is a lot of debate and skepticism to whether or not spark plug indexing
is worth the effort. Some studies have shown 1-2% of total engine power
can be lost through poor spark direction. Perhaps that does not sound
like enough power to be concerned about, but consider this: Chances
are that you, like the majority of enthusiasts, have spent a fair amount
of money on hotter coils, ignition boxes, better plugs, ignition wires,
cap and rotor, and the list goes on. In fact many heads these days boast
"revised spark plug angles" and "fast burn" chambers.
Heck, you probably even made the extra effort to set all your plug gaps
to the exact specification. Well if you believe all those things make
a difference, and they probably do, than it is only logical to think
that the spark needs to face the right direction in the combustion chamber!
Indexing spark plugs has been practiced by motoring enthusiasts for
many years. The technique has been used by racers to obtain maximum
combustion efficiency within each cylinder, and also to maintain consistent
combustion between all cylinders. The object of indexing spark plugs
is to ensure that a spark plugs ground electrode is not positioned in
such a way where it is shielding the spark from the fuel/air charge
or impeding the flame front as it travels away from the spark plug into
the chamber. The ideal position of the plug in the head is such that
the gap is facing towards the valves, so that the spark is fully visible
to, and aimed at, the center of the combustion chamber.
Unfortunately with today's mass produced plugs and cylinder heads, there
is absolutely no assurance that when you screw in a sparkplug it ends
up in the correct orientation. The majority of people take this for
granted, and probably feel the minimal horsepower gains is not worth
added effort. But then again, the guys winning races aren't just following
Indexing plugs is only valid on the conventional type of "gapped"
plugs that have the ground strap that extends off the threaded portion
of the plug. Some new style plugs, have no grounding electrode protruding
from the threaded bushing, or have multiple electrodes. These plugs
are self indexing, in that their position in the chamber is the same
no matter how they are installed. While this may be a small benefit,
the costs of these plugs is outrageous and has not been proven to be
worth the expense.
This article illustrates how indexing can be done in a relatively cheap
and easy manner. Of course there are methods of using special indexing
washers to also position the depth of the plug into the chamber. However
we'll stick to simply attaining proper spark orientation using nothing
more than a permanent marker.
On most cylinder heads, the spark plug enters the chamber at an angle.
This angle places the spark plug electrode near the wall of the combustion
chamber. I try to keep the grounding electrode close to the chamber
wall. This ensures that the plug gap will be fully exposed to the combustion
chamber and not hidden by the ground electrode. To do this, I look at
the plug and note the position of the grounding electrode in relation
to the plugs upper porcelain body. On the Autolites that I use, I reference
the ground electrode to the lettering on the porcelain body. For instance,
I might find that the grounding electrode is in line with the letter
"U" in the word Autolite that is written on the plug. This
is my reference that shows me where the gap is located when I screw
the plug into the head. After I have referenced the grounding electrode
to markings on the plug body, I screw the plug into the head, finger
tight, and note the position of my reference mark. I want my reference
mark to be positioned facing up and just slightly towards the corner
of the exhaust port for that cylinder. When this occurs, I know the
gap is fully exposed to the combustion chamber and the spark will not
be shielded, by the ground strap, from the fuel charge. If I find that
my reference mark does not line up within 20 degrees or so, I try another
cylinder until I find a cylinder that leaves the plug in a satisfactory
position. If I can't find a cylinder that allows the plug to be positioned
correctly, I have no use for that plug and set it aside for another
motor. I feel it is not critical to be 'dead on' with the plug orientation.
Within 20 to 30 degrees, either way, should be satisfactory. The main
idea is to have the gap open to the chamber, so that the air/fuel charge
is exposed to the maximum spark intensity in order to facilitate a fast
In order to ensure I can index a plug to all cylinders, I usually buy
several sets of plugs to work with. By buying several plugs, I am hoping
to get plugs that have been made at different factories, or different
threading machines, by getting plugs from different 'lots'. This way
I feel I have a better chance of getting a plug indexed to all cylinders.
After I have indexed a plug for each cylinder, I will scratch (or engrave)
the cylinder number into the plugs metal housing or one of the flat
sides of the hex nut surface for proper relocation. Indexing plugs is
one of those cheap and easy methods of ensuring that your ignition system
is delivering the maximum energy to all the combustion chambers. After
all, it is that tiny spark that gets all that horse power going. Make
sure your combustion chambers can see as much of the spark as possible
and you will be assured maximum power. F/M
The ideal position of the plug is such that the gap is facing
towards the valves, preferably the exhaust valve.
The object of indexing is to avoid this problem: when this plug
fires the spark is directed towards the chamber wall, rather than
the center of the combustion chamber.
Shown here are two Autolite (5144) plugs. Notice how the electrode
position has no correlation to the "Autolite" script
on the white porcelain section. There is also no guarantee the
threads are positioned the same on both plugs, or that the threads
in the head are consistent from chamber to chamber. In other words,
chances are good if you screw this plug in, it won't put the electrode
in the proper position.
Using a Sharpie pen, place a "index" mark on the plug
body. The mark lines up with the electrode gap (see photo 3).
Then screw the plug into the head. If the mark is facing up as
in this photo, and slightly towards the exhaust port, we know
the electrode is in the proper orientation to the center of the
chamber (as in photo 1.)
However if the index mark is facing downwards, as in this photo,
or more than 20 degrees off the corner of the exhaust port, then
you know the gap is not in the ideal location. Try this plug in
the other chambers to see if it works better elsewhere. If not,
set it aside and try another plug.
Another way to solve the problem is to use special copper indexing
washers (available from Moroso). These washers come in varying
thickness' and allow you to tailor the position and depth of the