WARNING!!!!: the wrong heat range plug can lead to insulator failure. That can lead to apex seal failure and that can lead to a severe loss of power. It is a very rare occurrence but it has happened.
Spark plugs last about 50 hours in rotary engines. They should be changed at that times as a spark plug that is too old can lead to a noticeable loss of power.
Picture by Joe Hull
Here are some comments by Jim Mederer owner of Racing Beat . Jim has more experience in rotary engines than any other person around. He has set many speed records at Bonneville and his rotary powered cars and engines have won many many car races over the past 37 years.
Many things affect heat range selection, so I can't give a "recommendation", only guidelines. Ultimately, for aircraft use, I believe you are better served with the coldest plug that doesn't foul excessively. This gets wrapped up with the power of the ignition, compression ratio, mixture, timing, cooling, plug gap, plug design, and a lot of environmental factors. As a rough cut, for a near-stock engine, 2 steps cold from the standard heat range is a reasonable starting point. And beware; Trailing plugs are more susceptible to detonation than Leading.
By the way, you cannot read the "color" of rotary plugs and tell anything definitive about mixture. Because of the lack of a cooling cycle, the plugs run so hot that, at best mixture (power), the plugs come out "white". You also cannot tune the engine with EGT or O2 – it varies all over the map. Only power matters.
A spark plug's heat range has no relationship to the actual voltage transferred through the spark plug. Rather, the heat range is a measure of the spark plug's ability to remove heat from the combustion chamber. The heat range measurement is determined by several factors; the length of the ceramic center insulator nose and its' ability to absorb and transfer combustion heat, the material composition of the insulator and center electrode material.
The insulator nose length is the distance from the firing tip of the insulator to the point where insulator meets the metal shell. Since the insulator tip is the hottest part of the spark plug, the tip temperature is a primary factor in pre-ignition and fouling. Whether the spark plugs are fitted in a lawnmower, boat, or a race car, the spark plug tip temperature must remain between 500C-850°C. If the tip temperature is lower than 500°C, the insulator area surrounding the center electrode will not be hot enough to burn off carbon and combustion chamber deposits. These accumulated deposits can result in spark plug fouling leading to misfire. If the tip temperature is higher than 850°C the spark plug will overheat which may cause the ceramic around the center electrode to blister and the electrodes to melt. This may lead to pre-ignition/detonation and expensive engine damage. In identical spark plug types, the difference from one heat range to the next is the ability to remove approximately 70°C to 100°C from the combustion chamber. A projected style spark plug firing tip temperature is increased by 10°C to 20°C.
The firing end appearance also depends on the spark plugs tip temperature. There are three basic diagnostic criteria for spark plugs: good, fouled and overheated. The borderline between the fouling and optimum operating regions (500&def;C) is called the spark plug self-cleaning temperature. The temperature at this point is where the accumulated carbon and combustion deposits are burned off. Keep in mind the insulator nose length is a determining factor in the heat range of a spark plug, the longer the insulator nose, the less heat is absorbed, and the further the heat must travel into the cylinder head water jackets. This means the plug has a higher internal temperature, and is said to be a hot plug. A hot spark plug maintains a higher internal operating temperature to burn off oil and carbon deposits, and has no relationship to spark quality or intensity.
Conversely, a cold spark plug has a shorter insulator nose and absorbs more combustion chamber heat. This heat travels a shorter distance, and allows the plug to operate at a lower internal temperature. A colder heat range is necessary when the engine is modified for performance, subjected to heavy loads, or is run at a high rpm for a significant period of time. Colder spark plugs remove heat quicker, reducing the chance of pre-ignition/detonation. Failure to use a cooler heat range in a modified application can lead to spark plug failure and severe engine damage.Below is a list of external influences on a spark plug's operating temperature. The following symptoms or conditions may have an effect on the actual temperature of the spark plug. The spark plug cannot create these conditions, but it must be able to cope with the levels of heat...if not, the performance will suffer and engine damage can occur.
Air/Fuel Mixtures seriously affect engine performance and spark plug operating temperatures.
Lean air/fuel mixtures cause plug tip and cylinder temperature to increase, resulting in pre-ignition, detonation, and possibly serious spark plug and engine damage
It is important to read spark plugs many times during the tuning process to achieve the optimum air/ fuel mixture
Higher Compression Ratios/Forced Induction will elevate spark plug tip and in-cylinder temperatures
can be increased by performing any one of the following
a) reducing combustion chamber volume (i.e.: domed pistons, smaller chamber heads, mill ing heads, etc.)
b) adding forced induction (Nitrous, Turbocharging or Supercharging)
c) camshaft change
As compression increases, a colder heat range plug, higher fuel octane, and careful attention to ignition timing and air/fuel ratios are necessary. Failure to select a colder spark plug can lead to spark plug/engine damage
Advancing Ignition Timing
Engine Speed and Load
Increases in firing-end temperature are proportional to engine speed and load. When traveling at a consistent high rate of speed, or carrying/pushing very heavy loads, a colder heat range spark plug should be installed
Ambient Air Temperature
This creates higher cylinder pressures/temperatures and causes an increase in the spark plug's tip temperature. So, fuel delivery should be increased.
As temperature increases, air density decreases, as does intake volume, fuel delivery should be decreased
Result is lower combustion pressures and temperatures, causing a decrease in the spark plug's temperature and a reduction in available power.
Air/fuel mixture should be leaner, depending upon ambient temperature.
The higher the altitude, the lower cylinder pressure becomes. As the cylinder temperature decreases, so does the plugs tip temperature
Many mechanics attempt to "chase" tuning by changing spark plug heat ranges
The real answer is to adjust air/fuel mixtures by rejetting in an effort to put more air back into the engine
Types of Abnormal Combustion
by hot spots in the combustion chamber...can be caused
(or amplified) by over advanced timing, too hot a spark plug, low octane fuel, lean air/fuel mixture, too high compression, or insufficient engine cooling
change to a higher octane fuel, a colder plug, richer fuel
or lower compression may be in order
You may also need to retard ignition timing, and check vehicle's cooling system
Pre-ignition usually leads to detonation; pre-ignition an detonation are two separate events
Can break insulators or break off ground electrodes
Pre-ignition most often leads to detonation
Plug tip temperatures can spike to over 3000°F during the combustion process (in a racing engine)
frequently caused by hot spots in the combustion chamber.
Hot spots will allow the air/fuel mixture to pre-ignite. As the piston is being forced upward by mechanical action of the connecting rod, the pre-ignited explosion will try to force the piston downward. If the piston can't go up (because of the force of the premature explosion) and it can't go down (because of the upward mo-tion of the connecting rod), the piston will rattle from side to side. The resulting shock wave causes an audible pinging sound. This is detonation.
Most of the damage than an engine sustains when "detonating" is from excessive heat
The spark plug is damaged by both the elevated temperatures and the accompanying shock wave, or concussion
A spark plug is said to have misfired when enough voltage has not been delivered to light off all fuel present in the combustion chamber at the proper moment of the power stroke (a few degrees before top dead center)
spark plug can deliver a weak spark (or no spark at all) for a
variety of reasons...defective coil, too much compression with
plug gap, dry fouled or wet fouled spark plugs, insufficient ignition timing, etc.
Slight misfires can cause a loss of performance for obvious reasons (if fuel is not lit, no energy is be-ing created)
Severe misfires will cause poor fuel economy, poor driveability, and can lead to engine damage
Will cause spark to leach to metal shell...no spark across plug gap will cause a misfire
Wet-fouled spark plugs must be changed...spark plugs will not fire
Dry-fouled spark plugs can sometimes be cleaned by bringing engine up to operating temperature
Before changing fouled
spark plugs, be sure to eliminate root
cause of fouling
NGK SPARKPLUG CODES
Here is the table of values for NGK spark plug names.
The breakdown is like this: [B] [CPR]  [E] [S] - 
Six fields. Some, e.g. the second field, are optional. Some fields may have multiple letters.
Field one: Thread diameter.
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 suppressor
Field three: Heat Range.
2 = hot, up to 10 = cold. There's no 1,
Field four: Thread reach.
E = 19mm F = tapered seat
H = 12.7mm (1.5") L = 11.2mm (7/16")
If this field is blank, an 18mm diameter plug has 12mm reach, and a 14mm plug has a 9.5mm (3/8") reach.
Field Five: Firing end construction.
A, B = special design (no details given)
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 engine
N = special ground electrode
P = platinum tip (premium)
Q = 4 ground electrodes
R = delta ground electrode for BMW
S = standard 2.6mm center electrode
T = 3 ground electrodes
V = fine-wire center electrode, gold palladium
VX = platinum tip (high performance)
W = tungsten electrode
X = booster gap
Y = v-groove center electrode
Field Six: (after the dash) Wide gap.
8 = .032" 9 = .036" 10 = .040"
11 = .044" there is no 12 13 = .050"
14 = .055" 15 = .060" 20 = .080"
Extended-life platinum tip plugs replacing the CR9EH-9 are available as CR9EHVX-9.
Resistor plugs are used for two reasons --
1. They cut down electrostatic interference.
2. They provide a sharper "edge" to the voltage spike, making for a stronger, shorter spark. On high RPM motors, this is important.
The projected insulator simply describes the shape of the plug head.
A projected insulator sticks out a little further into the combustion chamber.
You can reach NGK at 800-855-8151 ...... 714-855-8278
Combustion deposits are slight and not heavy enough to cause any detrimental effect on engine performance. Note the brown to greyish tan color, and minimal amount of electrode erosion which clearly indicates the plug is in the correct heat range and has been operating in a "healthy" engine.
May be caused by a foreign object that has accidentally entered the combustion chamber. When this condition is discovered, check the other cylinders to prevent a recurrence, since it is possible for a small object to "travel" from one cylinder to another where a large degree of valve overlap exists. This condition may also be due to improper reach spark plugs that permit the piston to touch or collide with the firing end.
Too much oil is entering the combustion chamber. This is often caused by piston rings or cylinder walls that are badly worn. Oil may also be pulled into the chamber because of excessive clearance in the valve stem guides. If the PCV valve is plugged or inoperative it can cause a build-up of crankcase pressure which can force oil and oil vapors past the rings and valve guides into the combustion chamber.
A clean, white insulator firing tip and/or excessive electrode erosion indicates this spark plug condition. k This is often caused by over advanced ignition, timing, poor engine cooling system efficiency (scale, stoppages, low level), a very lean air/fuel mixture, or a leaking intake manifold. When these conditions prevail, even a plug of the correct heat range will overheat.
Glazing appears as a yellowish, varnish-like color. This condition indicates that spark plug temperatures have risen suddenly during a hard, fast acceleration period. As a result, normal combustion deposits do not have an opportunity to "fluff-off" as they normally do. Instead, they melt to form a conductive coating and misfire will occur.
Usually one or a combination of several engine operating conditions are the prime causes of pre-ignition. It may originate from glowing combustion chamber deposits, hot spots in the combustion chamber due to poor control of engine heat, cross-firing (electrical induction between spark plug wires), or the plug heat range is too high for the engine or its operating conditions.
Rarely occurs in automotive engines, however, this condition is caused by similar conditions that produce splash fouling. Combustion deposits thrown loose may lodge between the electrodes, causing a dead short and misfire. Fluffy materials that accumulate on the side electrode may melt to bridge the gap when the engine is suddenly put under a heavy load.
This form of abnormal combustion has fractured the insulator core nose of the plug. The explosion that occurs in this situation apples extreme pressures on internal engine components. Prime causes include ignition time advanced too far, lean air/fuel mixtures, and insufficient octane rating of the gasoline.
Appears as "spotted" deposits on the firing tip of the insulator and often occurs after a long delayed tune-up. By-products of combustion may loosen suddenly when normal combustion temperatures are restored. During hard acceleration these materials shed from the piston crown or valve heads, and are thrown against the hot insulator surface.
A build-up of combustion deposits stemming primarily from the burning of oil and/or fuel additives during normal combustion ... normally non-conductive. When heavier deposits are allowed to accumulate over a longer mileage period, they can "mask" the spark, resulting in a plug misfire condition.
Soft, black, sooty deposits
easily identify this plug condition. This is most often caused by
an over-rich, air/fuel mixture.
This plug has served its useful life and should be replaced. The voltage required to fire the plug has approximately doubled and will continue to increase with additional miles of travel. Even higher voltage requirements, as much as 100% above normal, may occur when the engine is quickly accelerated. Poor engine performance and a loss in fuel economy are traits of a worn spark
SOURCE: Champion Spark Plugs