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.

Paul Lamar

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.

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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.

Jim Medere

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.

• Rich air/fuel mixtures cause tip temperature to drop, causing fouling and poor driveability

• 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

• Compression can be increased by performing any one of the following modifications:
   
  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

• Advancing ignition timing by 10° causes tip temperature to increase by approx. 70°-100° C

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

• As air temperature falls, air density/air volume becomes greater, resulting in leaner air/fuel mixtures.

• 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

Humidity

• As humidity increases, air intake volume decreases

• 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.

Barometric Pressure/Altitude

• Also affects the spark plug's tip 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

Pre-ignition

• Defined as: ignition of the air/fuel mixture before the pre-set ignition timing mark

• Caused 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

• A change to a higher octane fuel, a colder plug, richer fuel mixture,
  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

Detonation

• The spark plug's worst enemy! (Besides fouling)

• 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)

• Most 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

Misfires

• 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)

• A spark plug can deliver a weak spark (or no spark at all) for a variety of reasons...defective coil, too much compression with incorrect
  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

Fouling

• Will occur when spark plug tip temperature is insufficient to burn off carbon, fuel, oil or other deposits

• 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

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NGK SPARKPLUG CODES

Here is 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.

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

SOURCE: Champion Spark Plugs