for every two-hundredth advantage you can squeeze out of the motor.
There are two primary benefits. First, as Mark said, is that you want
the electrode out of the intake stream while the valve is open. Think
of the relative sizes of an intake valve and the side electrode of a
spark plug. The idea with indexing is to put the electrode so that the
skinny edge faces the intake stream, not the wide edge.
The other benefit is even more subtle, and the specific design of the
engine will determine which of these is most important for your car.
You want to index the head so that the opening of the spark plug provides
the best flame propagation in your combustion chamber. All CCs have an
optimum direction, based on the swirl induced by the intake charge and
the design of the chamber itself, in which the flame should propagate
for optimum power (or emissions, or efficiency, which ultimately work
out to pretty much the same thing -- getting the most out of each
cylinder's worth of air-fuel mix). Combustion is not the same as
explosion: the regular progression of the flame front through a
cylinder is a critical part of a car's power generating capability,
particularly at very high RPMs. It determines the pressure on the
piston, the temperature inside the combustion chamber, and ultimately
the power that the engine can produce. In some engines, it makes a
big difference whether you point the flame front in one direction or
another to start it off.
Again, this kind of thing only really matters when you've gotten down
to the really subtle details. Chances are you wouldn't notice this
on a street engine because it would be lost in the noise of a stock
ignition system, intake and exhaust system, camshaft profile and the
like. Only after you've degreed the cam, polished the combustion
chamber, figured out the optimum compression ratio, flow-ported the
heads, perfected the valves, balanced the air-fuel ratio, installed
the correct exhaust, and determined the optimum timing advance curve
and dwell angle for your engine does it make sense to think about
indexing the plugs. (Of course, it *IS* one of the cheaper things to
do, so you don't lose anything by doing it early.)
Forced to make an estimate of its efficiency, I'd say that you might
gain all of 1 to 2% at an absolute maximum by indexing plugs. Which
means it's definitely worth considering for a serious race effort
with a skilled driver and a properly built engine in a well-handling
chassis. I figure at that rate, I'll never need to worry about it
myself... :-)
--
"Do you ever wish you had a joystick with a big red button on it so
you could just nuke the person when you send a reply?" -- Kim
Scott Fisher/sfisher@wsl.pa.dec.com/DEC Western Software Labs/Palo Alto, CA
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