2. Rating and Testing

A. Rating

Rating is referred to as "heat range"

Heat range is the plug's thermal characteristics or its ability to transfer combustion heat from its firing end to the cylinder head.

Most heat is dissipated through the threaded and seating portion of the shell

Heat is transferred down the insulator nose to the seating or contact point of the insulator and shell, then out of the shell to the cylinder head.

A longer insulator nose = a hotter plug temp.

The insulator tip is the hottest part of the plug and therefore the part that could cause pre-ignition.  For a typical 4-cycle engine this takes place at approx. 1750 degrees Fahrenheit

The rating of a plug's heat range or its pre-ignition rating is done by an IMEP test.

B. Testing

1. IMEP TEST- SAE standard test # J551, single cylinder, constant comp. ratio, constant spark advance, and full throttle

IMEP rating is established by that power setting just below (1" baro.) the point where the plug reaches pre-ignition as     determined by a temp. sensitive indicator.  There is a dramatic increase in cylinder temp. within the first few cycles under pre-    ignition/detonation conditions.  Higher IMEP numbers (psi) = colder plug. IMEP numbers are only good for comparison of the plugs     ability to dissipate heat, not good for choosing a plug based on IMEP or BMEP of actual engine.  Even though the SAE IMEP test is     actually a load test the test engine design is so different and frictional losses are added back into the equation rendering the final figure useless for actual race engine IMEP or BMEP comparisons.

ex. BMEP = 150.8 X Torque/cubic in.   

so for a V-8 310 CI engine with 420 ft.lb. of torque

BMEP = 150.8 X 420/310 = 204.3 psi

IMEP rating for a C57C     = 450+

C63C    = 370

RV15YC     = 200

2. THERMOCOUPLE TESTING - used for establishing baseline temp. curve to determine most effective plug .

ex. idle temps - to determine anti-fouling needs

WOT temps - to determine maximum temps

Any change in the engine design i.e., type of cylinder head, cam, cam timing, comp. ratio, will change the temp. of the cylinder and therefore testing could be done.

Thermocouples are made in all common thread sizes and heat ranges

A thermocouple plug has a small temp sensitive element (a fused junction of platinum and platinum/10% rhodium) on the end of the insulator tip, being temp. sensitive the voltage at the junction increases as temp. increases and can be correlated to show exact temp. of plug at T.C. position.

3. FLAME ANALYZER or PRE-IGNITION TEST - basically a power source supplying a constant 10V to the spark plug with an oscilloscope showing  the voltage curve indicated by the current flowing through the ionization of the plug gap.

From this curve we can determine:

1.burn rate of fuel mixture

2. auto-ignition - this is determined by intermittently removing the electrical charge from the ignition system  to the spark plug and observing an ignition of the fuel at some point after the preset ignition time.

3. pre-ignition - when auto-ignition crosses to before set ignition timing.  This test indicates how close to pre-ignition a plug is, allowing as hot a plug as possible to be chosen.

4. plug fouling - the baseline current flow increases as the plug becomes more fouled

4.ELECTRODE EROSION TEST or ENDURANCE TEST - subjecting the electrodes to a highly corrosive environment  within the combustion chamber i.e., different fuels, lubricants, cylinder temps., ignition current, etc., to develop a product that will last longer.