Spark Plug Technical Information
for the DOHC 6G72 Engine in the
Mitsubishi 3000GT and Dodge Stealth

by Jeff Lucius

Topics

Introduction

Structure and Terminology

Thermal Characteristics

Heat Transfer

Heat Range

Factors Affecting Plug Temperature

Electrical Characteristics

Wasted Spark Distribution

Available Voltage Vs. Required Voltage

Factors Affecting Required Voltage

Part Numbers and Cross Reference Guide

Summary

Online Stores

References

Introduction

This is not another "All About Spark Plugs" web page. There are plenty of those and some are listed in the References below. This technical note focuses on the type of spark plugs used in the V6 DOHC 6G72 engine of the Mitsubishi 3000GT VR4 and Dodge Stealth R/T Twin Turbo, and the factors that affect the spark plug's thermal and electrical performance.

Structure and Terminology

The main parts of a spark plug are the insulator, the metal shell, and the two electrodes. The insulator is made of sintered aluminum oxide (Al₂O₃) and filler material, such as zirconia, in order to provide good heat conduction, mechanical strength, resistance to thermal shock, and anti-corrosion. In addition, the insulator must provide a high electrical resistance, about 1 Megohm at 400ºC (750ºF), between the center electrode and the shell. The metal shell has six sides (hexagonal) for use with a socket wrench during installation and removal. It is usually made of extruded steel and is zinc-plated for anti-corrosion and anti-seizure. The lower section of the shell is threaded for attaching the spark plug to the cylinder head. The length of the threaded section (called the reach, which is 19mm in the 6G72 engine) must closely match the thickness of the cylinder head as shown below. The electrodes are often a nickel alloy or nickel-steel alloy containing chromium, manganese, and silicon, which provides very good anti-corrosion and anti-erosion properties. The ground electrode is welded to the threaded section of the shell. The center electrode is welded to a copper core that runs the length of the electrode and connects to the spark plug wire terminal at the top. Many modern electrodes are coated with a precious metal, such as platinum or iridium, or have a disk of precious metal welded to the tips. Sometimes one or both electrodes are made of precious metal alloy. The stock spark plugs for the DOHC 6G72 engine have a small disk of platinum welded to the tips of both electrodes.

Resistor spark plugs
The standard spark plug used in modern engines, including the 6G72 engine, is a resistor-type spark plug, which is indicated by the letter "R" under the NGK letters stamped on the insulator. A 4,500 to 5,000-ohm ceramic resistor is built into the spark plug core to suppress spark-generated electromagnetic noise that can interfere with the car's on-board electronics and radio signals.

Thermal Characteristics

While the actual temperature of combustion is in the range of 2000ºC to 2500ºC (3600ºF to 4500ºF), heat transfer in the spark plug and certain engine operational factors combine to maintain a spark plug firing-end temperature range from a low of about 400ºC to 500ºC to a high of about 850ºC to 1000ºC (750-930ºF to 1560-1830ºF).

Heat Transfer
The spark plug's secondary function is to remove heat from the combustion chamber (the primary function is discussed below). The figure below shows a general breakdown of where heat is absorbed and dissipated by the spark plug. The actual distribution will vary depending on the operating conditions and cylinder head and spark plug design.

Spark plug heat transfer

Heat Range
Spark plugs are designed to work with the insulator tip and center electrode temperature within the ideal heat range of about 500ºC to about 850ºC (~930ºF to ~1560ºF). If the plug tip temperature exceeds about 1050ºC (~1920ºF), the air-fuel mixture in the combustion chamber tends to pre-ignite before a spark is produced. If the temperature is less than about 400ºC (~750ºF), the insulator tip and center electrode tend to foul with carbon and oil deposits, which can cause misfires, lowered available spark voltage, and poor driveability.

The spark plug heat range is a measure of how fast heat is transferred away from the insulator tip and center electrode through the threads into the cylinder head and cooling system. The length, surface area, and projection of the insulator nose, the volume of space between the insulator nose and the shell, the center electrode core material, and the thermal conductivity of the insulator material control the heat range. In the hotter heat-range spark plug, a longer insulator nose creates a longer heat flow path from the insulator tip to the threads and a smaller heat transfer area in the insulator, which allows heat to build up in the tip. For the same type of plug, one heat range represents about 70º to 100ºC (128º to 180ºF) of heat dissipating ability.

Factors Affecting Plug Temperature
The operational factors that affect spark plug temperature include the following.

Ignition timing advance - temperature increases as advance increases; 10º of advance can cause 70º to 100ºC (128º to 180ºF) of temperature increase; note that pre-ignition temperatures also elevate with more advance.
Compression pressure - temperature increases in direct proportion to cylinder pressure or as compression ratio or boost pressure increase; note that pre-ignition temperatures decrease as cylinder pressure increases.
Engine speed and load - temperature increases as engine speed or load increase because these change the cylinder pressure (or the intake charge density); with the highest temperatures occurring during wide open throttle (WOT) conditions.
Air-fuel ratio - temperature is at a maximum around 13:1 A/F; very rich mixtures can cause fouling.
Tightening torque - temperature increases slightly if the plug is loose, and decreases very slightly if the plug is too tight; proper torque for our spark plugs (14-mm flat seat with gasket in an aluminum head) is 18 to 21.6 lb-ft (24.4 to 29 Nm).

Electrical Characteristics

The spark plug's primary function is to generate a spark across the gap between electrodes in order to start the combustion process. Combustion is initiated by the flame kernel created from the heat and gas ionization caused by the electrical energy of the spark. The voltage necessary to produce a spark is called the required voltage. It is the job of the ignition system to provide sufficient voltage to the spark plugs, called the available voltage, at the correct time during the compression stroke (typically 5 to 30 degrees before top dead center, BTDC). If the required voltage exceeds the available voltage then a misspark or misfire occurs, that is, there is no spark. If the available voltage exceeds the required voltage then there is a voltage reserve, and the spark occurs when the required voltage level is met. The factors that affect the magnitude of the required voltage are discussed here, along with the wasted spark system used.

Wasted Spark Distribution
The cylinders are arranged and numbered as shown below. There is one spark plug located in the center of the top of the combustion chamber for each cylinder in the Mitsubishi 6G72, DOHC, 24-valve, turbocharged, 2.972-liter, V6 engine. Because each piston begins its combustion stroke 120º before the next numerically successive cylinder, there are three "pairs" of cylinders, 1-4, 2-5, and 3-6. Each cylinder pair moves up and down together and is at top dead center (TDC) at the same time. For each pair of cylinders, there is one ignition coil.
Mitsubishi 6G72 cylinder numbers

Each ignition coil fires a pair of spark plugs simultaneously; one with the piston before TDC on the compression stroke and the other one with the piston before TDC on the exhaust stroke. The spark during the exhaust stroke is wasted and the basically inert exhaust gas does not combust. An interesting consequence of this system is that the center electrode is positively charged for one spark plug of the pair and negatively charged for the other plug.

Wasted spark distribution

Available Voltage Vs. Required Voltage
The type of ignition system used and the engine operating conditions determine the voltage available to the spark plug. The solid-state (distributorless) inductive ignition used in the DOHC 3000GT and Stealth can produce a long-duration secondary voltage (up to about 160 milliseconds, ms) at a frequency as high as 30,000 times a minute. The available secondary voltage varies from a high near 25 kilovolt (kV) at low engine speeds to a low around 15 kV at high engine speeds. Available voltage decreases at high engine speed because of the reduced time for primary current build-up. The available voltage is also reduced if the spark plug insulator is fouled with carbon deposits, oil, or liquid gasoline and the electrical resistance between electrodes is reduced.

Factors Affecting Required Voltage
The following operational factors affect the required voltage.

Spark gap - the voltage needed to ionize the electrode gap is directly proportional to the gap width and the required voltage increases with a larger gap.
Compression pressure - required voltage increases in direct proportion to cylinder pressure or as the air density increases.
Engine load and speed - required voltage changes in direct proportion with engine torque and throttle opening because these change the cylinder air density.
Air-fuel ratio - required voltage is at a minimum with a stoichiometric (14.7) air-fuel mixture because the air is just damp enough to improve electrical conductivity; rich mixtures increase required voltage because grounding across the insulator increases slightly, and lean mixtures increase required voltage because the air is drier.
Electrode temperature - between 200º and 600ºC (~400º and ~1100ºF) required voltage decreases rapidly as temperature rises; above 600ºC required voltage falls at a much-slower rate.
Electrode diameter - required voltage increases as the electrode diameter increases and as the roundness of the electrode edges increases.
Electrode polarity - using the model that electrons flow from the negative electrode to the positive electrode and knowing that electrons are released more freely when metal is very hot (even though resistivity of metals generally increases with temperature), because the center electrode is hotter than the ground electrode, the required voltage can be lower when the center electrode has the negative polarity (true for half of our spark plugs).

These factors combine to determine the voltage required for a spark to occur. Because the available voltage decreases but the required voltage increases at higher engine speeds with the 3000GT/Stealth ignition, it is necessary to minimize the required voltage to reduce or eliminate misfires. Reducing the spark gap is the easiest way to reduce the required voltage. However, the gap can be reduced to one too small to be practical. At this point the available voltage must be increased. One way to accomplish this is to use coils with a higher output. An alternative method would be to replace the solid-state inductive ignition with a capacitor discharge ignition that provides a very-short duration (less than a ms) secondary voltage that has nearly constant magnitude of about 27 kV over the entire range of engine speeds.

Factors affecting required voltage

Part Numbers and Cross Reference Guide

The NGK, Denso, Bosch, and Champion part numbers are partially explained below. For complete part number keys refer to:
Autolite: misc/autolite_sparkplugnumberingsystem.pdf
Bosch: misc/bosch_sparkplugs_designationcodes.gif
Bosch: misc/bosch_sparkplugs_designationcodes.jpg
Champion: misc/champion_sparkplugs_designationcodes.jpg
Denso: http://www.densoiridium.com/identifyplugs.htm
NGK: misc/ngk-partnumberkey.pdf

Note: The SOHC 6G72 engine uses 13/16" hex-shell spark plugs. The recommended spark plug is the NGK BPR5ES-11 or the Denso W16EPR11.

NGK PFR6J-11
  P   Platinum tips
  F   14-mm thread, 19-mm reach, 5/8" hex shell
  R   Resistor type
  6   Heat range (6 is medium, 5 is hotter, 7 is colder)
  J   ISO projected insulator nose
-11  1.1-mm (0.043") spark gap (missing number means not pre-gapped)

NGK BCPR6ES-11
  B   14-mm thread
  C   5/8" hex shell
  P   JIS Projected insulator nose
  R   Resistor type
  6   Heat range (6 is medium, 5 is hotter, 7 is colder)
  E   19-mm reach
  S   Standard 2.5-mm diameter center electrode with copper core
-11  1.1-mm (0.043") spark gap (missing number means not pre-gapped)

Denso PK20PR-P11
PK   14-mm thread, 5/8" hex shell, 19-mm reach, platinum tips
20   Heat range (20 is medium like NGK 6, 16 is hotter, 22 is colder)
  P   ISO projected insulator nose
  R   Resistor type
-P   Platinum tipped plug for DIS
11   1.1-mm (0.043") spark gap (missing number means not pre-gapped)

Denso IK24ER-U11
   I   Iridium power plug
  K   14-mm thread, 5/8" hex shell
24   Heat range (24 is medium like NGK 6, 22 is hotter, 27 is colder)
  E   ISO projected insulator nose
  R   Resistor type
-U   U-groove center electrode
11   1.1-mm (0.044") spark gap

Bosch FR7DCX
  F   14-mm thread, 5/8" hex shell
  R   Resistor type
  7   Heat range (7 and 6 are medium, 8 is hotter, 5 is colder)
  D   3/4" reach, 3-mm projected insulator nose
  C   copper-core center electrode (P is for platinum-core)
  X   1.1-mm (0.043") spark gap (missing letter means not pre-gapped)

Champion RC8YC4
  R   Resistor type
  C   14-mm thread, 5/8" hex shell, 3/4" reach
  8   Heat range (8 and 9 are medium, 10 is hotter, 7 is colder)
  Y   standard projected core nose
  C   copper-core center electrode
  4   wide spark gap required for Federal and California standards (missing number means not pre-gapped)

The stock spark plug for the turbocharged, DOHC, 6G72 engine is NGK PFR6J-11 or Denso PK20PR-P11, which have platinum-tipped electrodes. While NGK plugs are probably the easier of the two to obtain, they both can be hard to find at the corner auto parts store. NGK and Denso spark plugs are available online at the stores listed below.
http://www.clubplug.net/
http://www.sparkplugs.com/
http://www.jagworks.com/

The selection of replacement spark plugs is somewhat limited from the other major manufacturers, which include AC Delco, Autolite, Bosch, Champion, and Motorcraft (the problem is either the hex size or lack of a resistor). The green areas in the table show the available replacements in the same heat range. The red areas are one heat range hotter. The blue areas one or more heat ranges colder. At this time, there are no heat range 8 or colder resistor-type plugs available in a 5/8" hex, 14-mm thread, and 19-mm reach. Because the standard 13/16" socket does not fit in the spark plug well in the cylinder head, we cannot use spark plugs with a 13/16" (20.6-mm) hex size made by NGK (such as BPR6ES, BPR7ES, BPR8ES, etc.) and the other manufacturers. If you can use a non-resistor spark plug, then the "racing" plugs listed in the gray area of the table below are also an option. If switching from the stock platinum spark plug to either a resistor or non-resistor plug, then attention may need to be paid to the spark gap.

A higher voltage spark can cause more-complete combustion and therefore increase cylinder pressure and produce more power. The electrode tip diameter is much smaller for platinum and iridium plugs, which require less voltage than wider conventional plugs to generate a spark at a given gap. Consequently, the spark gap is widened for precious metal plugs to return the required voltage to conventional-plug levels. When switching from a platinum-tipped electrode spark plug to a conventional nickel-alloy electrode spark plug, the spark gap can be reduced from the typical platinum-plug gap of about 0.043" to the more typical conventional plug gap of 0.032" to 0.034". However, if the ignition system can support it, the standard gap of 0.040" to 0.043" (1.0-1.1 mm) can be used to produce a stronger spark with nickel-alloy electrode spark plugs.

Spark Plug Cross Reference Guide for the Mitsubishi DOHC 6G72
NGK	NGK Heat Range	Resistor Type	NGK Iridium IX	Denso	Denso Iridium Power	Denso Iridium Tough	AC Delco	Bosch	Champion
BCPR5ES BCPR5ES-11	5	Yes	BCPR5EIX-11	Q16R-U Q16R-U11	IQ22	VQ22	CFR3CLS -	FR8DC FR8DCX	RC10YC RC10YC4
BKR5ES BKR5ES-11	5	Yes	BKR5EIX BKR5EIX-11	K16PR-U K16PR-U11	IK22	VK22	-	FR8DPX*	RC10YC RC10YC4
PFR6J-11*	6	Yes	BKR6EIX-11	PK20PR-P11*	IK24	VK24	-	FR7DPX* FR6DPX*	-
BCPR6ES BCPR6ES-11	6	Yes	BCPR6EIX-11	Q20R-U Q20R-U11	IQ24	VQ24	CFR2CLS -	FR7DC,FR6DC FR7DCX,FR6DCX	RC9YC RC8YC RC9YC4 RC8YC4
BKR6ES BKR6ES-11	6	Yes	BKR6EIX BKR6EIX-11	K20PR-U K20PR-U11	IK24	VK24	- FR2LS	FR7DP,FR6DP FR7DPX* FR6DPX*	RC9YC RC8YC RC9YC4 RC8YC4
BCPR7ES BCPR7ES-11	7	Yes	-	Q22PR-U Q22PR-U11	IQ27	VQ27	CFR1CLS -	FR5DC	RC7YC RC7YC4
BKR7ES-11	7	Yes	-	K22PR-U11	IK27	VK27	-	FR5DP*	RC7YC RC7YC4
BCP6ES BCP6ES-11	6	No	-	Q20-U11	IQ24	VQ24	-	F6DCX	-
BCP7ES	7	No	-	Q22P-U	IQ27	VQ27	-	F5DC	-
R5672A-8	8	No	-	-	IQ29	-	-	-	C63YC
R5672A-9	9	No	-	-	IQ31	-	-	-	C59YC
* Indicates platinum tips on electrodes (platinum core for Bosch). - All listed plugs have a 14-mm thread diameter (1.25-mm pitch), 19-mm (3/4") reach, and 16-mm (5/8") hex size. - NGK R5672A are a "V-power" projected tip type BK-ES plug with nickel alloy electrodes available in heat ranges 8, 9, and 10. - Denso lists the following heat range equivalence with NGK: (NGK=Denso) 5=16, 6=20, 7=22, 8=24, 9=27. Nevertheless, many 3S owners report that Denso heat range 24 acts similar to NGK heat range 6 in 3S cars and so that is presented above. - Bosch Platinum 2-, 3-, and 4-electrode plugs are not listed because many owners report problems with these in our engines. - Autolite offers the "52" replacement for the R5672A-8 and the "51" replacement for the R5672A-9. - Motorcraft offers the "AGSP32C" replacement for the BCPR5ES-11 and the "AGPR12C" replacement for the BCPR7ES. - Some Champion plug type=stock number: RC10YC4=346, RC9YC=344, RC9YC4=430, RC8YC4=345, RC7YC=340, C63YC=796, C59YC=792. - Information taken from various sources on the internet.

Summary

Spark plug electrodes wear the longer they are used because of oxidation when the temperature is above 870ºC (1600ºF) and from chemical erosion above 580ºC (1076ºF) from the fuel's sulfur content or additive's. At very high temperatures, the electrodes can even melt (nickel alloy melting point is 1200-1300ºC, 2200-2370ºF). Oil leaking into the cylinder can cause deposits on the electrodes and insulators from carbon, calcium, sulfur, barium, or zinc. Therefore, spark plugs must be periodically inspected, the electrodes cleaned (special care is required with platinum or iridium tips) and the spark gap corrected to the proper width. In addition, the engine's operating condition can be judged by inspecting the firing end of the plug. The links below provide more information on how to "read" spark plugs.

http://www.stealth316.com/2-sparkchart.htm
http://www.ngksparkplugs.com/techinfo/spark_plugs/faq/faqread2.asp
http://www.gnttype.org/techarea/engine/plugs.html
http://dodgeram.info/Engine-Gas/SparkPlugs/spkplghnbook.html

The ignition coil produces a secondary voltage that rises to a very high peak value then falls over a very-short time duration. When the secondary voltage reaches a magnitude that ionizes the air between the spark plug electrodes, a spark occurs. If the voltage the coil produces (the available voltage) exceeds the voltage required to make a spark then there is a voltage reserve, but the spark still occurs at the lower required-voltage level. If the required voltage is higher than the available voltage a very weak spark or no spark occurs. Because the available voltage decreases but the required voltage increases at higher engine speeds and higher boost levels, reducing the required voltage helps to eliminate misfires. The precious-metal spark plugs (the ones with platinum or iridium disks welded to the electrode tips) and smaller spark gaps help to reduce the required voltage. However, a higher-voltage spark, which can produce higher cylinder pressure and so more power, is generated by a larger gap. In order to use a larger spark gap without misfires, especially at higher boost levels, the ignition system may need to be upgraded to increase the available voltage.

Online Stores

http://www.clubplug.net/
http://www.sparkplugs.com/
http://www.jagworks.com/

References

Century Performance, Spark Plugs
Chrysler Corporation, 1988, 1990 Laser Technical Information Manual: Part number 81-699-9002, various pagination.
Chrysler Corporation, 1990, 1991 Stealth Technical Information Manual: Part number 81-699-0114, various pagination. Available on-line at http://www.stealth316.com/2-stim.htm.
Club Plug, Spark Plugs Online
Denso Corporation, Denso Iridium Power
Denso Corporation, Denso Iridium Power (site 2)
DodgeRam.info, Champion Spark Plug Handbook
Edelbrock, Champion Spark Plugs
Extreme Motorsports, NGK Spark Plug FAQ
Heinz Heisler, 1995, Advanced Engine Technology: SAE International, 794 p.
Magnacor, The Truth About Ignition Wire Conductors
Mitsubishi Motors Corporation, 1999, 1992 - 1996 3000GT Service Manual, Vol. 1, Body and Chassis: Pub No. MSSP-001B-96 (1/2), various pagination.
Mitsubishi Motors Corporation, 1998, 1999 3000GT Service Manual, Vol. 1 & 2: Pub No. MSSP-001B-99 (1/2), various pagination.
Monarch Products, Inc., Spark Plugs Online
NGK Spark Plug Co., Ltd., 1974, Engineering Manual For Spark Plugs: NGK, 36 p.
NGK Spark Plug Co., Ltd., NGK Iridium Spark Plugs
NGK Spark Plug Co., Ltd., NGK Spark Plug Tech Info
NGK Spark Plug Co., Ltd., NGK Spark Plug Tech Info (site 2)
NGK Spark Plug Co., Ltd., NGK Spark Plugs Part-Number Info
Racing Ohio, Spark Plugs
The Sentra Network, Spark Plugs Overview by NGK

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Except for the small gif and jpg images, the content, images, photographs, text, and multimedia displayed are Copyright ©2000-2006 by Jeff Lucius and K2 Software. All rights reserved. No part, section, image, photo, article, or whole of this site may be reposted or redisplayed without permission of the authors.
Page last updated January 25, 2006.

Spark Plug Technical Informationfor the DOHC 6G72 Engine in theMitsubishi 3000GT and Dodge Stealth