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1. Heat range.
A heat range refers to how much heat a spark plug is capable of removing from the combustion chamber. Selecting a spark plug with the
proper heat range will ensure that the tip will maintain a temperature high enough to prevent fouling, yet be cool enough to prevent
pre-ignition. If you modify a motor a lot it is best to start with a colder plug. If it ain't fouling its OK. On motocross bikes its unlikely you
would modify a motor enough to need the plug heatrange changed.
2. Resistor?
Always use a plug with a resistor. I will not affect power and will give the general public one less reason to want to ban bikes. Spark
plugs use resistors to suppress the emitted electrical "noise" from ignition systems. We must do this to comply with government
regulations on the amount of electrical "noise" allowed by ignition systems. Radio frequency interference (RFI) can also be suppressed
by using resistor (or inductor) wires or plug caps. We use these things to avoid interference with sensor signals and with your listening
radio. Sensor interference can occur if a sensor, or its wire is un-insulated and is routed close to the plug or
the plug wires. The resistor get rid of the initial voltage spike and its harmonics that cause the radio frequencies to be produced. Since
we rely on the trailing edge of this voltage spike for ignition, a resistor has a small effect on your power.
3. Size of the electrode.
This is what all the number at the end of a plug are for. Basically if you need to use a narrower electrode then much more expensive
metals (well they justify the cost using this reason) must be used as normal material would erode in no time. A narrower electrode is not
used to stop fouling and possibly could even foul easier as it only needs one little blob to foul it.
Basically the smaller the electrode (has less Capacitance for those into electrical stuff) then less voltage it needs to fire. The bigger the
electrode the more current it can flow. (good in cars with heaps of power available, huge sparks) So in most bike bikes when the ignition
power (current) is limited the ignition are generally set up for a small diameter electrode.
If dollars are short the standard "S" plugs will not do you engine any harm. It does however place extra stress on you electrical system as
it will be required to produce a higher voltage than normal. I have also seen dyno chart with very small increase in horsepower with the
"G", Whether in "reallife" you can tell any difference is unlikely. Like all thing a New "S" is much better than a rat shit "G" .
Below is a table of how to decode a NGK plug.
The "CM" is not listed but it would be a "EG" electrode. So they are basically a short "EG" plug because room is tight in a KTM 250. The
shorted body will not cause any hassles. If thread length is the same (very very important) and you can fit in an EG then that's ok as long
as the plug cap isn't hitting anything.
Here the table of values for NGK spark plug names.
The breakdown is like this: [B] [CPR] [6] [E] [S] - [11]
Six fields. Some, e.g. the second field, are optional. Some fields may have multiple letters.
Start with the code written on the plug. It will have something like BR8EQ, B8EG or B10EGV. This code can be broken down into
seperate codes that tell us exactly what the plug is designed for, like this:
A = 18mm
B = 14mm
C = 10mm
D = 12mm
Field Two: Construction
C = hex size 5/8"
K = hex size 5/8" with projected tip (ISO)
M = compact type
P = projected insulator type
R = resistor
SD = surface discharge for rotary engines
U = semi-surface discharge
Z = inductive supressor
Field Three: Heat Range
2 = HOTTEST
11 = COLDEST
Field Four: Thread Reach
E = 19mm
F = tapered seat
H = 12.7mm (1.5")
L = 11.2mm (7/16")
* NOTE *
If this field is blank, an 18mm plug has 12mm
reach,
and a 14mm plug has a 9.5mm (3/8") reach.
A, B = special design (Details unknown)
C = special ground electrode
G = racing use
GV = racing use, V-type
H = half thread
K = 2 ground electrodes for certain Toyotas
L = half heat range
LM = compact lawn mower type
M = 2 ground electrodes for Mazda rotary engines
N = special ground electrode
P = platinum tip (premium)
Q = 4 ground electrodes
R = delta ground electrode for BMW
S = standard 2.6mm centre electrode
T = 3 ground electrodes
V = fine wire centre electrode, gold palladium
VX = platinum tip (high performance)
W = tungston electrode
X = booster gap
Y = v-groove centre electrode
Field Six: Wide Gap
8 = .032"
9 = 0.36"
10 = .040"
11 = .044"
13 = .050"
14 = .055"
15 = .060"
20 = .080"